Northeast Fisheries Science Center Reference Document 03-03
A Report of the 36th Northeast Regional
Stock Assessment Workshop
Stock Assessment of Yellowtail Flounder
in the Cape Cod - Gulf of Maine Area
by Steven X. Cadrin1 and Jeremy King2
1National Marine Fisheries Serv., 166 Water St., Woods
Hole, MA 02543-1026
2Massachusetts Div. of Marine Fisheries, 50A Portside Dr.,
Pocasset, MA 02559
Print
publication date February 2003;
web version posted February 21, 2003
Citation: 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.
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Abstract
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.
INTRODUCTION
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.
Management
History
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.
Assessment History
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.
FISHERY DATA
Commercial Landings
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.
Discarded Catch
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.
Projections
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.
DISCUSSION
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).
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, 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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