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CONTENTS
Introduction
Fishery Data
Abundance and Biomass Indices
Mortality and Stock Size
Discussion
Acknowledgments
References

Northeast Fisheries Science Center Reference Document 03-02

A Report of the 36th Northeast Regional Stock Assessment Workshop: Stock Assessment of Yellowtail Flounder in the Southern New England - Mid-Atlantic Area

Steven X. Cadrin
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543

Web version posted February 20, 2003

Citation: Cadrin, S.X. 2003. Stock assessment of yellowtail flounder in the Southern New England - Mid-Atlantic area. U.S. Dep. Commer. Northeast Fish. Sci. Cent. Ref. Doc. 03-02; 101 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.

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Abstract

Southern New England and Mid Atlantic yellowtail flounder resources were previously assessed separately, but are combined for this assessment. The combined stock is overfished and overfishing is taking place. The current estimate of fishing mortality is high, much greater than the proposed FMSY proxy (F40%MSP = 0.26). Spawning stock biomass is low (2001 SSB = 1,900 mt), well below the proposed SSBMSY proxy (69,500 mt SSB). Recruitment has been poor for more than a decade. The age structure of the stock is truncated in comparison to MSY conditions.


INTRODUCTION

Yellowtail flounder, Limanda ferruginea, inhabit relatively shallow waters (20-100 m) of the northwest Atlantic from Labrador to Chesapeake Bay (Bigelow and Schroeder 1953, Scott and Scott 1988, Collette and Klein-MacPhee 2002).  A fishery for yellowtail flounder developed off southern New England in the 1930s, coincident with the increased use of otter trawls, a decline in winter flounder abundance, and demand for food products during World War II (Scott 1954, Royce et al. 1959).

The available information on yellowtail flounder stock structure off the northeast U.S. indicates separate stocks on Georges Bank, off Cape Cod, and from southern New England to the Mid-Atlantic Bight.  Distributional analyses indicate a relatively continuous distribution from the Mid Atlantic Bight to Nantucket Shoals, a concentration on Georges Bank, and a relatively separate concentration off Cape Cod (Royce et al. 1959).  Geographic patterns of landings over time suggest that yellowtail resources on Georges Bank on off southern New England are separate harvest stocks (McBride and Brown 1980).  Geographic variation indicates that yellowtail off Cape Cod comprise a separate phenotypic stock than resources to the south (Begg et al. 1999).  Tagging data indicate low dispersion from Cape Cod, Georges Bank and southern New England fishing grounds, but substantial movement from the Mid Atlantic to southern New England (Royce et al. 1959, Lux 1963).  Descriptive information on early life history stages and circulation patterns suggest that yellowtail spawn in hydrographic retention areas, but there may be some advection of eggs and larvae from Georges Bank and Cape Cod to southern New England and the Mid Atlantic Bight (Sinclair 1988).  In sumary, yellowtail flounder on Georges Bank appear to be a separate harvest stock, yellowtail off Cape Cod can be considered a separate phenotypic stock (with some question on the northern boundary of the stock area), but there is little evidence supporting separate stocks in southern New England and the Mid Atlantic Bight.

Management History

From 1950 to 1977, the International Commission for the Northwest Atlantic Fisheries managed yellowtail flounder resources in southern New England, Georges Bank and the Gulf of Maine (i.e., in ICNAF subarea 5).  Gear restrictions and total allowable catch were the primary management strategies of ICNAF, but minimum fish size, fishing effort and closed area and season regulations were also regulated.  Minimum trawl mesh size was 114 mm in the 1950s and 1960s.  National catch quotas were implemented for southern New England yellowtail flounder from 1971 to 1976, but these were exceeded in most years. 

Following the implementation of the Magnuson Fisheries Conservation and Management Act (FCMA) in 1976, U.S. yellowtail resources have been managed by the New England Fisheries Management Council (Table 1).  Groundfish regulations included minimum cod end mesh size, minimum fish size, seasonal area closures, mandatory reporting, trip limits and annual quotas.  Minimum size for yellowtail was increased from 28cm in 1982 to 30cm in 1986 and 33cm in 1989.  Minimum mesh size increased from 140 mm in 1991 (diamond and square mesh) to 140mm diamond-152mm square in 1994 and to 165mm in 1999.  A large area south of Nantucket Shoals was closed to fishing since December 1994.  Scallop dredge vessels were limited to possession of 136kg of yellowtail flounder since 1996.  Scallop dredge vessels were limited to possession of 136kg of yellowtail flounder since 1996, and in 1999 minimum twine top mesh was increased from 203mm to 254mm to reduce yellowtail bycatch.

Assessment History

The first quantitative stock assessment of yellowtail flounder was on the southern New England - Mid Atlantic resource and fishery.  Royce et al. (1959) evaluated landings, length and age composition, effort, and tagging data to conclude that fishing mortality was approximately 0.30 in the 1940s.  However, retrospective estimates of F during the 1940s were substantially greater (approximately 0.6, Lux 1969).  Lux (1964) concluded that the stock was not overfished during the 1950s, but age-based mortality estimates for the 1960s were high (Lux 1967[1] , 1969). 

Subsequent assessments of yellowtail flounder in the southern New England area excluded Mid-Atlantic catch and survey data, but indicated increasing F and declining stock size in the late 1960s (Brown and Hennemuth 1971a, 1971b; Pentilla and Brown 1973).  Starting in 1974, Mid Atlantic and southern New England yellowtail resources were treated as separate assessment and management units, but analyses for each area indicated high mortality and low stock size in the 1970s (Parrack 1974, Sissenwine et al. 1978, McBride and Sissenwine 1979, McBride et al. 1980, Clark et al. 1981).  In the early 1980s, there was indication of strong recruitment of yellowtail from surveys and commercial catches in both southern New England and Mid Atlantic areas, but discard rates were high and F exceeded Fmax in southern New England (McBride and Clark 1983, Clark et al. 1984, NEFC 1986). 

Assessment methods used for southern New England yellowtail progressed to a calibrated VPA in the late 1980s.  The 1988 assessment indicated high F in the 1970s and early 1980s and a strong 1980 cohort (F=0.60-1.48; NEFC 1989).  Later stock assessments showed another dominant cohort spawned in 1987, but F continually increased through the 1980s, and the stock was depleted to record low biomass in the early 1990s (Conser et al. 1991, Rago et al. 1994).  The VPA-based assessment of southern New England yellowtail was updated annually from 1997 to 1999, and assessments indicated a reduction in F in the late 1990s, but little rebuilding of stock biomass (NEFSC 1997, 1998; Cadrin 2000).  In 2000, an updated VPA was attempted, but was rejected as a basis for management advice because sampling in 1999 was inadequate to estimate catch at age reliably (Cadrin 2001b).  Therefore, recent assessments of southern New England yellowtail have been based on projections of observed catch from the 1999 VPA (Cadrin 2001b, NEFSC 2002).  An updated assessment of the southern New England yellowtail flounder stock was prepared concurrently with this assessment for the Groundfish Assessment Review Meeting (Cadrin 2002b).

An analytical assessment of Mid Atlantic yellowtail flounder has not been developed, and management advice has been based on descriptive summaries of landings and survey data.  Assessments of the Mid Atlantic yellowtail resource indicated similar trends in catch and survey indices as in southern New England (NEFC 1987, 1988; NEFSC 1991, 1992, 1993; Rago 1995; Overholtz and Cadrin 1998).  Based on survey biomass and exploitation ratios, the Mid Atlantic yellowtail resource was 2% of the BMSY proxy, and the exploitation rate greatly exceeded the FMSY proxy  (Cadrin 2001a).  An updated assessment of the Mid Atlantic yellowtail flounder stock was prepared concurrently with this assessment for the Groundfish Assessment Review Meeting (Cadrin 2002a).


1. Although Lux (1967) is titled, “Landings per unit effort, age composition and total mortality of yellowtail flounder (Limanda ferruginea) in subarea 5Z,” the southern New England analyses also include catch and effort data from statistical area 6.

FISHERY DATA

Commercial Landings

Commercial statistics for southern New England yellowtail flounder are from statistical areas 526, 537, 538, and 539, and mid Atlantic yellowtail are from statistical areas 611-623 (Figure 1).  U.S. commercial landings of yellowtail flounder were derived from dealer weighout reports and canvas data according to historical assessment reports (Royce et al. 1959, Brown and Hennemuth 1971, Sissenwine et al. 1978, McBride et al. 1980, McBride and Clark 1983, NEFC 1986, McBride 1989, Rago et al. 1994).  Total Mid Atlantic landings from canvas data were allocated to market category according to annual proportions in the weighout database.  Previous to 1994, landings were allocated to statistical area, month, and gear type according to interview data collected by port agents (Burns et al. 1983).  For 1994, landings reported by dealers were allocated to stock area using fishing vessel logbook data, by fishing gear, port, and season (Wigley, et al. 1998).  For 1995-1997, dealers’ reported landings were prorated to stock area using a modified proration that included dealer codes (NEFSC 1998).

Landings generally increased in southern New England during the 1930s and early 1940s and the fishery expanded to the Mid Atlantic in the early 1940s, with landings of 28,000mt in 1942 (Table 2, Figure 2).  Annual landings were around 10,000mt from 1943 to 1948 with approximately 10% from the Mid Atlantic.  A domestic industrial fishery developed in the late 1940s.  Landings decreased to less than 2,000mt in the mid 1950s.  Landings increased in southern New England in the late 1950s and again expanded to the Mid Atlantic in the 1960s.  A distant water fishery developed in the 1960s and total annual landings were greater than 20,000mt from 1963 to 1970.   The industrial and foreign fisheries were discontinued in the early 1970s.  Landings generally decreased since the 1970s, with temporary increases in the early 1980s and early 1990s.  Landings in 1995 were a record low 200 mt, and the proportion of landings from the Mid Atlantic generally increased from approximately 10% in the early 1990s to greater than 20% (e.g., in 1997, 70% of landings in the stock area came from the Mid Atlantic).  Landings slightly increased to greater than 1,000mt per year since 1999.

A summary of port samples (each consisting of approximately 100 lengths and 1 age sample per cm) are listed in Table 3.  Landings at age were derived by geographic region, half-year and market category, when possible.  Landings at age of southern New England yellowtail flounder are described in previous assessment documents (Conser et al. 1991; Rago et al. 1994; NEFSC 1997, 1998; Cadrin 2000; Cadrin 2002b).  Mid Atlantic landings were not sampled in several half-year periods, and age distributions of southern New England landings were assumed for Mid Atlantic landings in those periods by quarter and market category (2nd half of 1975, 2nd half of 1981, 2nd half of 1986, 2nd half of 1987, 2nd half of 1988, 1st half of 1989, 2nd half of 1990), or by half and market category for 2000 and 2001.  Landings at age and landed mean weights at age are reported in Table 4. In the early 1970s a substantial portion of landings were from older fish (e.g., 17% of 1973 landings were age-6 or older), but the age distribution of landings rapidly truncated, and the portion of age 6+ fish has generally been less than 3% since 1977.

Discarded Catch

Estimates of discards for the southern New England – Mid Atlantic yellowtail fishery for 1963-1969 were derived from interviews with vessel captains; historical discards were approximated by Brown and Hennemuth (1971a) from the 1963-1969 average discard rate (Table 5).  Discards for 1970-1977 were also based on interview data, however yellowtail interview data were suspect from 1978 to 1982 when trip limits were imposed (McBride et al. 1980, Clark et al. 1981).  Discards during 1978-1982 were estimated from observer data when available (Sissenwine et al. 1978), derived directly from field selectivity studies (McBride et al. 1980), or from application of selectivity estimates to survey size frequencies (McBride and Clark 1983).  Discards for 1983 were from interview data (Clark et al. 1984).  Discards at age from southern New England, 1984-1993 were from a combination of sea sampling, interviews and survey data (Conser et al. 1991, Rago et al. 1994).  Discards for 1994-2001 were derived from vessel logbooks (NEFSC 1997, 1998; Cadrin 2000).  Updated discard estimates for southern New England are listed in Table 5a.  Discards of Mid Atlantic yellowtail were from interview data for 1984-1993.  Mid Atlantic discards for 1994-2001 were derived from logbook data by gear for all trips that reported discards of any species (NEFSC 1998, Table 5b).

Discarded catch accounted for an average of 30% of total catch annually, but appears to have decreased to approximately 10% since 1995.  In 1969, discards peaked at 24,000mt, 40% of the total catch that year.  A substantial portion of recent discards are from the scallop dredge fishery.

Discards at age were estimated from observer lengths (Table 3) and survey ages 1994-2001.  Discards at age of southern New England yellowtail flounder are described in previous assessment documents (Conser et al. 1991; Rago et al. 1994; NEFSC 1997, 1998; Cadrin 2000; Cadrin 2002b).  Age distribution of discards in southern New England were assumed for Mid Atlantic discards for 1973 to 1993 (Table 6).  Discards were primarily ages 1 and 2 during from the 1970s through the early 1990s, but shifted to age 2 and 3 in the early 1990s, coincident with regulated mesh size increases.

Estimates of total catch at age reflect the landings at age in that they indicate a relatively wide age distribution in the catch in the early 1970s (e.g., approximately 10% of the catch was age-6 or older from 1973 to 1975; Figure 3, Appendix A).  Subsequent catch at age was dominated by the 1980 and 1987 cohorts, but few fish older than age-6 contributed to the catch.  Mean weights at age of older fish (age 4+) generally increased in the mid 1970s, were relatively light during the mid 1980s, and generally increased in recent years (Figure 4).  Mean weight of age-1 yellowtail generally decreased in the 1990s, presumably from discards of small yellowtail in the scallop fishery.


ABUNDANCE AND BIOMASS INDICES

Stock Abundance and Biomass Indices

The NEFSC spring and autumn bottom trawl surveys have sampled offshore strata since 1963 and 1968, respectively (Despres et al. 1988).  However, the southern–most offshore strata (61-76) were not sampled until 1967.  Therefore southern strata were included in the spring survey index, 1968-2002 and the winter survey index 1992-2002 (strata 1, 2, 5, 6, 9, 10, 69, 73, 74; Figure 5), but excluded from the fall survey index, 1963-2001 (strata 1, 2, 5, 6, 9, 10).  Nearly all yellowtail caught by the survey in the southern New England – Mid Atlantic stock area (99%) are in the spring and winter strata sets.  The strata set for the NEFSC scallop survey was determined as all strata that were consistently sampled in the stock area (14, 15, 18, 19, 22-28, 30, 31, 33, 35, and 46).

Indices of abundance and biomass indicate relatively high stock size in the 1960s and early 1970s, followed by a rapid decrease in the mid 1970s (Table 6, Figure 6).  Stock biomass increased temporarily in the early and late 1980s with the recruitment of the strong 1980 and 1987 cohorts.  Recent distributions of yellowtail catches in surveys are illustrated in Figure 7.  The average portion of yellowtail biomass in the Mid Atlantic region has been 45% of the total southern New England – Mid Atlantic yellowtail biomass (Figure 8).  Age distribution of yellowtail in surveys indicates abundant cohorts in the 1960s and early 1970s, strong year classes in 1980 and 1987, and relatively truncated age structure since the early 1970s (Table 7, Figure 9).

Correspondence among survey indices was assessed using correlations among normalized observations for the VPA time series 1973-2001 [Ln(x/mean);  Table 8].  Normalized indices of catch per tow at age are illustrated in Figure 10.  Correlations among survey series were generally low for the winter survey, particularly for older ages, presumably because it is a short series with little contrast.  Correlations between spring and fall survey series were strongest at ages 2-4 (r=0.71-0.82).


MORTALITY AND STOCK SIZE

Virtual Population Analysis

Abundance estimates from virtual population analysis of catch of age-1 to age-7+, 1973-1997, were calibrated using an ADAPT algorithm (Gavaris 1988) that estimated age 2-5 survivors in 2002 and survey catchability coefficients (q) using nonlinear least squares of survey observation errors.  Abundance at age was calibrated with survey indices of abundance: spring survey indices (age-1 to age-7+) and winter indices (age-1 to age-5) were calibrated to January abundance, and fall survey indices (age-1 to age-7+) were calibrated to mean abundance.  The instantaneous rate of natural mortality (M) was assumed to be 0.2 based on tag returns (Lux 1969), relationships of Z to effort (Brown and Hennemuth 1971a), and the oldest individual sampled in the stock area (age-14).  Although catches of yellowtail older than age-8 are rare in commercial or research catches, the stock has been heavily exploited for seven decades.  Maturity at age for southern New England yellowtail flounder was reported by O’Brien et al. (1993) from 1985-1990 NEFSC spring survey samples.  Calibration output is reported in Appendix A. Model Residuals are plotted in Figure 11.

Results show that the stock was abundant in the early 1970s with a relatively wide age structure (11% of the population in 1973 was age 6 or older), but was quickly truncated by the late 1970s (<2% age 6+ from 1978 to 2001; Figure 12c).  Fishing mortality generally increase in the 1970s and 1980s to a peak of 2.3 in 1991 and 1992, averaged 1.6 during the 1990s, and appears to have decreased to 0.68 in 2000 and increased to 0.91 in 2001 (Figure 12a).  Recruitment was generally strong in the 1970s and moderate during the 1980s, with two exceptional year classes in 1980 and 1987.  Recruitment has been low during the 1990s.  Spawning biomass was high in the early 1970s, decreased in the late 1970s, and increased briefly in the early and late 1980s with recruitment of the 1980 and 1987 cohorts.  Spawning biomass decreased to a record low 622mt in 1994, gradually increased to 2,100mt in 2000, and decreased to 1,900mt in 2001.  Retrospective analysis indicates a strong pattern of underestimating F, and overestimating SSB in recent years (Figure 13).

Biomass Dynamics

Given the problems in estimating recent catch at age in the southern New England area (Cadrin 2000) an age-aggregated production model (ASPIC, Prager 1994) was fit to total catch and survey biomass indices.   Results are reported in Appendix B.  Initial trials did not fit the winter survey biomass series, presumably because it is relatively short and does not have much contrast, nor did the model fit the catch rate data from Lux (1969).  Alternative analyses that assumed that stock biomass was at the carrying capacity in 1935 had very similar results.

Results of the biomass dynamics model indicate that biomass decreased during the 1960s and early 1970s to about 10% of the biomass estimated for the early 1960s (Figure 14).  Similar to the age-based analysis, the biomass dynamics model indicates brief periods of rebuilding in the early and late 1980s and a further decrease to extremely low biomass in the mid 1990s.  However, the biomass dynamics model indicates a slightly faster rate of rebuilding in recent years than indicated by the age-based analysis.

Biological Reference Points

Yield and biomass per recruit were calculated assuming the observed partial recruitment and mean weight at age for 1994-2001 (Thompson and Bell 1934).  Results are reported in Table 9 and illustrated in Figure 15.  A comparison of recently observed age distributions with the age distribution expected at F40% shows a relative truncation in current age structure (Figure 16).

Applying the approach used to estimate MSY proxies for southern New England yellowtail (NEFSC 2002), FMSY is approximated as F40% (0.26).  The SSBMSY proxy is 69,500mt, calculated as the product of 40%MSP (1.129 kg spawning biomass) and average long-term recruitment  (61.57 million).  The average long-term recruitment was derived as the fall survey age-1 index divided by the catchability coefficient estimated by ADAPT  (8.08E-5).  The MSY proxy is 14,200mt, derived as the product of yield per recruit at F40% (0.230 kg) and average recruitment. 

Such MSY reference point proxies are highly sensitive to the assumed value of recruitment.  For example, different periods of observed recruitment produce a wide range of SSBMSY and MSY proxies (Figure 17).  Alternatively, SSBMSY and MSY can be approximated using stochastic long-term projections assuming recent average weights at age and partial recruitment (1994-2001), and the distribution of long term recruitment.  Results suggest that at an F of 0.26, the long-term average catch is 13,100mt, and long-term average SSB is 64,500mt (Figure 18).  For comparison, the estimate of BMSY from biomass dynamics analysis is 104,700mt of total biomass, FMSY is 0.19 on total biomass, and MSY is 20,300mt.

Projections

Stochastic age-based projections that assume a 15% reduction in F from 2001 to 2002 and recruitment similar to that experienced in the last decade suggest that the stock cannot rebuild to BMSY by 2009 even if F in 2003-2010 is zero.  If the same hindcast recruitment values used to derive the reference points are assumed for projections, there stock is expected to have approximately a 50% chance of rebuilding to SSBMSY by 2009 with an F of 0.08 (Figure 19, Appendix A).  However, long-term recruitment levels are not likely in the short-term, because SSB is extremely low, and retrospective patterns indicate that projections may be overly optimistic.


DISCUSSION

Although estimation of catch at age from Mid Atlantic area is independent of the estimation for southern New England waters, and the Mid Atlantic area is well sampled by the surveys, temporal patterns of abundance and mortality for the combined area is similar to previous assessments of the Southern New England stock, though scaled up to account for Mid Atlantic catches.  However, the estimate of 2001 fishing mortality for the southern New England assessment (Cadrin 2002b) is half of the F estimated for the combined southern New England-Mid Atlantic stock area.

Unlike stocks of yellowtail flounder that have recently rebuilt from low stock sizes on Georges Bank and the Grand Banks, fishing mortality on southern New England-Mid Atlantic yellowtail has not been substantially reduced and is still well above the rate that will allow rapid rebuilding.  Accordingly, the rate of rebuilding has been slow.  Projections suggest that fishing mortality should be reduced to near zero to achieve rebuilding by the specified management target of 2009.

Some difficulties in the assessment of southern New England yellowtail (e.g., the retrospective pattern of the VPA) also persist in this assessment of the combined southern New England-Mid Atlantic area.  However, the overfished and overfishing status is not affected by the retrospective bias.  The retrospective pattern suggests that the VPA is not well calibrated.  It appears that the surveys do not monitor small differences in relative abundance at such low densities.  Perhaps the winter survey, which samples yellowtail more efficiently than the spring and fall surveys, will help to monitor stock rebuilding as more contrast is observed in the relatively short series.


ACKNOWLEDGMENTS

Tom Nies compiled information on management history.  Vaughn Silva provided age determinations for recent years.  Jay Burnett and Paul Kostovick helped to restore historical age data.  Ralph Mayo assigned areas to commercial length samples and provided software for observer data.  Susan Wigley provided software for survey and logbook data.  Paul Nitschke offered input on discard estimation.  Mark Terceiro provided input on many assessment decisions and chaired the Working Group meeting, and drafted the Working Group discussion.  Kathy Sosebee served as rapporteur for the assessment meeting.  Andrew Payne chaired the Stock Assessment Review Committee.  I thank all Working Group and Review Committee participants.


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