CONTENTS Summary Introduction Fishery Data Biological Data Research Survey Indices Estimates of Mortality and Stock Size Biological Reference Points Projections Conclusions Acknowledgments Literature Cited
Northeast Fisheries Science Center Reference Document 03-09
Stock Assessment of Summer Flounder for 2003
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Web version posted August 7, 2003Citation: Terceiro, M. 2003. Stock assessment of summer flounder for 2003. Northeast Fish. Sci. Cent. Ref. Doc. 03-09; 179 p.
Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.SUMMARY
This assessment of the summer flounder (Paralichthys dentatus) stock along the Atlantic coast (Maine to North Carolina) is an update through 2002/2003 of commercial and recreational fishery catch data, research survey indices of abundance, and the analyses of the data. For 2002, commercial and recreational fishery quotas were 6,612 mt and 4,408 mt, respectively, for a total of 11,020 mt. The reported commercial landings used in this assessment for 2002 were 6,407 mt, while estimated recreational landings were 3,610 mt, for a 2002 landings total of 10,017 mt.
An analytical assessment (virtual population analysis, VPA) of commercial and recreational total catch at age (landings plus discards) was conducted. Indices of recruitment and stock abundance were developed from Northeast Fisheries Science Center winter, spring and autumn, Massachusetts spring and autumn, Rhode Island annual, Connecticut spring and autumn, New Jersey annual, and Delaware annual trawl survey data. Recruitment indices were also developed from young-of-year surveys conducted by the states of North Carolina, Virginia, and Maryland.
The stock assessment indicates that the summer flounder stock is not overfished and overfishing is not occurring relative to the current biological reference points. The fishing mortality rate has declined from 1.32 in 1994 to 0.23 in 2002, below the overfishing definition reference point (Fthreshold = Ftarget = Fmax = 0.26). There is an 80% chance that the 2002 F was between 0.21 and 0.28. The estimate of F for 2002 may understate the actual fishing mortality; retrospective analysis shows that the current assessment method tends to underestimate recent fishing mortality rates (e.g., by about 40% over the last three years). Total stock biomass has increased substantially since 1989, and on January 1, 2003 was estimated to be 56,100 mt, 5% above the biomass threshold of 53,200 mt. There is an 80% chance that total stock biomass in 2003 was between 51,000 and 63,000 mt.
Spawning stock biomass (SSB; Age 0+) declined 72% from 1983 to 1989 (18,800 mt to 5,200 mt), but has increased eight-fold, with improved recruitment and decreased fishing mortality, to 42,200 mt in 2002. Retrospective analysis shows a tendency to slightly overestimate the SSB in the most recent years. The arithmetic average recruitment from 1982 to 2002 is 40 million fish at age 0, with a median of 35 million fish. The 2002 year class is currently estimated to be about average at 38 million fish. There is no consistent retrospective pattern in the estimation of the abundance of age 0 fish over the last three years. If the landings for 2003 do not exceed the Total Allowable Landings (TAL) and the proportion of catch discarded does not increase, the TAL in 2004 would need to be 12,790 mt (28.2 million lbs) to meet the target F rate of Fmax = 0.26 with 50% probability.
The Stock Assessment Workshop (SAW) Southern Demersal Working Group met on June 9, 2003 to assess the status of summer flounder. The following scientists and managers participated in the meeting:
Paul Caruso -- MADMF
Sarah McLaughlin -- NOAA Fisheries NERO
Chris Moore -- MAFMC
Paul Nitschke -- NOAA Fisheries NEFSC
David Simpson -- CTDEP
Mark Terceiro (Chair) -- NOAA Fisheries NEFSC
Although they were unable to attend the meeting, Najih Lazar of the RIDFW, Anne Mooney of the NYDEC, Don Byrne of the NJFGW, Stew Michels of the DEDFW, Steve Doctor of the MDDNR, Chris Bonzak of the Virginia Institute of Marine Science (VIMS), Rob O'Reilly of the VMRC, and Carter Watterson of the NCDMF provided research survey and/or fisheries catch data that were used in the assessment.
The following terms of reference were addressed for summer flounder:
1. Characterize the commercial and recreational catch including landings and discards.
2. Estimate fishing mortality, spawning stock biomass, and total stock biomass for the current year and characterize the uncertainty of those estimates.
3. Where appropriate, estimate a TAC and/or TAL based on stock status and target mortality rate for the year following the terminal assessment year.
4. Provide short term projections (2-3 years) of stock status based on the target fishing mortality rate.
For assessment purposes, the previous definition of Wilk et al. (1980) of a unit stock extending from Cape Hatteras north to New England has been accepted in this and previous assessments (e.g., NEFSC 2002). The joint Mid-Atlantic Fishery Management Council (MAFMC) Atlantic States Marine Fisheries Commission (ASMFC) Fishery Management Plan (FMP) for summer flounder has as a management unit all summer flounder from the southern border of North Carolina, northeast to the U.S.-Canadian border. A recent summer flounder genetics study, which revealed no population subdivision at Cape Hatteras (Jones and Quattro, 1999), is consistent with the definition of the management unit. A recent consideration of summer flounder stock structure incorporating new tagging data concluded that evidence supported the existence of stocks north and south of Cape Hatteras, with the stock north of Cape Hatteras possibly composed of two distinct spawning aggregations, off New Jersey and Virginia-North Carolina (Kraus and Musick, 2003). The conclusions of Kraus and Musick (2003) are consistent with the current assessment stock unit.
Amendment 1 to the FMP in 1990 established the overfishing definition for summer flounder as the fishing mortality rate equal to Fmax, initially estimated as 0.23 (NEFC 1990). Amendment 2 in1992 set target fishing mortality rates for summer flounder for 1993-1995 (F = 0.53) and 1996 and beyond (Fmax = 0.23). Major regulations enacted under Amendment 2 to meet those fishing mortality rate targets included: 1) an annual fishery landings quota, with 60% allocated to the commercial fishery and 40% to the recreational fishery, based on the historical (1980-1989) division of landings, with the commercial allocation further distributed among the states based on their share of commercial landings during 1980-1989; 2) commercial minimum landed fish size limit at 13 in (33 cm), as established in the original FMP; 3) a minimum mesh size of 5.5 in (140 mm) diamond or 6.0 in (152 mm) square for commercial vessels using otter trawls that possess 100 lb (45 kg) or more of summer flounder, with exemptions for the flynet fishery and vessels fishing in an exempted area off southern New England (the Northeast Exemption Area) during 1 November to 30 April; 4) permit requirements for the sale and purchase of summer flounder; and 5) annually adjustable regulations for the recreational fishery, including seasons, a 14 in (36 cm) minimum landed fish size, and possession limits.
Amendment 3 to the FMP revised the western boundary of the Northeast Exemption Area to 72°30'W (west of Hudson Canyon), increased the large mesh net possession threshold to 200 lbs during 1 November to 30 April, and stipulated that only 100 lbs could be retained before using a large mesh net during 1 May to 31 October. Amendment 4 adjusted Connecticut's commercial landings of summer flounder and revised the state-specific shares of the commercial quota accordingly. Amendment 5 allowed states to transfer or combine the commercial quota. Amendment 6 allowed multiple nets on board commercial fishing vessels if properly stowed, and changed the deadline for publication of overall catch limits and annual commercial management measures to 15 October and the recreational management measures to 15 February.
The results of previous assessments indicated that summer flounder abundance was not increasing as rapidly as projected when Amendment 2 regulations were implemented. In anticipation of the need to drastically reduce fishery quotas in 1996 to meet the management target of Fmax , the MAFMC and ASMFC modified the fishing mortality rate reduction schedule in 1995 to allow for more stable landings from year to year while slowing the rate of stock rebuilding. Amendment 7 to the FMP set target fishing mortality rates of 0.41 for 1996 and 0.30 for 1997, with a target of Fmax = 0.23 for 1998 and beyond. Total landings were to be capped at 8,400 mt (18.51 million lbs) in 1996-1997, unless a higher quota in those years provided a realized F of 0.23.
Amendment 12 in 1999 defined overfishing for summer flounder to occur when the fishing mortality rate exceeds the threshold fishing mortality rate of FMSY. Since FMSY could not be reliably estimated for summer flounder, Fmax = 0.24 was used as a proxy for FMSY, and was also defined as the target fishing mortality rate. The stock was defined to be overfished when the total stock biomass falls below the minimum biomass threshold of one-half of the biomass target, BMSY. Because BMSY could not be reliably estimated, the biomass target was defined as the product of total biomass per recruit and contemporary (1982-1996) median recruitment, estimated to be 153,350 mt (338 million lbs), with the biomass threshold defined as 76,650 mt (169 million lbs). In the 1999 stock assessment (Terceiro 1999), these reference points were updated using estimates of median recruitment (1982-1998) and mean weights at age (1997-1998), providing a biomass target of 106,444 mt (235 million lbs) and biomass threshold of 53,222 mt (118 million lbs). The Terceiro (1999) reference points were retained in the 2000 and 2001 stock assessments (NEFSC 2000, MAFMC 2001a) because of the stability of the input data. Concurrent with the development of the 2001 assessment, the MAFMC and ASMFC convened the ASMFC Summer Flounder Overfishing Definition Review Committee to review the reference points. The work of the Committee was reviewed by the MAFMC Scientific and Statistical Committee (SSC) in August 2001. The SSC recommended that the FMSY proxy of Fmax = 0.26 remain for 2002, and endorsed the recommendation of SARC 31 (NEFSC 2000) which stated that "...the use of Fmax as a proxy for FMSY should be reconsidered as more information on the dynamics of growth in relation to biomass and the shape of the stock recruitment function become available (MAFMC 2001b).
The most recent previous stock assessment completed in 2002 (SARC 35; NEFSC 2002) found that the summer flounder stock was overfished and overfishing was occurring relative to the current biological reference points. The fishing mortality rate had declined from 1.32 in 1994 to 0.27 in 2001, marginally above the overfishing definition reference point (Fthreshold = Ftarget = Fmax = 0.26). Total stock biomass in 2001 was estimated to be 42,900 mt, 19% below the biomass threshold (53,200 mt). In the review of the 2002 stock assessment, SARC 35 concluded that updating the biological reference points was not warranted at that time (NEFSC 2002).
Commercial Fishery Landings
Total U.S. commercial landings of summer flounder from Maine to North Carolina peaked in 1979 at nearly 18,000 mt (40 million lbs, Table 1). The reported landings in 2002 of 6,407 mt (about 14.1 million lbs) were about 3% under the 2002 quota of 6,612 mt (14.6 million lbs). Since 1980, 70% of the commercial landings of summer flounder have come from the Exclusive Economic Zone (EEZ; greater than 3 miles from shore). The percentage of landings attributable to the EEZ was lowest in 1983 and 1990 at 63% and was highest in 1989 at 77%. Large variability in summer flounder landings exist among the states, over time, and the percent of total summer flounder landings taken from the EEZ has varied widely among the states.
Northeast Region (Maine to Virginia)
Annual commercial landings data for summer flounder in years prior to 1994 were obtained from trip-level detailed landings records contained in master data files maintained by the NEFSC (the weighout system; 1963-1993) and from summary reports of the Bureau of Commercial Fisheries and its predecessor the U.S. Fish Commission (1940-1962). Beginning in 1994, landings estimates were derived from mandatory dealer reports under the current NMFS Northeast Region (NER) summer flounder quota monitoring system.
Prior to 1994, summer flounder commercial landings were allocated to NEFSC 3-digit statistical area according to interview data (Burns et al. 1983). During 1994-2002, dealer landings were allocated to statistical area using fishing Vessel Trip Reports (VTR data) according to the general procedures developed by Wigley et al. (1997), in which a matched set of dealer and VTR data is used as a sample to characterize the statistical area distribution of monthly state landings. A comparison of the distribution of landings by state and month as indicated by the dealer, VTR, and matched set data for 1994-2002 is presented in Tables 2-10. Since the implementation of the annual commercial landings quota in 1993, the commercial landings have become concentrated during the first calender quarter of the year, with 46% of the landings taken during the first quarter in 2002 (Table 10).
The distribution of Northeast Region (ME to VA) 1992-2002 landings by three-digit statistical area is presented in Table 11. Areas 537-539 (Southern New England), areas 611-616 (New York Bight), areas 621, 622, 625, and 626 (Delmarva region), and areas 631 and 632 (Norfolk Canyon area) have generally accounted for over 80% of the NER commercial landings. A summary of length and age sampling of summer flounder landings collected by the NEFSC commercial fishery port agent system in the NER is presented in Table 12. For comparability with the manner in which length frequency sampling in the recreational fishery has been evaluated, sampling intensity is expressed in terms of metric tons of landings (mt) per 100 fish lengths measured. The sampling is proportionally stratified by market category (jumbo, large, medium, small, and unclassified), with the sampling distribution generally reflecting the distribution of commercial landings by market category. Overall sampling intensity has improved markedly since 1995, from 165 mt per 100 lengths to 30-60 mt per 100 lengths, and temporal and geographic coverage has generally improved as well (Tables 13-21).
The age composition of the NER commercial landings for 1994-2002 was generally estimated semiannually by market category and (usually) 1-digit statistical area (e.g., area 5 or area 6), using standard NEFSC procedures (market category length frequency samples converted to mean weights by length-weight relationships; mean weights in turn divided into landings to calculate numbers landed by market category; market category numbers at length apportioned to age by application of age-length keys, on semiannual area basis). For 2000-2002, sampling was generally sufficient to make quarterly estimates of the age composition in area 6 (in some cases, by division) for the large and medium market categories.
The distribution of 1994-2002 length frequency samples by market category, 1- and 2-digit statistical area (division), and calendar quarter is presented in Tables 13-21. NER landed numbers at age were raised to total NER (general canvas) commercial landings when necessary by assuming that landings not accounted for in the weighout/mandatory reporting system had the same age composition as that sampled, as follows: calculate proportion at age by weight; apply proportions at age by weight to total NER commercial landings to derive total NER commercial catch at age by weight; divide by mean weights at age to derive total NER commercial landed numbers at age. The proportion of large and jumbo market category fish in the NER landings has increased since 1996, while the proportion of small market category landings has become very low. The mean size of fish landed in the NER commercial fishery has been increasing since 1993, and was 0.9-1.0 kg (2.0-2.2 lbs) during 2000-2002, typical of an age 3 summer flounder (Tables 22-23).
The North Carolina winter trawl fishery accounts for about 99% of summer flounder commercial landings in North Carolina. A separate landings at age matrix for this component of the commercial fishery was developed from North Carolina Division of Marine Fisheries (NCDMF) length and age frequency sampling data. The NCDMF program samples about 10% of the winter trawl fishery landings annually, at a mean (2000-2002) rate of between 6 and 8 mt of landings per 100 lengths measured (Table 24). All length frequency data used in construction of the North Carolina winter trawl fishery landings at age matrix were collected in the NCDMF program; age-length keys from NEFSC commercial data and NEFSC spring survey data (1982-1987) and NCDMF commercial fishery data (1988-2002) were combined by appropriate statistical area and semiannual period to resolve lengths to age. Fishery regulations in North Carolina also changed between 1987 and 1988, with increases in both the minimum mesh size of the codend and minimum landed fish size taking effect. It is not clear whether the change in regulations or the change in keys, or some combination, is responsible for the decreases in the numbers of age-0 and age-1 fish estimated in the North Carolina commercial fishery landings since 1987. Landed numbers at age and mean weights at age from this fishery are shown in Tables 25-26.
Commercial Fishery Discards
In a previous assessment, analysis of variance of fishery observer data for summer flounder was used to identify stratification variables for an expansion procedure to estimate total landings and discards from the observer data kept and discard rates (weight per day fished) in the commercial fishery. Initial models included year, quarter, fisheries statistical division (2-digit area), area (divisions north and south of Delaware Bay), and tonnage class as main effects. Quarter and division consistently emerged as significant main effects without significant interaction with the year (NEFSC 1993). The estimation procedure expands transformation bias-corrected geometric mean catch (landings and discards) rates in year, quarter, and division strata by total days fished (days fished on trips landing any summer flounder by any mobile gear, including fish trawls and scallop dredges) to derive fishery landings and discards. The use of fishery effort as the multiplier (raising factor) allows estimation of landings from the fishery observer data for comparison with dealer reported landings, to help judge the potential accuracy of the procedure and/or sample data.
For strata with no fishery observer sampling, catch rates from adjacent or comparable strata were substituted as appropriate (except for Division 51, which generally has very low catch rates and negligible catch). Estimates of discard were stratified by 2 gear types (scallop dredges; trawls) for years when data were adequate (1992 and later years). Estimates at length and age were stratified by gear only for 1994-2000 and 2002, again due to sample size considerations. Only 11 fish were sampled from the sea scallop dredge fishery 2001, and so the scallop dredge discards were assumed to have the same length and age composition as the trawl fishery discards in 2001.
While estimates of catch rates from the NER fishery observer data were used in this assessment to estimate total discards, catch rate information is also reported in the VTR data. A comparison of discard to total catch ratios for the fishery observer and VTR data sets for trawl and scallop dredge gear indicates similar discard rates from the two data sources. Overall fishery observer and VTR discard to total catch ratios for 1994-2002 were generally within 10% of each other; 2001 was an exception, with an overall discard to total catch ratio of 49% in the fishery observer data and 29% in the VTR data. Discard rates of summer flounder in the scallop dredge fishery were much higher than in the trawl fishery (Tables 27-28).
The change in mid-1994 from the interview/weighout data reporting system to the VTR/mandatory dealer report system required a change in the estimation of effort (days fished) to estimate total discards. An initial examination of days fished and catch per unit effort (CPUE; landings per day fished) for cod conducted at SAW 24 (NEFSC 1997a) compared these quantities as reported in the full weighout and VTR data sets (DeLong et al., 1997). This comparison indicated a shift to a higher frequency of short trips (trips with one or two days fished reported), and to a mode at a lower rate of CPUE. It was not clear at SAW 24 if these changes were due to the change in reporting system (units reported not comparable), or real changes in the fishery, and so effort data reported by the VTR system were not used quantitatively in the SAW 24 assessments. In the SAW 25 assessment for summer flounder (NEFSC 1997b), a slightly different comparison was made. The port agent interview data for 1991-93 and merged dealer/VTR data for 1994-1996 (the matched set data), which under each system serve as the "sample" to characterize the total commercial landings, were compared in relative terms (percent frequency). For summer flounder, the percent frequency of short trips (lower number of days fished per trip) increased during 1991-1996, but not to the degree observed for cod, and the mode of CPUE rates for summer flounder increased in spite of lower effort per trip. For the summer flounder fishery, these may reflect actual changes in the fishery, due to increased restrictions on allowable landings per trip (trip landings limits might lead to shorter trips) and stock size increases (higher CPUE). As for cod, however, the influence of each of these changes (reporting system, management changes, stock size changes) has not been quantified. Total days fished in the summer flounder fishery were comparable between 1989-1993 period and 1994 (Tables 29-37; WO DF and WO/VTR DF). With increasing restrictions on the fishery in 1995-2002 (lower landings quota, higher stock size, and thus increasing impact of trips limits and closures), total days fished declined relative to the early 1990s (Tables 38-53). Questions will remain about the accuracy of the VTR data . However, because the effort measure is critical to the estimation of discards for summer flounder, the VTR data were used as the best data source to estimate summer flounder fishery days fished for 1994-2002.
Two adjustments were made to the dealer/VTR matched data subset days fished estimates to fully account for summer flounder fishery effort during 1994-2002. First, the landings to days fished relationship in the matched set was assumed to be the same for unmatched trips, and so the days fished total in each discard estimation stratum (2-digit area and quarter) was raised by the dealer to matched set landings ratio. This step in the estimation accounted for days fished associated with trips landing summer flounder, and provided an estimate of discard for trips landing summer flounder (Tables 36-53, variable OB EST DISC 1).
Given the restrictions on the fishery however, there is fishing activity which results in summer flounder discard, but no landings, especially in the scallop dredge fishery. The days fished associated with these trips was accounted for by raising strata discard estimates by the ratio of the total days fished on trips catching any summer flounder (trips with landings and discard, plus trips with discard only) to the days fished on trips landing summer flounder (trips with landings and discard) (Tables 36-53, variable NO KEPT RATIO), for VTR trips reporting discard of any species (DeLong et al. 1997). For this step , it is necessary to assume that the discard rate (as indicated by the fishery observer data, which includes trips with discard but no landings, and which is used in previous estimation procedure steps) is the same for trips with only discards as for trips with both landings and discards.
Discard estimates for 1989-2002 are summarized in Tables 29-53 (variable OB EST DISC MT). Discards as a proportion of the fishery observer data estimated landings (OB EST LAND MT) were highest in 2001 (53%), and lowest in 1995 and 1996 (5 and 7%). Estimates of landings from observer data ranged from +35% (1996) to -69% (2001) of the reported landings in the fisheries, with discards ranging from 41% (1990) to 6% (1995) of the reported landings. Total discards estimated for 2000, 2001, and 2002 were 18%, 16%, and 9% of the reported landings. Scallop dredge fishery discard to landed ratios are much higher than trawl fishery ratios, purportedly because of closures and trip limits. Although the scallop dredge landings of summer flounder are less than 5% of the total, the discards of summer flounder are of the same order of magnitude as in the trawl fishery.
These discard estimates were based only on the days fished data for ports in the NER during 1989-1996, and so it was necessary to raise the discard estimate to account for discarding occurring outside the NER reporting system (i.e., NER state reporting systems such as Connecticut and Virginia, and North Carolina). To determine the proper raising factor, landings accounted for by the NER reporting system (which result from the fishing effort on which the fishery observer discard estimate is based) were compared with total NER landings, plus that portion of North Carolina landings from the EEZ (it is assumed that only the North Carolina fishery in the EEZ would experience significant discard, as mesh regulations in state waters have resulted in very low discards in state waters since implementation of the regulation in 1989; R. Monaghan, NCDMF; personal communication, June 30, 1997). As a result of this exercise, the total discard estimates were raised by 11 to 38% for the 1989-1996 period. Since 1996, all states' landings and are included in the NER dealer reporting system, so no raising is necessary to account for missing landings. As recommended by SAW 16 (NEFSC 1993), a commercial fishery discard mortality rate of 80% was assumed to develop the final estimate of discard mortality (Table 54).
Existing fishery observer data were used to develop estimates of commercial fishery discard for 1989-2002. However, adequate data (e.g., interviewed trip data, survey data) are not available to develop summer flounder discard estimates for 1982-1988. Discard numbers were assumed to be very small relative to landings during 1982-1988 (because of the lack of a minimum size limit in the EEZ), but to have increased since 1989 with the implementation of fishery regulations under the FMP. It was recognized that not accounting directly for commercial fishery discards in 1982-1988 would result in an underestimation of fishing mortality and population sizes in these years.
NEFSC fishery observer length frequency samples were converted to sample numbers at age and sample weight at age frequencies by application of NEFSC survey length-weight relationships and fishery observer, commercial fishery, and survey age-length keys. Sample weight proportions at age were next applied to the raised fishery discard estimates to derive fishery total discard weight at age. Fishery discard weights at age were then divided by fishery observer mean weights at age to derive fishery discard numbers at age. Classification to age for 1989-1993 was done by semiannual (quarters 1 and 2 pooled, quarters 3 and 4 pooled) periods using NEFSC fishery observer age-length keys, except for 1989, when first period lengths were aged using combined commercial landings (quarters 1 and 2) and NEFSC spring survey age-length keys. For 1994-2002, only NEFSC winter, spring, and fall survey age-length keys were used, since fishery observer age-length keys were not yet available and commercial landings age-length keys contained an insufficient number of small summer flounder (<40 cm = 16 inches) that comprise most of the discards. Fishery observer sampling intensity is summarized in Table 54. Estimates of discarded numbers at age, mean length and mean weight at age are summarized in Tables 55-57.
The reason for discarding in the trawl and scallop dredge fisheries has been changing over time. During 1989 to 1995, the minimum size regulation was recorded as the reason for discarding summer flounder in over 90% of the observed trawl and scallop dredge tows. In 1999, the minimum size regulation was provided as the reason for discarding in 61% of the observed trawl tows, with quota or trip limits given as the discard reason in 26% of the observed tows, and high-grading in 11% of the observed tows. In the scallop fishery in 1999, quota or trip limits was given as the discard reason in over 90% of the observed tows. During 2000-2002, minimum size regulations were identified as the discard reason in 40-45% of the observed trawl tows, quota or trip limits in 25-30% of the tows, and high grading in 3-8%. In the scallop fishery during 2000-2002, quota or trip limits was given as the discard reason for over 99% of the observed tows. As a result of the increasing impact of trip limits, fishery closures, and high grading as reasons for discarding, the age structure of the summer flounder discards has also changed, with a higher proportion of older fish being discarded (Table 55).
Recreational Fishery Landings
Summary landings statistics for the summer flounder recreational fishery (catch type A+B1) as estimated by the National Marine Fisheries Service (NMFS) Marine Recreational Fishery Statistics Survey (MRFSS) are presented in Tables 58-59. Recreational fishery landings decreased 39% by number and 32% by weight from 2001 to 2002, as the fishery landed 82% (3,610 mt, 8.0 million lbs) of the 4,408 mt (9.7 million lbs) harvest limit established for 2002.
Length frequency sampling intensity for the recreational fishery for summer flounder was calculated by MRFSS subregions (North - Maine to Connecticut; Mid - New York to Virginia; South - North Carolina) on a metric tons of landings per hundred lengths measured basis (Burns et al. In Doubleday and Rivard, 1983). For 2002, aggregate sampling intensity averaged 112 mt of landings per 100 fish measured (Table 60).
MRFSS sample length frequency data, NEFSC commercial age-length data, and NEFSC survey age-length data were examined in terms of number of fish measured/aged on various temporal and geographical bases. Correspondences were made between MRFSS intercept date (quarter), commercial quarter, and survey season (spring and summer/fall), and between MRFSS subregion, commercial statistical areas, and survey depth strata to integrate data from the different sources. Based on the number, size range, and distribution of lengths and ages, a semiannual (quarters 1 and 2; quarters 3 and 4), subregional basis of aggregation was adopted for matching of commercial and survey age-length keys with recreational length frequency distributions to convert lengths to ages.
The recreational landings historically have been dominated by relatively young fish. Over the 1982-1996 period, age 1 fish accounted for over 50% of the landings by number; summer flounder of ages 0 to 4 accounted for over 99% of landings by number. No fish from the recreational landings were determined to be older than age 7. With increases in the minimum size during 1997-2001 (to 14.5 in [37 cm] in 1997, 15 in [38 cm] in 1998-1999, generally 15.5 in [39 cm] in 2000, and various state minimum sizes from 15.5 [38 cm] to 17.5 in [44 cm] in 2001-2002) and reductions in fishing mortality, the age composition of the recreational landings now includes mainly fish at ages 2 and 3. The number of summer flounder of ages 4 and older landed by the recreational fishery in 2002 (16% of the landings by number) was the highest in the time series (Table 61).
Limited MRFSS length sampling for larger fish resulted in a high degree of variability in mean length for older fish, especially at ages 5 and older. Attempts to estimate length-weight relationships from the MRFSS biological sampling data provided unsatisfactory results. As a result, quarterly length (mm) to weight (g) relationships from Lux and Porter (1966) were used to calculate annual mean weights at age from the estimated age-length frequency distribution of the landings.
Recreational Fishery Discards
MRFSS catch estimates were aggregated on a subregional basis for calculation of the proportion of live discard (catch type B2) to total catch (catch types A+B1+B2) in the recreational fishery for summer flounder. Examination of catch data in this manner shows that the live discard has varied from about 18% (1985) to about 81% (1999, 2001-2002) of the total catch (Table 62).
To account for all removals from the summer flounder stock by the recreational fishery, some assumptions about the biological characteristics and hooking mortality rate of the recreational live discard needed to be made, because biological samples are not routinely taken of MRFSS catch type B2 fish. In previous assessments, data available from New York Department of Environmental Conservation (NYDEC) surveys (1988-92) of New York party boats suggested the following: 1) nearly all (>95%) of the fish released alive from boats were below the minimum regulated size (during 1988-92, 14 in [36 cm] in New York state waters); 2) nearly all of these fish were age 0 and age 1 summer flounder; and 3) age 0 and 1 summer flounder occurred in approximately the same proportions in the live discard as in the landings. It was therefore assumed that all B2 catch would be of lengths below regulated size limits, and be either age 0 or age 1 in all three subregions during 1982-1996. Catch type B2 was allocated on a semi-annual, subregional basis in the same ratio as the annual age 0 to age 1 proportion observed in the landings during 1982-1996. Mean weights at age were assumed to be the same as in the landings during 1982-1996.
The minimum landed size in federal and most state waters increased to 14.5 in (37 cm) in 1997, to 15.0 in (38 cm) in 1998-1999, and to 15.5 in (39 cm) in 2000. Applying the same logic used to allocate the 1982-1996 recreational released catch to size and age categories during 1997-2000 implied that the recreational fishery released catch included fish of ages 2 and 3. Investigation of data from the CTDEP Volunteer Angler Survey (VAS) for 1997-1999 and from the American Littoral Society (ALS) for 1999, and comparing the length frequency of released fish in these programs with the MRFSS data on the length frequency of landed fish below the minimum size, indicated this assumption was valid for 1997-1999 (MAFMC 2001a). The CTDEP VAS and ALS data, along with data from the NYDEC Party Boat Survey (PBS) was used to validate this assumption for 2000. For 1997-2000 all B2 catch was assumed to be of lengths below regulated size limits, and therefore comprised of ages 0 to 3. Catch type B2 was allocated on a sub-regional basis in the same ratio as the annual age 0 to age 3 proportions observed in the landings at lengths less than 37 cm in 1997, 38 cm in 1998-1999, and 39 cm in 2000 (Table 63).
In 2001, many states adopted different combinations of minimum size and possession limits to meet management requirements. As a result, minimum sizes for summer flounder ranged from 15.5 in (39 cm) in Federal, VA, and NC waters, 16 in (41 cm) in NJ, 16.5 in (42 cm) in MA, 17 in (43 cm) in MD and NY, to 17.5 in (44 cm) in CT, RI, and DE. Examination of data provided by MD sport fishing clubs, the CTDEP VAS, the ALS, and the NYDEC PBS indicated that the assumption that fish released are those smaller than the minimum size remained valid for 2001, and so catch type B2 was characterized by the same proportion at length as the landed catch less than the minimum size in the respective states. The differential minimum sizes by state continued in 2002. For 2002, increased samples of the recreational fishery discards by the CT VAS and NYDEC PBS allowed direct characterization the length frequencies of the discards for these states (Table 63).
Studies conducted to estimate hooking mortality for striped bass and black sea bass suggest a hooking mortality rate of 8% for striped bass (Diodati and Richards 1996) and 5% for black sea bass (Bugley and Shepherd, 1991). Work by the states of Washington and Oregon with Pacific halibut (a potentially much larger flatfish species, but otherwise morphologically similar to summer flounder) found "average hooking mortality...between eight and 24 percent" (IPHC, 1988). An unpublished tagging study by the NYDEC (Weber MS 1984) on survival of released sublegal summer flounder caught by hook-and-line suggested a total, non-fishing mortality rate of 53%, which included hooking plus tagging mortality as well as deaths by natural causes (i.e., predation, disease, senescence). Assuming deaths by natural causes to be about 18%, (an instantaneous rate of 0.20), an annual hooking plus tagging mortality rate of about 35% can be derived from the NYDEC results. In the SARC 25 (NEFSC 1997b) and earlier assessments of summer flounder, a 25% hooking mortality rate was assumed for summer flounder released alive by anglers.
However, two recent investigations of summer flounder recreational fishery release mortality suggest that a lower release mortality rate is more appropriate. Lucy and Holton (1998) used field trials and tank experiments to investigate the release mortality rate for summer flounder in Virginia, and found rates ranging from 6% (field trials) to 11% (tank experiments). Malchoff and Lucy (1998) used field cages to hold fish angled in New York and Virginia during 1997 and 1998, and found a mean short term mortality rate of 14% across all trials. Given the results of these release mortality studies conducted specifically for summer flounder, a 10% release mortality rate was adopted in the Terceiro (1999) stock assessment and has been retained in all subsequent assessments (NEFSC 2000, MAFMC 2001a, NEFSC 2002).
Ten percent of the total B2 catch at age is added to estimates of summer flounder landings at age to provide estimates of summer flounder recreational fishery discard at age (Table 63), total recreational fishery catch at age in numbers (Table 64) and mean weights at age (Table 65). In 2002, the number of fish discarded and assumed dead in the recreational fishery (Table 63: 1.3 million fish, 676 mt) was 41% by number and 19% by weight of the total landed (Table 61: 3.2 million fish; Table 60: 3,610 mt) in the recreational fishery.
Total Catch Composition
NER commercial fishery landings and discards at age, North Carolina winter trawl fishery landings and discards at age, and MRFSS recreational fishery landings and discards at age totals were summed to provide a total fishery catch at age matrix for 1982-2002 (Table 66; Figure 1). The percentage of age-3 and older fish in the total catch in numbers has increased during the last decade from only 4% in 1993 to 41% in 2002. Overall mean lengths and weights at age in the total catch were calculated as weighted means (by number in the catch at age) of the respective mean values at age from the NER commercial (Maine to Virginia), North Carolina commercial, and recreational (Maine to North Carolina) fisheries (Tables 67-68; Figure 2). The recreational fishery component of the total summer flounder catch has generally increased since 1995 (Table 69; Figure 3).
Work performed for the SAW 22 assessment (NEFSC 1996b) indicated a major expansion in the size range of 1-year old summer flounder collected during the 1995 and 1996 NEFSC winter bottom trawl surveys, and brought to light differences between ages determined by NEFSC and NCDMF fishery biology staffs. Age structure (scale) exchanges were performed after the SAW 22 assessment to explore these differences. The results of the first two exchanges, which were reported at SAW 22 (NEFSC 1996b), indicated low levels of agreement between age readers at the NEFSC and NC DMF (31 and 46%). In 1996, research was conducted to determine inter-annular distances and to back-calculate mean length at age from scale samples collected on all NEFSC bottom trawl surveys (winter, spring and fall) for comparison with NCDMF samples. While mean length at age remained relatively constant from year to year, inter-annular distances increased sharply in the samples from the 1995-1996 winter surveys, and increased to a lesser degree in samples from other 1995-1996 surveys. As a result, further exchanges were suspended pending the resolution of an apparent aging problem.
Age samples from the winter 1997 bottom trawl survey, aged utilizing both scales and otoliths by only by one reader, indicated a similar pattern as the previous two winter surveys (i.e., several large age 1 individuals), and some disagreement between scale and otolith ages obtained from the same fish. Because of these problems, a team of five experienced NEFSC readers was formed to re-examine the scales aged from the winter survey. After examining several hundred scales, the team determined that re-aging all samples from 1995-1997 would be appropriate, including all winter, spring, and fall samples from the NEFSC and MA DMF bottom trawl surveys and all samples from the commercial fishery. The age determination criteria remained the same as those developed at the 1990 summer flounder workshop (Almeida et al. 1992) and described in the aging manual utilized by NEFSC staff (Dery 1997). Only those fish for which a 100% agreement of all group members was attained were included in the revised database, however. The data from the re-aged database were used in analyses in the SAW 25 assessment (NEFSC 1997b).
A third summer flounder aging workshop was held at the NEFSC in February, 1999, to continue the exchange of age structures and review of aging protocols for summer flounder (Bolz et al. 2000). Participants at this workshop concluded that the majority of aging disagreements in recent NEFSC-NCDMF exchanges arose from the interpretation of marginal scale increments due to highly variable timing of annulus formation, and from the interpretation of first year growth patterns and first annulus selection. The workshop recommended regular samples exchanges between NEFSC and NCDMF, and further analyses of first year growth. An exchange of NEFSC and NCDMF aging structures for summer flounder will again be conducted in 2003. Recently, Sipe and Chittenden (2001) concluded that sectioned otoliths were the best structure for aging summer flounder over the age range from 0 to 10 years. Since 2001, both scales and otoliths have routinely been collected in all NEFSC trawl surveys for fish larger than 60 cm, and studies are underway to determine the best structure to use for aging these large summer flounder.
The maturity schedule for summer flounder used in the 1990 SAW 11 and subsequent stock assessments through 1999 was developed by the SAW 11 Working Group using NEFSC Fall Survey maturity data for 1978-1989 and mean lengths at age from the NEFSC fall survey (G. Shepherd, NEFSC, personal communication, July 1, 1990; NEFC 1990; Terceiro 1999). The SAW 11 work indicated that the median length at maturity (50th percentile, L50) was 25.7 cm for male summer flounder, 27.6 cm for female summer flounder, and 25.9 cm for the sexes combined. Under the aging convention used in the SAW 11 and subsequent assessments (Smith et al. 1981, Almeida et al. 1992, Szedlmayer and Able 1992, Bolz et al. 2000), the median age of maturity (50th percentile, A50) for summer flounder was determined to be 1.0 years for males and 1.5 years for females. Combined maturities indicated that at peak spawning time in the autumn, that 38% of age-0 fish are mature, 72% of age-1 fish are mature, 90% of age-2 fish are mature, 97% of age-3 fish are mature, 99% of age-4 fish are mature, and 100% of age-5 and older fish are mature. The maturities for age-3 and older were rounded to 100% in the SAW 11 and subsequent assessments.
In the series of summer flounder assessments, it has been noted that the NEFSC maturity schedules have been based on simple gross morphological examination of the gonads and therefore may not accurately reflect (i.e., may overestimate) the true spawning potential of the summer flounder stock (especially for age-0 and age-1 fish). It should also be noted, however, that spawning stock biomass (SSB) estimates based on age-2 and older fish show the same long term trends in SSB as estimates which include age 0 and 1 fish in the spawning stock. A research recommendation that the true spawning contribution of young summer flounder to the SSB be investigated has been included in summer flounder stock assessments since 1993 (NEFSC 1993). In light of the completion of a URI study to address this research recommendation, the maturity data for summer flounder for 1982-1998 were examined in the 2000 assessment (NEFSC 2000) to determine if changes in the maturity schedule were warranted.
The research at the University of Rhode Island (URI) by Drs. Jennifer Specker and Rebecca Rand Merson (hereafter referred to collectively as the "URI 1999" study) attempted to address the issue of the true contribution of young summer flounder to the spawning stock. The URI 1999 study examined the histological and biochemical characteristics of female summer flounder oocytes (1) to determine if age-0 and age-1 female summer flounder produce viable eggs, and (2) to develop an improved guide for classifying the maturity of summer flounder collected in NEFSC surveys (Specker et al. 1999, Merson et al. 2000, Merson et al. In review). The URI 1999 study examined 333 female summer flounder (321 aged fish) sampled during the NEFSC Winter 1997 Bottom Trawl Survey (February 1997) and 227 female summer flounder (210 aged fish) sampled during the NEFSC Autumn 1997 Bottom Trawl Survey (September 1997) using radioimmunoassays to quantify the biochemical cell components characteristic of mature fish.
The NEFSC and URI 1999 maturity determinations disagreed for 13% of the 531 aged fish, with most (10%) of the disagreement due to NEFSC mature fish classified as immature by the URI 1999 histological and biochemical criteria. The URI 1999 criteria indicated that 15% of the age-0 fish were mature, 82% of the age-1 fish were mature, 97% of the age-2 fish were mature, and 100% of the age 3 and older fish were mature. When the proportions of fish mature at length and age were estimated by probit analysis, median length at maturity (50th percentile, L50) was estimated to be 34.7 cm for female summer flounder, with the following proportions mature at age: age-0: 30%, age-1: 68%, age-2: 92%, age-3: 98%, and age-4: 100%. Median age of maturity (50th percentile, A50) was estimated to be about 0.5 years.
SARC 31 (NEFSC 2000) considered 5 options for the summer flounder maturity schedule for the 2000 stock assessment:
1) No change, use the maturity schedule for combined sexes as in the SAW 11 and subsequent assessments (rounded to 0.38, 0.72, 0.90, 1.00, 1.00, and 1.00 as in the SAW 25 and Terceiro (1999) assessment analyses).
2) Consider only age-2 and older fish of both sexes in the SSB.
3) Knife edged, age-1 and older maturity for both sexes. This would eliminate age-0 fish of both sexes from the SSB, and assume that the proportions mature at age-1 "round" to 100%.
4) NEFSC 1982-1989, 1990-1998 for both sexes, assuming a 1:1 sex ratio in deriving a combined schedule.
5) NEFSC 1982-1989, 1990-1998 for males, URI 1999 for females, assuming a 1:1 sex ratio in deriving a combined schedule.
The 5 options produce the following maturity schedules for both sexes combined:
Option Age 0 1 2 3 4 5+ 1 0.38 0.72 0.90 1.00 1.00 1.00 2 0.00 0.00 0.90 1.00 1.00 1.00 3 0.00 1.00 1.00 1.00 1.00 1.00 4 0.45, 0.45 0.88, 0.82 0.97, 0.93 1.00, 0.98 1.00, 0.99 1.00, 1.00 5 0.29, 0.31 0.74, 0.76 0.95, 0.94 0.99, 0.98 1.00, 1.00 1.00, 1.00
SARC 31 concluded that some contribution to spawning from ages 0 and 1 should be included, eliminating options 2 and 3. The differences among remaining options 1, 4, and 5 were considered to be relatively minor, and so the SAW 11 schedule (Option 1) was retained for the subsequent (MAFMC 2001a, NEFSC 2002) and current assessments. SARC 31 recommended that more biochemical and histological work should be done for additional years to determine if the results of the URI 1999 study will be applicable over the full VPA time series. SARC 31 also noted the need for research to explore whether the viability of eggs produced by young, first time spawning summer flounder is comparable to the viability of eggs produced by older, repeat spawning summer flounder.
RESEARCH SURVEY INDICES
Long-term trends in summer flounder abundance were derived from a stratified random bottom trawl survey conducted in spring by NEFSC between Cape Hatteras and Nova Scotia since 1968 (Clark 1979). NEFSC spring survey indices suggest that total stock biomass last peaked during 1976-1977, and the 2003 index (2.42 kg/tow) was at a new historical high, about 20% above the peak 1976 value of 2.00 kg/tow (Table 70, Figure 4). Age composition data from the NEFSC spring surveys indicate a substantial reduction in the number of ages in the stock between 1976-1990 (Table 71). Between 1976-1981, fish of ages 5-8 were captured regularly in the survey, with the oldest individuals aged 8-10 years. Between 1982-1986, fish aged 5 and older were only occasionally observed in the survey, and by 1986, the oldest fish observed in the survey were age 5. In 1990 and 1991, only three age groups were observed in the survey catch, and there was an indication that the 1988 year class was very weak. Since 1991, the survey age composition has expanded significantly. There is strong evidence in the 1998-2002 NEFSC spring surveys of increasing abundance of age-3 and older fish, due to increased survival of the 1994 and subsequent year classes. Mean lengths at age in the NEFSC spring survey are presented in Table 72.
Summer flounder are frequently caught in the NEFSC autumn survey at stations in inshore strata (< 27 meters = 15 fathoms = 90 feet) and at offshore stations in the 27-55 meter depth zone (15-30 fathoms, 90-180 feet) at about the same level as in the spring survey (Table 70). Furthermore, the autumn survey catches age-0 summer flounder in abundance, providing an index of summer flounder recruitment (Table 73, Figure 5). Autumn survey indices suggest improved recruitment since the late 1980s, and an increase in abundance of age-2 and older fish since 1995. The NEFSC autumn surveys indicate that the 1995 year class was the most abundant in recent years, and that subsequent, weaker year classes are experiencing increased survival (Table 73). Mean lengths at age in the NEFSC autumn survey are presented in Table 74.
A new series of NEFSC winter trawl surveys was initiated in February 1992 to provide improved abundance indices for flatfish, including summer flounder. The surveys target flatfish when they are concentrated offshore during the winter. A modified 36 Yankee trawl is used that differs from the standard trawl employed during the spring and autumn surveys in that long trawl sweeps (wires) are added before the trawl doors to better herd fish to the mouth of the net, and the large rollers used on the standard gear are absent with only a chain "tickler" and small spacing "cookies" are present on the footrope.
The design and conduct of the winter survey (timing, strata sampled, and the use of the modified 36 Yankee trawl gear) has resulted in greater catchability of summer flounder compared to the other surveys. Most fish area captured in survey strata 61-76 (27-110 meters; 15-60 fathoms) off the Delmarva and North Carolina coasts . Other concentrations of fish are found in strata 1-12, south of the New York and Rhode Island coasts, in slightly deeper waters. Significant numbers of large summer flounder are often taken along the southern flank of Georges Bank (strata 13-18).
Indices of summer flounder abundance from the winter survey indicate stable stock size during 1992-1995, with catch per tow values ranging from 10.9 in 1995 to 13.6 in 1993 (Tables 70 and 75). For 1996, the winter survey index increased by 290% over 1995, from 10.9 to 31.2 fish per tow. The largest increases in 1996 occurred in the Mid-Atlantic Bight region (offshore strata 61-76), where increases up to an order of magnitude occurred in several strata, with the largest increases in strata 61, 62, and 63 off the northern coast of North Carolina. Most of the increased catch in 1996 consisted of age-1 summer flounder from the 1995 year class. In 1997, the index dropped to 10.3 fish per tow, due to the lower numbers of age-1 (1996 year class) fish caught. Since 1998, the Winter trawl survey indices have increased, with the Winter 2003 survey number and weight per tow indices the highest in the time series (Tables 70 and 75, "Figure 4). As with the other two NEFSC surveys, there is strong evidence in recent winter surveys of increased abundance of age-3 and older fish relative to earlier years in the time series (Table 76). Mean lengths at age in the NEFSC winter survey are presented in Table 77.
Spring and fall bottom trawl surveys conducted by the Massachusetts Division of Marine Fisheries (MADMF) show a decline in abundance in numbers of summer flounder from high levels in 1986 to record lows in 1990 (MADMF fall survey), and 1991 (MADMF spring survey). In 1994, the MADMF survey indices increased to values last observed during 1982-1986, but then declined substantially in 1995, although the indices remain higher than the levels observed in the late 1980s. Since 1996, both the MADMF spring and fall indices have increased to record high levels (Tables 78-79, Figure 6). The MADMF also captures a small number of age-0 summer flounder in a seine survey of estuaries, and these data constitute an index of recruitment (Table 80, Figure 7).
Spring and fall bottom trawl surveys are conducted by the Connecticut Department of Environmental Protection (CTDEP). The CTDEP surveys show a decline in abundance in numbers of summer flounder from high levels in 1986 to record lows in 1989. The CTDEP surveys indicate recovery since 1989, and evidence of increased abundance at ages 2 and older since 1995. The 2002 spring and autumn indices were the highest in the respective time series (Tables 81-82, Figure 8). An index of recruitment from the autumn series is available (Table 82, Figure 5).
Rhode Island DFW
Standardized bottom trawl surveys have been conducted since 1979 during the spring and fall months in Narragansett Bay and state waters of Rhode Island Sound by the Rhode Island Department of Fish and Wildlife (RIDFW). Indices of abundance at age for summer flounder have been developed from the autumn survey data using NEFSC autumn survey age-length keys. Survey indices show that the 1984-1987, 1999, 2000, and 2002 year classes are all strong. The autumn survey reached a time series high in 2002 (Table 83, Figure 6). An abundance index has also been developed from a set of fixed stations sampled monthly during 1990-2002. Age-1 indices from this series indicate that strong year classes recruited to the stock in 1996, 1999, 2000, and 2002, with age 2+ abundance peaking in 2000 (Table 84). Recruitment indices are available from both the autumn (Figure 7) and monthly fixed station surveys.
New Jersey BMF
The New Jersey Bureau of Marine Fisheries (NJBMF) has conducted a standardized bottom trawl survey since 1988. Indices of abundance for summer flounder incorporate data collected from April through October. The NJBMF survey mean number per tow indices and frequency distributions were converted to age using the corresponding annual NEFSC combined spring and fall survey age-length keys. Indices of the 1995 year class at age-0 and at older ages in subsequent years indicate that this cohort is the strongest in the time series. Indices of the 1996-2001 year classes are below average, while the 2002 year class is average. The NJBMF survey indices reached a peak in 2002 (Table 85, Figure 8). Age 0 recruitment indices are available from the NJBMF survey (Figure 5).
The Delaware Division of Fish and Wildlife (DEDFW) has conducted a standardized bottom trawl survey with a 16 foot headrope trawl since 1980, and with a 30 foot headrope trawl since 1991. Recruitment indices (age 0 fish; one index from the Delaware estuary proper for 1980 and later, one from the inland bays for 1986 and later) have been developed from the 16 foot trawl survey data. Indices for age-0 to age-4 and older summer flounder have been compiled from the 30 foot headrope survey. The indices use data collected from June through October (arithmetic mean number per tow), with age 0 summer flounder separated from older fish by visual inspection of the length frequency. The 16 foot headrope survey indices suggest poor recruitment in 1988 and 1993, improved recruitment in 1994-1995, and above average recruitment in 2000 (Tables 86-87, Figure 7). The 30 foot headrope survey indices suggest stable stock sizes over the 1991-2001 time series, with strong recruitment in 1991, 1994, 1995, and 2000. The 2002 index from the 30 foot survey was a time series low, presumably reflecting decreased availability to the survey, rather than a true decrease in abundance (Table 88, Figure 8).
The Maryland Department of Natural Resources (MDDNR) has conducted a standardized trawl survey in the seaside bays and estuaries around Ocean City, MD since 1972. Samples collected during May to October with a 16 foot bottom trawl have been used to develop a recruitment index for summer flounder for the period 1972-2002. This index suggests that weakest year class in the time series recruited to the stock in 1988, and the strongest in 1972, 1983, 1986, and 1994. The 2000 and 2001 indices were about average, while the 2002 index was below average (Table 89, Figure 9).
Virginia Institute of Marine Science
The Virginia Institute of Marine Science (VIMS) conducts a juvenile fish survey using trawl gear in Virginia rivers and the mainstem of Chesapeake Bay. The time series for the rivers began in 1979. With the Bay included, the series is available only since 1988, but many more stations are included. Trends in the two time series are very similar. An index of recruitment developed from the rivers only series suggests weak year classes recruited to the stock in 1987 and 1999, with strong year classes recruiting during 1980-1984, and 1990. Recruitment indices since 1990 have been below average (Table 90, Figure 9).
North Carolina DMF
The NCDMF has conducted a stratified random trawl survey using two 30 foot headrope nets with 3/4" mesh codend in Pamlico Sound since 1987. An index of recruitment developed from these data suggests weak year classes recruited to the stock in 1988 and 2000, with strong year classes in 1987, 1992, and 1996, 2001, and 2002 (Table 91, Figure 9). The survey normally takes place in mid-June, but in 1999 was delayed until mid-July. The 1999 index is therefore inconsistent with the other indices in the time series, and the 1999 value was excluded from the VPA calibration in the SARC 31 assessment (NEFSC 2000).
ESTIMATES OF MORTALITY AND STOCK SIZE
Natural Mortality Rate
The instantaneous natural mortality rate (M) for summer flounder was assumed to be 0.2 in all analyses, although alternative estimates of M were considered in the SAW 20 assessment (NEFSC 1996a). In the SAW 20 work, estimates were derived with the methods described by: 1) Pauly (1980) using growth parameters derived from NCDMF age-length data and a mean annual bottom temperature (17.5oC) from NC coastal waters; 2) Hoenig (1983) using a maximum age for summer flounder of 15 years; and 3) consideration of age structure expected in unexploited populations (5% rule, 3/M rule, e.g., Anthony 1982). SAW 20 (NEFSC 1996a) concluded that M = 0.2 was a reasonable value given the mean (0.23) and range (0.15-0.28) obtained from the various analyses, and this value for M has been used in all subsequent assessments.
The non-equilibrium surplus production model incorporating covariates (ASPIC; Prager 1994, 1995) can be used to estimate maximum sustainable yield (MSY) and other biological reference points. An ASPIC analysis applied to summer flounder using various state and federal agency survey biomass indices (the 1998 analysis) was previously reviewed by the NEFMC Overfishing Review Panel (Applegate et al. 1998). Based on total weighted mean squared error (MSE), the NEFSC spring and autumn biomass indices gave the best fit to the data in that analysis. However, the Overfishing Review Panel concluded that biological reference points estimated in the 1998 analysis for summer flounder were unreliable, due to the short time series of reliable catch estimates and lack of dynamic range in the input data (Applegate et al. 1998).
An ASPIC analysis using projected catch and NEFSC survey biomass indices through 1999 was reviewed in the 1999 assessment (Terceiro 1999). Model results were examined for sensitivity by employing a Monte Carlo search routine and by initializing over a broad range the values of MSY (10,000 to 50,000 mt) and the intrinsic rate of increase (r : 0.12 to 1.25). The ratio of initial to current biomass (B1 ratio) was assigned a starting value of 0.50. Overall, the 1999 ASPIC model results for summer flounder were not well defined and suggested the possibility of numerous local minima in the sums of squared errors (SSE) response surface. The Monte Carlo search algorithm was employed in an attempt to provide a better search of the SSE response surface, and the this generated a range of estimates of MSY from 19,000 mt to 58,000 mt and of r from 0.49 to 1.08. Due to the number of iterations needed to reach convergence (>25) and the probable number of local minima, these results also appeared to be unreliable. Thus, biological reference points for summer flounder estimated by the 1999 ASPIC analysis were not considered to be robust, and the ASPIC analysis has not been repeated in the assessment.
Virtual population analysis
Fishing mortality rates in 2002 and stock sizes in 2003 were estimated using the ADAPT method for calibration of the VPA (Parrack 1986, Gavaris 1988, Conser and Powers 1990) as implemented in the NOAA Fisheries Toolbox (NFT) version 2.1 VPA. As recommended by the MAFMC S&S Committee during the review of the Terceiro (1999) assessment and by the National Research Council review of the summer flounder assessment (NRC 2000), ages 0-6 were included in the analysis as true ages, with ages 7 and older combined as a plus group. An instantaneous natural mortality rate of M = 0.2 was assumed for all ages in all years. Maturities at age for all years were 38% for age-0, 72% for age-1, 90% for age-2, and 100% for ages 3 and older. Stock sizes in 2003 were directly estimated for ages 1-6, while the age 7+ group was calculated from Fs estimated in 2002. Fishing mortality on the oldest true age (6) in the years prior to the terminal year was estimated from back-calculated stock sizes for ages 3-6. Fishing mortality on the age 7+ group was assumed equal to the fishing mortality for age 6. Winter, spring, and mid-year (e.g., RIDFW monthly fixed station, DEDFW, and NJBMF) survey indices and all survey recruitment (age-0) indices were compared to population numbers of the same age at the beginning of the same year. The recruitment indices available from the research surveys are summarized in Table 92. Fall survey indices were compared to population numbers one year older at the beginning of the next year. Tuning indices were unweighted.
A number of exploratory VPA runs using different combinations of research survey tuning indices were used to examine the sensitivity of the summer flounder VPA. The inclusion of each survey index was considered based on a pre-calibration correlation analysis among all indices, a post-calibration correlation analysis among the indices and resulting VPA estimates of stock size, and an examination of the VPA diagnostics (including the partial variance accounted for by each index, patterns in residuals, and the mean squared residual (MSR) of the calibrated solution). Survey indices with trends that did not reasonably match corresponding patterns in abundance as estimated by other indices and/or the VPA (as evidenced by poor correlation, high partial variance in tuning diagnostics, or patterns in residuals) were eliminated from the VPA tuning configuration.
The final run (run F03_1) included the same set of indices (n=41) in terms of source and age range as used in the 2002 SARC 35 assessment (NEFSC 2002). In addition to a run including all available indices (F03_ALL) and the run chosen as final (F03_1), the results from two other runs were also considered (Table 93). The NEFSC survey indices generally had the lowest partial variances within the VPA and demonstrated similar rank order of stock sizes at age (significant correlation among indices at age), but sometimes indicated patterns in stock size dissimilar to those in the state surveys. Therefore runs were also examined that contrasted the VPA solutions provided by NEFSC (F03_NEC) versus state survey (F03_STATE) series. Run F03_NEC had the smallest MSR of the six runs considered (about 12% smaller than final run F03_2), but due in part to fewer degrees of freedom, provided less precise (by about 50%) 2003 stock size estimates. Run F03_STATE had the largest MSR (Table 93). The output for the final 2003 assessment VPA (run F03_1) is presented in Table 94.
The annual partial recruitment of age-1 fish decreased from near 0.50 during the first half of the VPA time series to less than 0.30 since 1994, and to about 0.20 during 2000-2002; the partial recruitment of age-2 fish has decreased from 1.00 in 1993 to about 0.80 during 2000-2002 (Table 94). These decreases in partial recruitment at age are in line with expectations given recent changes in commercial and recreational fishery regulations. For these reasons, summer flounder are currently considered to be fully recruited to the fisheries at age 3, and fully recruited fishing mortality is expressed as the unweighted average of fishing mortality at age for ages 3 to 5.
Fishing mortality calculated from the average of the currently fully recruited ages (3-5) has been high, varying between 0.94 and 2.15 during 1982-1997 (55%-82% exploitation), far in excess of the revised FMP Amendment 12 overfishing definition, Fthreshold = Ftarget = Fmax = 0.26 (21% exploitation). Fishing mortality has declined substantially since 1997 and was estimated to be 0.23 (18% exploitation) in 2002, the lowest observed in the 21-year VPA time series (Figure 10).
Summer flounder spawn in the late autumn and early winter (peak spawning on November 1), and age 0 fish recruit to the fishery during the autumn after they are spawned. For example, summer flounder spawned in autumn 1987 (from the November 1, 1987 spawning stock biomass) recruit to the fishery in autumn 1988, and appear in VPA tables as age 0 fish in 1988. This assessment indicates that the 1982 and 1983 year classes were the largest of the VPA series, at 74 and 80 million fish, respectively. The 1988 year class was the smallest of the series, at only 13 million fish. The 2002 year class is estimated at 38 million fish, above the time series median of 35 million (Table 94, Figure 11 and Figure 12).
Total stock biomass has increased substantially since 1989, and at the beginning of 2003 total stock biomass was estimated to be 56,100 mt. Spawning stock biomass (SSB; Age 0+) declined 72% between 1983 and 1989 (18,800 mt to 5,200 mt), but has increased eight-fold, with improved recruitment and decreased fishing mortality, to 42,200 mt in 2002 (Table 94, Figures 11-12). In general, the abundance of summer flounder age 2 and older has increased substantially since the early 1990s. The age structure of the spawning stock has thus also expanded, with 80% at ages 2 and older, and 19% at ages 5 and older. Under equilibrium conditions at Fmax, about 85% of the spawning stock biomass would be expected to be ages 2 and older, with 50% at ages 5 and older (Figure 13).
A bootstrap procedure (Efron 1982) was used to evaluate the precision of the final VPA estimates with respect to random variation in tuning data (survey abundance indices). The procedure does not reflect uncertainty in the catch-at-age data. Five hundred bootstrap iterations were used to generate distributions of the 2002 fishing mortality rate and the 2003 total stock biomass. Histogram plots of the distribution of the terminal year VPA estimates visually indicate the amount of variability. The cumulative probability can be used to evaluate the risk of making a management decision based on the estimated value. For fishing mortality, the cumulative plot indicates the probability that the fishing mortality rate in 2002 was greater than a given level when measurement errors are considered (e.g., some target fishing mortality rate). For stock biomass, the cumulative plot indicates the probability that biomass at the beginning of 2003 was less than a given level (e.g., some desired minimum stock biomass).
The precision and bias of the 2002 fishing mortality rates, 1 January 2003 stock sizes, 1 November 2002 spawning stock biomass, 2002 mean stock biomass, and 1 January 2003 total stock biomass estimates are presented in Table 95. Bias was less than 10% for all parameters estimated. The bootstrap estimate of the 2003 total stock biomass was relatively precise, with a corrected CV of 9%. The bootstrap mean (56,717 mt) was slightly higher than the VPA point estimate (56,088 mt). The bootstrap results suggest a high probability (>90%) that total stock biomass in 2003 was at least 50,600 mt, reflecting only variability in survey observations (Table 95, Figure 14).
The corrected coefficients of variation for the Fs in 2002 on individual ages were 23% for age 0, 19% for age 1, 15% for age 2, 16% for age 3, 21% for age 4, 31% for age 5, 13% for age 6, and 13% for ages 7 and older. The distribution of bootstrap Fs was not strongly skewed, resulting in the bootstrap mean F for 2002 (0. 2392) being slightly higher than the point estimate from the VPA (0.2310). There is a 80% chance that F in 2002 was between about 0.21 and 0.28, given variability in survey observations (Table 95, Figure 14).
Retrospective analysis of the summer flounder VPA was carried out for terminal catch years 1998-2001. This "internal" retrospective analysis indicates a pattern of underestimation of fully recruited F (ages 3-5) for 1999-2001, continuing the pattern observed in the last three assessments (NEFSC 2000, MAFMC 2001a, NEFSC 2002). Fishing mortality was underestimated by 51% for 1999 (0.41 versus 0.84), by 40% for 2000 (0.60 versus 0.36), and by 20% for 2000 (0.35 versus 0.28), relative to the current VPA estimates. Spawning stock biomass has been generally been overestimated in recent years, ranging from 20% for 1999 and 2000 to 7% for 2001, relative to the current VPA estimates. There is no consistent retrospective pattern in the estimation of the abundance of age 0 fish over the last three years (Table 96, Figure 15). Comparison with previous assessments ("historical retrospective") shows a tendency to substantially underestimate fully-recruited fishing mortality (ages 2-4, for comparability across assessments) and slightly overestimate the SSB (ages 0-7+) from the the mid-1990s through 2000 (Figure 16). The 2002 (NEFSC 2002) and 2003 assessments provide the most consistent sequential estimates of fishing mortality and SSB since the 1996 and 1997 assessments.
BIOLOGICAL REFERENCE POINTS
The calculation of biological reference points based on yield per recruit for summer flounder using the Thompson and Bell (1934) model was detailed in the 1990 SAW 11 assessment (NEFC 1990). The 1990 analysis estimated Fmax = 0.23. In the 1997 SAW 25 assessment (NEFSC 1997b), an updated yield per recruit analysis reflecting the partial recruitment pattern and mean weights at age for 1995-1996 estimated that Fmax = 0.24. The analysis in the Terceiro (1999) assessment, reflecting partial recruitment and mean weights at age for 1997-1998, estimated that Fmax = 0.263 (Figure 17).
The Overfishing Definition Review Panel (Applegate et al. 1998) recommended that the MAFMC base MSY proxy reference points on yield per recruit analysis, and this recommendation was adopted in formulating the FMP Amendment 12 reference points (see Introduction), based on the 1999 assessment (Terceiro 1999). The 1999 assessment yield per recruit analysis indicated that Fthreshold = Ftarget= Fmax = 0.263, yield per recruit (YPR) at Fmax was 0.55219 kg/recruit, and January 1 biomass per recruit (BPR) at Fmax was 2.8127 kg/recruit. The median number of summer flounder recruits estimated from the 1999 VPA for the 1982-1998 period was 37.844 million fish. Based on this recruitment, maximum sustainable yield (MSY) was estimated to be 20,897 mt (46 million lbs) at a biomass (BMSY) of 106,444 mt (235 million lbs). The biomass threshold, one-half BMSY, was therefore estimated to be 53,222 mt (118 million lbs; Figure 18). The Terceiro (1999) reference points were retained in the 2000 and 2001 stock assessments (NEFSC 2000, MAFMC 2001a) because of the stability of the input data. In the review of the 2002 stock assessment, SARC 35 concluded that updating these reference points was not warranted (NEFSC 2002), and therefore the reference points were not updated in this assessment either.
Stochastic projections were made to provide forecasts of stock size and catches in 2003-2005 consistent with target reference points established in the FMP. The projections assume that recent patterns of discarding will continue over the time span of the projections. Different patterns that could develop in the future due to additional trip and bag limits and fishery closures have not been evaluated. The partial recruitment pattern (including discards) used in the projections was estimated as the geometric mean of F at age for 2000-2002, reflecting recent conditions in the fisheries. Mean weights at age were estimated as the geometric means of 2000-2002 values. Separate mean weight at age vectors were developed for the January 1 biomass, landings, and discards.
One hundred projections were made for each of the 500 bootstrapped realizations of 2003 stock sizes from the final 2003 VPA, using algorithms and software described by Brodziak and Rago (MS 1994) as implemented in the NFT AGEPRO version 3.01. Recruitment during 2003-2004 was generated randomly from a cumulative density function of the VPA recruitment series for 1982-2002 (median recruitment = 35.368 million fish). Other input parameters were as in Table 97; uncertainty in partial recruitment patterns, discard rates, or components other than survey variability was not considered.
If landings in 2003 are 10,570 mt (23.3 million lbs) and discards are 1,100 mt (2.4 million lbs), the forecast estimates a median (50% probability) F in 2003 = 0.25 and a median total stock biomass on January 1, 2004 of 63,600 mt, above the biomass threshold of ½ BMSY = 53,200 mt. Landings of 12,790 mt (28.2 million lbs) and discards of 1,300 mt (2.9 million lbs) in 2004 provide a median F in 2004 = 0.26 and a median total stock biomass level on January 1, 2005 of 70,500 mt. Landings of 14,500 mt (32.0 million lbs) and discards of 1,400 mt (3.1 million lbs) in 2005 provide a median F in 2005 = 0.26 and a median total stock biomass level on January 1, 2006 of 75,800 mt (Table 97, Figure 18 and Figure 19).
The summer flounder stock is not overfished and overfishing is not occurring relative to the current biological reference points. The fishing mortality rate has declined from 1.32 in 1994 to 0.23 in 2002, below the overfishing definition reference point (Fthreshold = Ftarget = Fmax = 0.26). There is an 80% chance that the 2002 F was between 0.21 and 0.28. The estimate of F for 2002 may understate the actual fishing mortality; retrospective analysis shows that the current assessment method tends to underestimate recent fishing mortality rates (e.g., by about 40% over the last three years).
Total stock biomass has increased substantially since 1989, and on January 1, 2003 was estimated to be 56,100 mt, 5% above the biomass threshold (53,200 mt). There is an 80% chance that total stock biomass in 2003 was between 51,000 and 63,000 mt. Spawning stock biomass (SSB; Age 0+) declined 72% from 1983 to 1989 (18,800 mt to 5,200 mt), but has increased eight-fold, with improved recruitment and decreased fishing mortality, to 42,200 mt in 2002. Retrospective analysis shows a tendency to slightly overestimate the SSB in the most recent years. The age structure of the spawning stock has expanded, with 80% at ages 2 and older, and 19% at ages 5 and older. Under equilibrium conditions at Fmax, about 85% of the spawning stock biomass would be expected to be ages 2 and older, with 50% at ages 5 and older.
The arithmetic average recruitment from 1982 to 2002 is 40 million fish at age 0, with a median of 35 million fish. The 2002 year class is currently estimated to be about average at 38 million fish. There is no consistent retrospective pattern in the estimation of the abundance of age 0 fish over the last three years.
If the landings for 2003 do not exceed the TAL and the proportion of catch discarded does not increase, the total allowable landings (TAL) in 2004 would need to be 12,790 mt (28.2 million lbs) to meet the target F rate of Fmax = 0.26 with 50% probability. As noted above, retrospective analysis suggests that the assessment tends to underestimate fishing mortality rates in the most recent years.
The following major data and analytic needs for future assessments were identified in the SARC 35 review of the 2002 assessment (NEFSC 2002) and in the preparation of the 2003 assessment:
1) Expand the NEFSC fishery observer program for summer flounder, with special emphasis on a) comprehensive areal and temporal coverage, b) adequate length and age sampling, and c) continued sampling after commercial fishery areal and seasonal quotas are reached and fisheries are limited or closed, and d) sampling of summer flounder discard in the scallop dredge fishery. Maintaining adequate observer coverage will be especially important in order to monitor a) the effects of implementation of gear and closed/exempted area regulations, both in terms of the response of the stock and the fishermen, b) potential continuing changes in "directivity" in the summer flounder fishery, as a results of changes in stock levels and regulations, and c) discards of summer flounder in the commercial fishery once quota levels have been attained and the summer flounder fishery is closed or restricted by trip limits.
2) Evaluate the amount of observer data needed to reliably estimate discards of summer flounder in all components of the fishery
3) Conduct further research to better determine the discard mortality rate of recreational and commercial fishery summer flounder discards.
4) Develop a program to annually sample the length and age frequency of summer flounder discards from the recreational fishery.
5) RIDFW monthly fixed station survey length frequencies are currently converted to age using length cut-offs points. Investigate the utility of applying the appropriate NEFSC or MADMF age-length keys to convert the RIDFW monthly fixed station survey lengths to age.
6) Explore the possibility of weighting survey indices used in VPA calibration by the areal coverage (e.g., in square kilometers) of the respective seasonal surveys.
7) Explore the sensitivity of the VPA calibration to the addition of 1 and/or a small constant to values of survey series with "true zeros."
8) Statistically analyze changes in mean weights at age in the catch and NEFSC surveys. Determine if using mean weights at age in the survey are more appropriate for estimating the BMSY proxy. Explore the sensitivity of the mean weights of the catch and partial recruitment pattern from a longer time series (1997 to 2001) to the re-estimated BMSY proxy. ) As the NEFSC fall survey age structure expands, investigate the use of survey mean weights at age for stock weights at age in yield per recruit, VPA, and projection analyses.
9) Monitor changes in life history (growth and maturity) as the stock rebuilds.
10) Evaluate use of a forward calculating age-structured model for comparison with VPA. Forward models would facilitate use of expanding age/sex structure and allow inclusion of historical data. If sex-specific assessments are explored, the implications on YPR should also be investigated.
11) Explore the sensitivity of the VPA results to separating the summer flounder stock into multiple components.
12) Evaluate trends in the regional components of the NEFSC surveys and contrast with the state surveys that potentially index components of the stock.
13) Use NEFSC fishery observer age-length keys for 1994 and later years (as they become available) to supplement NEFSC survey data in aging the commercial fishery discard.
Major sources of assessment uncertainty
The SARC 35 review of the 2002 assessment (NEFSC 2002) identified the following major sources of uncertainty:
1) The landings from the commercial fisheries used in this assessment assume no under reporting of summer flounder landings. Therefore, reported landings from the commercial fisheries should be considered minimal estimates.
2) The recreational fishery landings and discards used in the assessment are estimates developed from the Marine Recreational Fishery Statistics Survey (MRFSS). While the estimates of summer flounder catch are considered to be among the most reliable produced by the MRFSS, they are subject to possible error. The proportional standard error (PSE) of estimates of summer flounder total landings in numbers has averaged 7%, ranging from 26% in 1982 to 3% in 1996, during 1982-2002.
3) The intensity of fishery observer sampling of the commercial scallop dredge fishery (outside of exempted area fisheries) was particularly low in 2001. This level of observer coverage likely was insufficient to accurately characterize summer flounder discards.
4) The length and age composition of the recreational discards are based on data from a limited geographic area (Long Island, New York, 1988-1992; Connecticut, 1997-2001, New York party boats 2000-2001, ALS releases focused in New York and New Jersey, 1999-2001). Sampling of recreational fishery discards on a annual, synoptic basis is needed.
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