CONTENTS Introduction Fishery Data Abundance and Biomass Indices Mortality and Stock Size Discussion Acknowledgments References
Northeast Fisheries Science Center Reference Document 03-03
Steven X. Cadrin1 and Jeremy King2
A Report of the 36th Northeast Regional Stock Assessment Workshop: Stock Assessment of Yellowtail Flounder in the Cape Cod - Gulf of Maine Area
1National Marine Fisheries Serv., 166 Water St., Woods Hole, MA 02543-1026
2Massachusetts Div. of Marine Fisheries, 50A Portside Dr., Pocasset, MA 02559
Web version posted February 21, 2003Citation: Cadrin, S.X.; King, J. 2003. Stock assessment of yellowtail flounder in the Cape Cod - Gulf of Maine area. U.S. Dep. Commer. Northeast Fish. Sci. Cent. Ref. Doc. 03-03.
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.
Cape Cod yellowtail flounder were previously assessed as a unit stock, but are now combined with those in the Gulf of Maine. The Cape Cod / Gulf of Maine stock is overfished and overfishing is occurring. Current fishing mortality is high (2001 Fages 3-4=0.75) and much greater than the proposed FMSY proxy (F40%MSP=0.17). Spawning stock biomass declined in the early 1990s, and began increasing in 1998 to 3,200 mt in 2001, but is much less than the proposed SSBMSY proxy (12,600 mt SSB). With the exception of the strong 1987 yearclass, recruitment has been relatively stable, but early indications suggest that the 2000 cohort is extremely low. The age structure of the stock is truncated in comparison to MSY conditions.
Yellowtail flounder, Limanda ferruginea, inhabit the continental shelf of the northwest Atlantic from Labrador to Chesapeake Bay (Bigelow and Schroeder 1953, Collette and Klein-MacPhee 2002). Off the U.S. coast, commercially important concentrations are found on Georges Bank, off southern New England, and off Cape Cod (statistical areas 514 and 521; Figure 1). Cape Cod yellowtail inhabit shallow water (10-60 m) relative to offshore stocks of yellowtail (Lux 1964). Spawning occurs during spring and summer, peaking in late May. Larvae are pelagic for a month or more, then develop demersal form and settle to the bottom. Yellowtail flounder on the Cape Cod grounds generally mature at age-3 (O’Brien et al. 1993) and grow to 58 cm total length.
A New England fishery for yellowtail flounder developed in the 1930s, coincident with a decline in winter flounder abundance, and the fishery expanded from southern New England to Georges bank and the Cape Cod grounds in the late 1930s and early 1940s (Royce et al. 1959, Lux 1964). On the Cape Cod grounds, yellowtail are generally caught in multi-species groundfish fisheries (principally by otter trawls) from late fall to spring, with some landings by gillnets in the winter and spring, but may also be specifically targeted in certain seasons (Royce et al. 1959).
Historically, landings from the Cape Cod grounds were a small portion of the total U.S. yellowtail landings. However, during the collapse of Georges Bank and southern New England stocks in the early 1990s (NEFSC 1994), the Cape Cod stock was the most productive of the U.S. yellowtail stocks (Overholtz and Cadrin 1998).
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 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 (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 summary, yellowtail on the Cape Cod grounds can be considered a separate phenotypic stock (with some question on the northern boundary of the stock area). There is little evidence supporting separate stocks on the Cape Cod grounds and in the northern Gulf of Maine.
Over the past 25 years, the fishery for yellowtail flounder in federal waters has been managed under several regimes. From 1971 to 1976, national quotas were allocated by the International Commission for Northwest Atlantic Fisheries. From 1977 to 1982, the New England Fishery Management Council Atlantic Groundfish Fishery Management Plan established optimum yield thresholds for yellowtail west of 69° longitude (which included Cape Cod and southern New England yellowtail stocks) and imposed minimum mesh size, spawning closures, and trip limits (Table 1). In 1982, the Council adopted an Interim Groundfish Plan, which established a minimum size limit of 28 cm (11 in) and a minimum mesh size of 130 mm (5 1/8"; with exemptions). In 1983, the minimum mesh size was increased to 140 mm (5.5"; with exemptions) In 1986, the Council’s Multispecies Fishery Management Plan increased the minimum legal size to 30 cm (12 in) and imposed seasonal area closures. Amendment #4 to the Plan further increased the minimum legal size to 33 cm (13 in) in 1989. In 1993, finfish exclusion devices were required in the northern shrimp fishery to reduce groundfish bycatch. Amendments #5, #6, and #7 (1994-1996), limited days at sea, closed areas year-round, further increased minimum mesh size to 142 mm (6 in diamond or square; with fewer exemptions), imposed trip limits for groundfish bycatch in the sea scallop fishery, and prohibited small-mesh fisheries from landing groundfish. Framework #25 was an annual adjustment to the Multispecies Plan which prohibited bottom trawling in two areas of yellowtail habitat on the Cape Cod grounds in 1998: Massachusetts Bay was closed in March, and the waters off Cape Ann were closed in April. Other sections of the western Gulf of Maine were closed in May and June. The ‘western Gulf of Maine closure’ is too deep to protect yellowtail flounder. Amendment #9 was adopted in 1998 to revise the overfishing definition according to Sustainable Fisheries Act requirements. . In 1999, minimum twine top mesh of scallop dredges was increased from 203mm to 254mm to reduce yellowtail bycatch.
The portion of the Cape Cod yellowtail stock found within the Massachusetts territorial sea is managed by the Massachusetts Division of Marine Fisheries under a suite of management measures. Since 1931, many coastal areas have been closed to bottom trawling year-round (e.g. Winthrop Head to Gloucester), or seasonally (e.g. Boston to Provincetown and Gloucester to New Hampshire). The state has had a succession of more stringent size limits beginning with a 11" minimum size in 1982. The size limit increased to 12" in 1986 and then to 13" in 1988. In 1986, 5" mesh codends were required for trawling within the 20 fathom contour in waters north of Cape Cod. In 1986, a winter flounder spawning closure to trawling and gillnetting extending approximately one to two miles from shore was established in waters from the New Hampshire border to Provincetown from February 1 to April 30 (extended to May 31 in 1990). In 1989, small mesh trawling was restricted to permitted fisheries targeting specific species. In 1991, minimum mesh size throughout the net was increased to 5 1/2" north and east of Cape Cod. Since November 1, 1992 a year-round night closure to mobile gear has abbreviated fishing effort by curtailing "trip fishing". Beginning in 1993, a Coastal Access Permit was required to fish mobile gear. The mesh size was increased again in 1994 to 6". A moratorium on new applicants for this permit was enacted in 1994 stemming an increase in effort into state waters. In 1995, the size limit for vessels fishing mobile gear was reduced from 90' registered length to 72' length over all. From 1995-1999, small mesh trawling in state waters north of Cape Cod was limited to an experimental whiting fishery with drastic ground gear modifications for bycatch reduction, prohibitions on groundfish retention and intensive sea sampling. Scallop dredge fisheries have been limited to 10' combined maximum dredge width since 1990. Gillnet fisheries in Massachusetts have a permit moratorium, 2400' maximum net length, 6" minimum mesh size and seasonally closed areas.
Yellowtail resources on the Cape Cod fishing grounds and in the northern Gulf of Maine have been assessed and managed separately. The Cape Cod yellowtail resource was initially assessed by descriptive summaries of catch, effort, catch samples, survey indices, yield per recruit modeling, and estimates of total mortality rate (Z) from survey and commercial age samples. The stock was more stable than the Georges Bank or southern New England stocks from the 1940s to the 1960s, based on patterns of landings and commercial catch rates (Royce et al. 1959, Lux 1964). However in the early 1970s, effort began to increase, and catch rates began to decline (Parrack 1974). Estimates of fishing mortality rate (F) during the 1970s were at or above the estimated level of maximum yield per recruit (Howe 1975). Although yield remained stable relative to offshore stocks, catch rates were at the lowest levels observed by the late 1970s (Sissenwine et al. 1978). For a brief period in the mid 1970s, the stock appeared to be stable (McBride and Sissenwine 1979). However, by the late 1970s, peak catches produced high mortality rates, the age structure appeared to be truncated, and catch rates continued to decrease (McBride et al. 1980, McBride and Sissenwine 1980, Clark et al. 1981). Despite some indications of good recruitment in early 1980s (McBride and Clark 1983, Clark et al. 1984), landings and relative abundance generally decreased in the 1980s (NEFC 1986). The 1987 year class was dominant and contributed to some rebuilding, however, the most recent descriptive assessment of Cape Cod yellowtail concluded that the stock was overexploited (Rago 1994). An age-based assessment indicated that F was high (>0.7) from 1985 to 1997 and biomass was much less than BMSY (Cadrin et al. 1999). Updated assessments in 1999 and 2000 each indicated a reduction in F in the last year of the assessment (Cadrin and King 2000, Cadrin 2001), but the revised estimate of 1998 F remained high (1.0, Cadrin 2001). An updated assessment of the Cape Cod yellowtail flounder stock was prepared concurrently with this assessment for the Groundfish Assessment Review Meeting (Cadrin and King 2002).
Yellowtail flounder in the northern Gulf of Maine have not been analytically assessed. Royce et al. (1959) compiled yellowtail landings statistics for the scattered shoals in the northern Gulf of Maine in the 1940s, and Lux (1964) updated landings statistics through 1961. McBride and Sissenwine (1980) reported a substantial increase in yellowtail flounder landings from the northern Gulf of Maine during the 1970s, and described the sparse survey information available for yellowtail in the northern Gulf of Maine. This assessment combines catch and survey information from the Cape Cod grounds and the northern Gulf of Maine for a single-stock analysis.
Commercial statistics for Cape Cod yellowtail flounder are from statistical areas 514 and 521, and northern Gulf of Maine yellowtail are from statistical areas 511, 512, 513 and 515 (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, Lux 1964, Sissenwine et al. 1978, McBride et al. 1980, McBride and Clark 1983, NEFC 1986). 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).
Annual landings generally increased from less than 1,000mt in the mid 1930s to a peak of 5,600mt in 1980 (Table 2, Figure 2). Landings decreased to approximately 1,200mt per year in the late 1980s, but peaked again in 1990 at 3,200mt with recruitment of the strong 1987 yearclass. Landings decreased to 800mt in 1993 and remained low through the 1990s, but rapidly increased to greater than 2,400mt in 2000 and 2001.
Landings at age of Cape Cod yellowtail flounder are described in Cadrin et al. (1999), Cadrin and King (2000, 2002) and Cadrin (2001), and sample sizes are reported in Table 3. Very few port samples are available for the northern Gulf of Maine yellowtail fishery (six samples from 1969, 1976, 1983, 1987, 1988 and 1991) and all market categories were not sampled in any year. Therefore, the age distribution of Cape Cod yellowtail landings, by half and market category, were assumed for northern Gulf of Maine landings. Landings at age, by region, are listed in Table 4.
Discards were estimated using discard to kept observations from 1989-2001 sea sampling for the trawl and gillnet fisheries and discard per effort for the shrimp and scallop fisheries as described in Cadrin et al. (1999). Discards of Cape Cod yellowtail flounder for 1985-1997 are described in Cadrin et al. (1999), and for 1998-2001 by Cadrin and King 2002 (Table 5a). Discards for the northern Gulf of Maine averaged 38% of Gulf of Maine yellowtail landings, primarily from the trawl fishery and the shrimp fishery prior to the Nordmore grate requirement in 1993 (Table 5b). Discards for 1985-1988 were approximated by assuming a 38% annual discard ratio.
Discards at age of Cape Cod yellowtail flounder are described in Cadrin et al. (1999) and Cadrin and King (2002; Table 6a). Discards at age for yellowtail in the northern Gulf of Maine were estimated using length observations from sea sampling (Table 6b; using pooled-year samples by half and gear for unsampled discards) and survey age-length keys for 1989-2001 by half-year. The proportion discard at age from the Cape Cod grounds were assumed for 1985-1988 discards in the northern Gulf of Maine. Total catch at age is dominated by age-3 and indicates a strong 1987 yearclass (Appendix A, Figure 3). Mean weight at age of catch was relatively stable from 1985 to 1996, but has increased for ages 2+ in recent years (Figure 4).
ABUNDANCE AND BIOMASS INDICES
Stock Abundance and Biomass Indices
NEFSC survey strata for the Cape Cod grounds are offshore strata 25-27 and inshore strata 56-66 and strata for the northern Gulf of Maine are offshore strata 39 and 40 (Figure 5). The NEFSC spring and autumn bottom trawl surveys have sampled offshore strata since 1963 and 1968, respectively (Despres et al. 1988). However, sampling of inshore strata north of Cape Cod began in 1977. Yellowtail are consistently sampled in offshore stratum 27 by the spring survey, but were only caught in 4 years since 1963 by the fall survey. Therefore, the spring index includes offshore stratum 27, but the fall survey does not. The Massachusetts survey has been conducted since 1978 and consistently catches yellowtail in strata 17-36 (Howe 1989).
Survey biomass indices are somewhat noisy, but generally indicate high biomass in the late 1970s and early 1980s, a decline in the 1980s and a rapid increase in the late 1990s (Figure 6). The rapid increases in fall 1999 or spring 2000 do not appear to result from strong recruitment, because catches of all ages increased. Large survey catches were distributed throughout Cape Cod and Massachusetts Bays, Stellwagen Bank and Jeffreys Ledge (Figure 7).
The portion of survey biomass from northern Gulf of Maine is variable, but averages 11% throughout the survey time series (Figure 8). There appears to have been low abundance of yellowtail in the northern Gulf of Maine during the late 1960s, early 1970s, and middle 1980s. Age distribution of survey catches are potted in Figure 9 and listed in Table 8.
Correspondence among survey indices was assessed using correlations among normalized observations [Ln(x/mean); Table 7]. Correlations among survey series were weak to moderate with strongest correlations among indices for ages 2-4 (r=0.12 to 0.69). Normalized indices of catch per tow at age are illustrated in Figure 10.
MORTALITY AND STOCK SIZE
Virtual Population Analysis
Estimates of abundance from virtual population analysis of catch at age-1 to age-5+, 1985-2001, were calibrated using an ADAPT algorithm (Gavaris 1988) that estimated age-2 to age-4 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 were calibrated to January abundance at age, and fall survey indices were calibrated to abundance at age for January of the next year. 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 1971), 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 Cape Cod 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 indicate that F on ages 3+ decreased from a peak of 1.3 in 1988 to 0.28 in 1993, then increased to an annual average of 0.61 from 1995 to 2000 and was 0.75 in 2001 (Figure 12). With the exception of the strong 1987 year class (29 million at age-1), recruitment has been stable, averaging 10 million at age 1. However, early indications are that the 2000 yearclass is well below average. Spawning biomass averaged 1,000mt during the late 1980s increased to a peak of 3,800mt in 1991 as the 1987 cohort matured, decreased to 1,600mt in 1998, and gradually increased to 3,200 mt in 2001. Retrospective analysis indicates a pattern of underestimating F, and overestimating SSB in the last five years (Figure 13).
Bootstrap analysis indicates that abundance estimates in 2002 were estimated with moderate precision (CVs=0.26-0.51). The 80% confidence limit for 2001 F is 0.59-0.95, and the 80% confidence limit for 2001 SSB is 2,500-4,000mt.
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 shown in Figure 14. A comparison of recently observed age distributions with the age distribution expected at F40% shows a relative truncation in current age structure (Figure 15).
Applying the approach used to estimate MSY proxies for Cape Cod yellowtail (NEFSC 2002), FMSY is approximated as F40%MSP (0.17). The SSBMSY proxy is 12,600mt, calculated as the product of 40%MSP (1.192kg spawning biomass) and average recruitment (10.5 million). The MSY proxy is 2,300mt, derived as the product of yield per recruit at F40%MSP (0.213kg) and average recruitment.
Stochastic projections at 85% of status quo F in 2002 and F=0.06 for 2003-2009 there is a 50% probability of rebuilding to SSBMSY by 2009 (Appendix A, Figure 16). However, retrospective patterns indicate that projections may be optimistic.
Although there is little evidence to separate yellowtail flounder in the northern Gulf of Maine from those on the Cape Cod fishing grounds, the lack of samples in the Gulf of Maine, and the resulting need to use Cape Cod samples to characterize the entire catch produce a catch at age matrix that is very similar to that used in previous assessments of Cape Cod yellowtail flounder, though slightly greater to account for Gulf of Maine catch. Therefore, the same patterns of abundance at age are indicated in this combined assessment. Furthermore, the peculiarities of the Cape Cod assessment persist in this assessment. For example, the retrospective pattern of overestimating abundance at older ages (i.e., age 5+) continues. The apparent lack of older fish in the catch and surveys continues to produce extremely high F on older ages. Despite the high estimates of F, recruitment appears to have been stable, and SSB has recently increased.
The possibility that older fish are moving from the fishing and survey areas, giving the false impression of high mortality, was investigated. Size distributions from the longest time series of survey data (fall survey, offshore strata 25, 26, 39 and 40; Figure 17) show that some larger fish were sampled in the assessment strata in the 1960s, but recent length distributions are considerably smaller. More large fish were also sampled in the earliest years of the Massachusetts survey (Figure 18). The Gulf of Maine summer survey, which sampled the inshore strata of the western Gulf of Maine (1977-1981, inshore strata 68-90; Figure 19) caught a similar size distribution of yellowtail as the assessment strata. Survey catches in the central and eastern Gulf of Maine also caught a similar size distribution of yellowtail as the assessment strata (Figure 20), but inconsistently and at much lower densities than those in the assessment strata (e.g., since 1963, yellowtail were only caught twice in stratum 28, six surveys in stratum 29, six surveys in stratum 37 and once in stratum 38). Therefore, the assessment strata appear to reflect the size distribution throughout the Gulf of Maine, and no large yellowtail were sampled anywhere in the Gulf of Maine in recent years.
The initial ADAPT calibration was configured for catch at age for age-1 to age-6+ and exhibited a severe retrospective pattern for SSB and F. A comparison of ADAPT retrospective patterns from Cape Cod-Gulf of Maine and Cape Cod only exhibited little difference. The low numbers of age 5 in the catch and surveys did not appear to be sufficient to reliably estimate F on age 5. As a result, an alternative ADAPT configuration which truncated the catch at age to age-5+ was considered. Estimation of abundance for the truncated catch at age required that age 3 be considered fully recruited for calculation of F on the oldest true age. The final ADAPT run reduced the magnitude of the retrospective patterns for fully recruited F and spawning biomass. The results revealed a high sensitivity to the calibration change. The fully recruited F decreased while spawning stock biomass increased.
Including a flat-topped selectivity pattern at age 3+ could mask high F’s at true fully recruited ages. The original formulation, which estimated F on age 3, suggested that age 3 yellowtail were partially recruited. Assessments of yellowtail flounder in other U.S. management areas (Georges Bank and southern New England-Mid Atlantic, where yellowtail growth is faster) indicate partial recruitment at age-3. A comparison of observed length distribution at age-3 and length selectivity at various mesh sizes indicated only partial retention of age-3 yellowtail. However, mesh selectivity is only one component of fishery selectivity and other factors, such as temporal-spatial elements of the fishery, also influence fishery selectivity. In addition, the mean weights of a plus group at age-5 and older may be difficult to characterize because they continue to grow substantially after age 5.
Yield and SSB per recruit were re-estimated assuming full recruitment at age-3 in order to be consistent with the revised ADAPT configuration. Examination of stock-recruit observations for Cape Cod-Gulf of Maine yellowtail and fishing mortality rates at various levels of replacement suggests that the stock can replace itself at F greater than F40% (i.e.Fmed > F40% MSP), and F40% may be a conservative proxy for FMSY. However, extrapolating recruitment at high stock sizes from the VPA time series may overestimate productivity of the stock at higher SSB. The stock recruitment relationship is similar to the Georges Bank stock prior to recovery, in that most stock recruitment points were above the F40% replacement line. This suggests that a short-term perspective of the stock recruitment relationship may not represent the potential productivity of the Cape Cod-Gulf of Maine stock. The 36th Northeast Stock Assessment Review Committee (SARC) concluded that there is currently no justification for changing the F40% reference point.
Contributions from the Georges Bank or Southern New England stocks of yellowtail flounder to the Cape Cod-Gulf of Maine stock may occur through both adult movement and recruitment impacts. Given the relative sizes of the stocks, especially the Georges Bank and Cape Cod stocks, any transfer among stocks could overwhelm the recruitment signal from reproduction within the Cape Cod-Gulf of Maine area (Hart and Cadrin 2003). Given the difficulty in estimating fully-recruited fishing mortality with consistency and estimating reliable patterns of mortality and abundance, independent estimates of mortality may be needed to verify estimates from ADAPT. Mark-recapture studies could be used to estimate mortality as well as mixing rates with adjacent areas.
The sharp increase in catch and survey indices from 1999 to 2001 are difficult to interpret, because increases were at all ages and throughout the stock area. Perhaps the rolling closures may have increased both survey and fishery catchability. Surrounding closures may have redirected effort onto Stellwagen Bank. However, sharp increases also occurred in historic landings (Figure 2).
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, chaired the Working Group meeting, and drafted the Working Group discussion. Andrew Payne chaired the Stock Assessment Review Committee. We thank all Working Group and Review Committee participants
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