Northeast Fisheries Science Center Reference Document 01-08
TRAC
Advisory Report on Stock Status
A
Report of the Fourth Meeting of the Transboundary Resources
Assessment Committee,
St.
Andrews Biological Station, St. Andrews,
New Brunswick,
April 17-20,
2001
by Transboundary Resources Assessment Committee
National Marine Fisheries Service, 166 Water St., Woods Hole MA 02543
Print
publication date July 2001;
web version posted July 27, 2001
Citation: Transboundary Resources Assessment Committee. 2001. TRAC advisory report on stock status: a report of
the fourth meeting of the Transboundary Resources Assessment Committee (TRAC), St. Andrews Biological
Station, St. Andrews, New Brunswick, April 17-20, 2001. Northeast Fish. Sci. Cent. Ref. Doc. 01-08; 18 p.
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INTRODUCTION
This Advisory Report on Stock Status provides
results of the fourth meeting of the Transboundary Resources Assessment
Committee (TRAC), held in St. Andrews, N.B.Canada April 17-20, 2001.
The meeting was attended by 29 Canadian and USA scientists, managers
and industry representatives. The primary purpose of the meeting
was to review assessments of Georges Bank cod, haddock and yellowtail
flounder management units and to prepare documentation required for
management.
The TRAC process was initiated in 1998 as a forum for
joint peer review of assessments for transboundary stocks of mutual
interest to Canada and the United States. The process includes two
working bodies, these being the Transboundary Assessment Working Group,
or TAWG, consisting of Canadian and USA scientists and industry members,
and the TRAC itself. The TAWG produces the assessments and addresses
research recommendations and other needs as requested by the TRAC.
The TRAC, consisting of Canadian and USA scientists, managers and industry
members, reviews these assessments and other products and generates
final reports on stock status. For Canada, this takes the form of Stock
Status Reports (available from DFO Canada) for use in preparing management
advice for the current fishing year (Canada typically opens its Georges
Bank groundfish fishery June 1). For the USA, the primary products
are advisory reports which are used in preparing management advice
for the subsequent fishing year (here, the 2002 Fishing Year). These
advisory reports consider stock status only, since projections of fishing
mortality and stock size for the 2002 fishing year will require additional
commercial landings information for 2001 and policy guidance on fishing
mortality rates which will not be available until autumn.
The TAWG met in Woods Hole, MA March 19-22, 2001 and completed assessments
for Georges Bank and South (Northwest Atlantic Fisheries Organization
or NAFO Division 5Z and Subarea 6) cod, Georges Bank (5Z) haddock,
and Georges Bank (5Z) yellowtail flounder, together with assessments
for Eastern Georges Bank (5Zj,m) cod and haddock (Canadian and USA
management units for Georges Bank yellowtail are the same). In addition,
the TAWG also evaluated approaches used by both countries for determining
biological reference points describing stock status. Statistical areas
and stock boundaries for the USA assessments are given in Figure
1. The Canadian 5Zj and 5Zm areas coincide with USA statistical
reporting areas 551-61 and 552-62, respectively.
The present meeting of the TRAC peer-reviewed these 5 assessments
and completed advisory and stock status reports (advisory reports for
the USA management units are given below). Complete assessment documents
for the USA assessments will be published in the Northeast Fisheries
Science Center reference document series. The TRAC also further reviewed
procedures for determining biological reference points. The Proceedings
of this meeting and other supporting documentation will be published
separately.
The evaluations of resource status presented in the following advisory
reports are as mandated by the Sustainable Fisheries Act or SFA (PL
104-297) of 1996, now part of the Magnuson-Stevens Fisheries Conservation
and Management Act or MSFCMA. National Standard Guidelines for the
SFA require use of "status determination criteria" in the
form of threshold levels for such evaluations. Overfishing is said
to be occurring if fishing mortality (F) exceeds the Maximum Fishing
Mortality Threshold (MFMT, or FThreshold); and a stock is
considered to be overfished if it falls below the Minimum Stock Size
Threshold (MSST, or BThreshold). The latter is usually 1/4
to 1/2 BMSY, the stock biomass level which would be expected to produce
maximum sustainable yield or MSY. Definitions in the following advisories
are as proposed by the Overfishing Definition Review Panel (Applegate et
al.1998), now in effect under Amendment 9 to the New England Fishery
Management Council's Northeast Multispecies Fishery Management Plan
or FMP. Because overfished stocks must be rebuilt, the Panel also proposed
control laws or rules which specify how F should vary with biomass
and provide both threshold and target levels determined by stock biomass
levels relative to BMSY. Note that FThreshold is
a "limit" reference point indicating the point at which Fshould
be constrained substantially and the probability for exceeding this
value should be low. "Target" levels are reference points
designed to ensure that F exceeds the threshold only rarely. Control
rules for Georges Bank and South cod, Georges Bank haddock, and Georges
Bank yellowtail flounder, and evaluations of stock status for 2000
in relation to these control rules, are given in the following advisory
reports.
These stocks are managed under the FMP by both indirect and direct
effort controls including a moratorium on permits and days-at-sea restrictions.
Canadian management measures include total allowable catches (TACs)
based on an F0.1 management strategy (further information including
projections of harvests in 2001 and stock size in 2002 is given in
the DFO Stock Status Reports for 2001 referenced below). Measures for
both countries are designed to keep F at levels which will promote
stock rebuilding.
For further information
Applegate, A., S. Cadrin, J. Hoenig, C. Moore, S. Murawski and E.
Pikitch. 1998. Evaluation of existing overfishing definitions and recommendations
for new overfishing definitions to comply with the Sustainable Fisheries
Act. Final Report of the Overfishing Definition Review Panel to the
New England Fishery Management Council, 179 p.
DFO 2001. Eastern Georges Bank cod. DFO Sci. Stock Status Report A3-04
(2001).
DFO 2001. Eastern Georges Bank haddock. DFO Sci. Stock Status Report
A3-08 (2001).
DFO 2001. Yellowtail flounder on Georges Bank. DFO Sci. Stock Status
Report A3-15 (2001).
Restrepo, V.R., P.M. Mace and F. Serchuk. 1999. The precautionary
approach: a new paradigm or business as usual? p. 61-70 In:
NMFS. 1999. Our living oceans. Report on the status of U.S. living
marine resources, 1999. U.S. Dep. Comm., NOAA Tech Memo. NMFS-F/SPO-41,
301 p.
DEFINITIONS OF TECHNICAL TERMS
ADAPT: A computer program used to optimally fit
a virtual population analysis (VPA, see below) to abundance data.
Biological Reference Points: Specific values for
variables that describe the state of a fishery, used to evaluate
its status. These may include "target" reference points, corresponding
to a desired result (e.g., maintaining a healthy biomass level),
and "limit" reference points, or "thresholds" carrying an unacceptably
high risk if exceeded. Examples are F0.1, FMSY,
FThreshold, and Fmax .
Biomass-weighted F: An estimate of fishing mortality
in which F estimates for each age group are weighted by corresponding
stock biomass at age. This calculation is needed to make average
F estimates from age structured assessments comparable to those obtained
from surplus production modeling of all stock components.
BMSY: The long-term average stock biomass
level required to achieve Maximum Sustainable Yield or MSY, when
the stock is fished at FMSY.
BThreshold (Minimum Stock Size Threshold or MSST): One
of two Status Determination Criteria specified in the National Standard
Guidelines. At stock sizes below BThreshold, the stock
is considered to be overfished.
Control Rule: A protocol for specifying harvest
rates in relation to stock status and limit and target reference
points. Technically, a harvest strategy which, if implemented, would
be expected to result in a long-term average catch approximating
MSY. In the National Standard Guidelines, the "MSY Control Rule" is
used to determine the limit fishing mortality, or Maximum Fishing
Mortality Threshold (MFMT). Control rules are also known as "decision
rules" or "harvest control laws" in some of the scientific literature.
Exploitation Pattern: The distribution of fishing
mortality over the age composition of the fish population, determined
by the type of fishing gear, areal and seasonal distribution of fishing,
and the growth and migration of the fish. The pattern is expressed
as a series of values ranging from 0.0 to 1.0.
Exploitation Rate: The proportion of a population
at the beginning of the year that is caught during the year. For
example, if 720,000 fish were caught during the year from a population
of 1 million fish alive at the beginning of the year, the annual
exploitation rate would be 0.72 or 72%. Note that this rate cannot
exceed unity; obviously, more fish cannot die than were originally
present.
Fishing Mortality Rate (F): That part of the total
mortality rate applying to a fish population that is caused by fishing.
Fishing mortality is usually expressed as an instantaneous rate,
and can range to values exceeding unity, such as 2.0 or higher.
Fmax: The fishing mortality rate that
results in the maximum level of yield per recruit. This is the point
beyond which growth overfishing begins.
FMSY: The fishing mortality rate that
produces MSY by taking a constant fraction of a stock that is fluctuating
around BMSY.
F0.1: The fishing mortality rate at
which the increase in yield per recruit in weight for an increase
in a unit of effort is 10 percent of the yield per recruit produced
by the first unit of effort on the unexploited stock (i.e., the slope
of the yield-per-recruit curve for the F0.1 rate is one-tenth
the slope of the curve at its origin).
FTarget: The fishing mortality rate
which (with some specified probability level) will prevent FThreshold from
being exceeded.
FThreshold (Maximum Fishing Mortality Threshold
or MFMT): One of two Status Determination Criteria, specified
in the National Standard Guidelines. Exceeding FThreshold for
one year or more constitutes overfishing.
Fully-recruited F: An estimate of fishing mortality
for all age groups fully vulnerable to fishing.
Maximum Sustainable Yield (MSY): The largest long-term
average catch or yield that can be taken from a stock or stock complex
under prevailing ecological and environmental conditions.
Maximum Spawning Potential Reference Points: Reference
points based on some fraction of maximum spawning potential (MSP)
as determined from spawning stock biomass per recruit models, used
to define overfishing. MSP is the spawning stock biomass per recruit
in the absence of fishing; it is then reduced to a percentage of
the maximum as F increases.
Mean Biomass: The product of mean abundance (numbers)
and the average weight of individual fish. Mean abundance is calculated
from abundance at the beginning of the year and the annual mortality
rate, while average weights are derived from population size and
weight at age data.
Mortality Rates: The rates at which fish die from
fishing and/or natural causes. Biologists tend to work with instantaneous rates,
in which time intervals are sufficiently short so as to allow separation
of the primary components as instantaneous fishing mortality (F)
and instantaneous natural mortality (M). Together the two equal instantaneous
total mortality (Z); i.e. Z = F+M.
Natural Mortality Rate (M): That part of total
mortality applying to a fish population that is caused by factors
other than fishing. It is common practice to consider all sources
together since they usually account for much less than fishing mortality.
It is usually expressed as an instantaneous rate as noted above.
Nominal Catch: The sum of the catches that are
landed (expressed as live weight or equivalents). Nominal catches
do not include unreported discards.
Overfishing/overfished: According to the National
Standard Guidelines, "overfishing occurs whenever a stock or stock
complex is subjected to a rate or level of fishing mortality that
jeopardizes the capacity of a stock or stock complex to produce MSY
on a continuing basis." Overfishing is occurring if FThreshold is
exceeded for a year or more. An "overfished" stock has been reduced
below BThreshold requiring management actions to rebuild
to the MSY level within an acceptable time frame.
Partial Recruitment: Patterns of relative vulnerability
of fish of different sizes or ages due to the combined effects of
gear selectivity and availability to the fishery.
Recruitment: The number of fish that survive (from
birth) to a specific age or grow to a specific size.
Spawning Stock Biomass (SSB): The total weight
of all sexually mature fish in the population.
Spawning Stock Biomass Per Recruit (SSB/R): The
expected lifetime contribution to the spawning stock biomass for
a recruit. For a given exploitation pattern, rate of growth, and
rate of natural mortality, an expected equilibrium value of SSB/R
can be calculated for each level of F and compared to the maximum
level of SSB/R that would be realized if there were no fishing.
Status Determination Criteria: Objective and measurable
criteria used to determine if overfishing is occurring or the stock
is in an overfished state according to National Standard Guidelines.
TAC: Total allowable catch is the total regulated
catch from a stock in a given time period, usually a year.
Virtual Population Analysis or VPA: A retrospective
analysis of the catches from a given year class which provides estimates
of fishing mortality and stock size at each age. A VPA takes natural
mortality into account as well as removals from fishing. The method
requires accurate statistics of catch by year, information on natural
mortality, and a reasonably accurate estimate of F in the terminal
year of the time series. This technique is used extensively in fishery
assessments, since the conditions for its use are so common; many
fisheries are heavily exploited, the annual catches for a year class
can generally be determined, and the natural mortality rate is known
within a fairly small range and is low compared with the fishing
mortality rate.
Year class (or cohort): Fish in a stock born in
the same year. For example, the 1987 year class of cod includes all
cod born in 1987, which would be age 1 in 1988, age 2 in 1989, etc.
Yield per recruit (Y/R or YPR): The average expected
yield in weight from a single recruit. Y/R is calculated assuming
that F is constant over the life span of a year class. The calculated
value is also dependent on the exploitation pattern, rate of growth,
and natural mortality rate.
A. GEORGES BANK COD ADVISORY REPORT
Figure A: HTML Format - PDF
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State of Stock: Mean biomass in 2000 is above BTHRESHOLD (1/4
BMSY =27,000 mt) and biomass weighted fishing mortality is
below FMSY (0.32), therefore, the stock is not overfished
and overfishing is not occurring, according to the Sustainable Fisheries
Act (SFA) status determination criteria. Fully recruited fishing mortality
declined from a record high of 1.47 (72% exploitation) in 1994 to 0.22
(18% exploitation) in 2000, slightly above F0.1. (0.18) (Figure
A1). Biomass weighted fishing mortality in 2000 was about twice the target
F specified by the SFA control rule (Figure
A4). Although mean biomass
has increased from the record low in 1995 it is still only about 36%
of BMSY (108,000 mt; Amendment 9) in 2000 (Figure A2 and A4).
Spawning stock biomass has increased from the time series low in 1995
to 29,000 mt in 2000 (41% of the Amendment 7 rebuilding target) (Figure
A2). Since the 1990 year class, the sizes of recruiting year classes
have all been well below average. The 1997 and 2000 year classes are
estimated to be the weakest in the time series. Without improved recruitment
biomass rebuilding is unlikely. (Figure A2).
Catch and Status Table (weights in '000 mt, recruitment in millions):
Georges Bank Cod
Year |
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000 |
Max1 |
Min1 |
Mean1 |
Total commercial landings2 |
23.1 |
15.2 |
7.9 |
8.9 |
10.4 |
8.8 |
9.9 |
9.2 |
57.2 |
7.9 |
29.5 |
US commercial landings2 |
14.6 |
9.9 |
6.8 |
7.0 |
7.5 |
6.9 |
8.1 |
7.6 |
40.1 |
6.8 |
21.5 |
Canada commercial landings |
8.5 |
5.3 |
1.1 |
1.9 |
2.9 |
1.9 |
1.8 |
1.6 |
17.8 |
1.1 |
8.0 |
Discards |
Discards occur but reliable estimates not presently
available |
US recreational landings3 |
1.9 |
1.0 |
1.3 |
0.6 |
0.8 |
0.5 |
0.4 |
1.1 |
8.7 |
0.4 |
2.1 |
Catch used in assessment |
23.1 |
15.2 |
7.9 |
8.9 |
10.4 |
8.8 |
9.9 |
9.2 |
57.2 |
7.9 |
29.5 |
Mean Biomass |
35.6 |
28.1 |
26.6 |
30.3 |
34.4 |
32.8 |
35.6 |
38.5 |
135.2 |
26.6 |
75.2 |
Spawning stock biomass4 |
29.4 |
19.2 |
18.1 |
20.5 |
22.9 |
25.2 |
27.4 |
29.0 |
92.6 |
18.1 |
54.9 |
Recruitment (age 1) |
8.4 |
6.3 |
4.3 |
7.6 |
10.3 |
3.2 |
7.3 |
4.9 |
42.8 |
3.2 |
15.7 |
F (ages 4-8, fully recruited, unweighted) |
1.15 |
1.47 |
0.63 |
0.44 |
0.83 |
0.59 |
0.45 |
0.22 |
1.47 |
0.22 |
0.65 |
Exploitation rate |
63% |
71% |
43% |
34% |
52% |
41% |
33% |
18% |
71% |
18% |
42% |
F (age 1+, biomass weighted) |
0.66 |
0.55 |
0.30 |
0.30 |
0.31 |
0.27 |
0.28 |
0.24 |
0.66 |
0.24 |
0.40 |
1Over period 1978-2000. 2US landings for 1994-2000
are provisional. 3Not used in assessment. 4At beginning
of the spawning
season (i.e., March 1).
Stock Identification and Distribution: The Georges
Bank cod stock is distributed primarily from the Northeast Peak of Georges
Bank to Nantucket Shoals, with minor occurrence in the Southern New England
and Mid-Atlantic regions. The distribution on the Northeast Peak spans
the U.S.-Canada boundary.
Catches: Commercial landings increased in the late
1970s and early 1980s, peaking at a record high 57,200 mt in 1982. During
1983-1986, landings declined, but subsequently increased through 1990
(Figure
A1). Total commercial landings declined to a record low of 7,900
mt in 1995 and have since remained relatively stable. Landings were 9,200
mt in 2000 (Figure
A1). Recreational landings have ranged from 400 mt
to 8,700 mt accounting for 1-19% of the total cod catch.
Data and Assessment: An analytical assessment (VPA)
of 1978-2000 commercial landings-at-age data was conducted assuming M=0.2.
The VPA was calibrated using the ADAPT method with recruitment and abundance
indices from NEFSC spring and autumn and Canadian spring bottom trawl
survey catch-per-tow-at-age data. Discards and recreational catches were
not included in the VPA. Precision associated with the estimates of fishing
mortality, spawning stock biomass, and mean biomass in 2000 was evaluated
(Figures
A5 and A6 and A7).
Biological Reference Points: FMSY (weighted
by stock biomass at age) is estimated to be 0.32 and the MSY is estimated
to be 35,000 mt from a BMSY of 108,000 mt based on a surplus
production model. Yield and SSB per recruit analyses , with an assumed
M of 0.20, indicate that F0.1 = 0.18 (15% exploitation), and
the corresponding SSB per recruit is 40% of the maximum (Figure
A3).
SFA Control Rule: When stock biomass is greater than
BMSY fishing mortality is not to exceed FTHRESHOLD =
FMSY;when biomass is between BMSY and 1/2 BMSY F
will be defined by a 10 year rebuilding time period and when biomass
is between ½ BMSY and 1/4 BMSY F will be defined
by a five year rebuilding time period. F weighted by biomass is the metric
for the SFA control rule.
Fishing Mortality: Fully recruited fishing mortality
(ages 4-8,unweighted) doubled between 1979 and 1985 from 0.35 (27% exploitation)
to 0.74 (48% exploitation), declined to 0.48 (35% exploitation)
in 1986-1987, then increased to a record high of 1.47 (72% exploitation)
in 1994. F has since declined to 0.22 (18% exploitation) in 2000 (Figure
A1). There is an 80% probability that F in 2000 was between 0.18 and
0.25 (Figure
A5). Biomass weighted F(ages 1+) has remained relatively
stable since 1995 while fully recruited F has declined since 1997, indicating
that younger cod are being exploited more heavily, particularly the 1996
and 1997 year classes (Figure A1).
Recruitment: Strong year classes were produced in 1980,
1983, and 1985. The 1990 year class was slightly above average, and all
subsequent year classes have been well below average. The 1997 and 2000
year classes are the weakest year classes in the time series (1978-2000)
(Figure
A2).
Stock Biomass: Spawning stock biomass declined by about
40% between 1980 and 1985 (from 92,600 mt to 55,000 mt), increased in
1988 (74,000 mt), and then declined to a record low in 1995 (18,100 mt).
SSB has since gradually increased to 29,000 mt in 2000, as a result of
growth and survival of recent year classes rather than improved recruitment
(Figure
A2). There is an 80% probability that the SSB in 2000 was between
25,000 mt and 32,000 mt (Figure
A6). Mean biomass trends were similar
to SSB trends. There is an 80% probability that mean biomass in 2000
was between 33,000 mt and 43,000 mt (Figure
A7)
Special Comments:
- Retrospective analysis indicates a pattern of inconsistencies in
the last year in which estimates of F are less than revised estimates
of F and estimates of SSB are more than revised estimates of SSB.
- Low levels of sampling of U.S. landings of large cod and discards
contribute to the uncertainty in estimates of the size and age composition
of the U.S. catch. U.S. port sampling for large cod has been chronically
low, threatening to undermine the integrity of the stock assessment.
VTRs and at sea observations by the Sea Sampling program are inadequate
to reliably estimate the quantity of discard or characterize the size
and age compositions.
- There is a problem of mixing reference points and metrics of stock
status that are based on different models (surplus production vs. dynamic
pool) e.g. FMSY = 0.32 corresponds to a maximum spawning
potential (MSP) of 16.5% at fully recruited F = 0.47, which is less
than 20% MSP at fully recruited F = 0.41.
Sources of Information: O'Brien, L and N.J. Munroe.
2001. Assessment of the Georges Bank Atlantic cod stock for 2000, NEFSC
Ref. Doc. 01-xx.
Overfishing Definition Review Panel. 1998. Evaluation of existing overfishing
definitions and recommendations for new over fishing definitions to comply
with the SFA.
O'Brien L. and S.X. Cadrin. 1999. Assessment of the Georges Bank Atlantic
cod stock for 1998, NEFSC Lab. Ref. 99-03.
B. GEORGES BANK HADDOCK ADVISORY REPORT
Figure B: HTML Format - PDF
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State of Stock: Spawning stock biomass in 2000 was
greater than the SSBthreshold (½ BMSY proxy = 53,000
mt, which is 50% of the spawning stock biomass expected to produce maximum
sustainable yield) and fully recruited fishing mortality in 2000 was
less than Fthreshold (FMSY proxy or F0.1 =
0.26); therefore, the stock is not overfished and overfishing is not
occurring according to the Sustainable Fisheries Act status determination
criteria. Fully recruited fishing mortality has been reduced from pre-1994
levels, but has increased slightly in each of the past three years and
is approaching the target fishing mortality for a rebuilt stock specified
by the SFA control rule (Figure B4). The stock is recovering from an
overexploited condition and spawning stock biomass has increased more
than 5-fold from 1993 record low levels due to higher survivorship and
improved recruitment since 1992. The 2000 estimate (64,100 mt) is approximately
61% of the spawning stock biomass estimated to produce maximum sustainable
yield (105,000 mt). The age structure of the population is continuing
to expand and age 4+ biomass is at its highest level since 1982. The
1996 year class (20.7 million fish at age 1) has recruited to the fishery
and comprised approximately 25% of the spawning stock biomass and 34%
of the landings in 2000. Although there is uncertainty about the estimated
sizes of the 1998 (39.6 million fish at age 1) and 2000 (57.3 million
fish at age 1) year classes, they are currently estimated to be the largest
since 1978. These year classes provide opportunities for both moderate
increases in harvest and continued rebuilding of spawning stock biomass.
Catch and Status Table (weights in '000 mt, recruitment in millions):
Georges Bank Haddock
| Year |
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000 |
Max1 |
Min1 |
Mean1 |
| Landings |
US commercial2 |
0.7 |
0.2 |
0.2 |
0.3 |
0.9 |
1.8 |
2.8 |
3.4 |
52.9 |
0.2 |
12.2 |
Canada landings |
3.7 |
2.4 |
2.1 |
3.6 |
2.7 |
3.4 |
3.7 |
5.4 |
18.3 |
0.5 |
5.1 |
Other commercial |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
82.63 |
0.03 |
10.73 |
| Total landings |
4.4 |
2.6 |
2.3 |
4.0 |
3.6 |
5.2 |
6.5 |
8.8 |
150.4 |
2.3 |
21.0 |
| Discards |
US commercial discards |
N/A |
0.5 |
0.1 |
0.3 |
0.6 |
0.1 |
N/A |
N/A |
N/A |
N/A |
N/A |
| Catch used in assessment |
4.4 |
3.1 |
2.4 |
4.3 |
4.3 |
5.3 |
6.5 |
8.8 |
150.4 |
2.4 |
21.8 |
| Spawning stock biomass4 |
11.3 |
15.1 |
24.4 |
30.2 |
37.5 |
45.0 |
53.4 |
64.1 |
180.5 |
11.0 |
48.2 |
| Recruitment (age 1) |
13.9 |
13.5 |
10.7 |
11.0 |
20.7 |
13.8 |
39.6 |
19.6 |
471.9 |
0.4 |
9.25 |
| F (ages 4-7, fully recruited unweighted) |
0.42 |
0.35 |
0.14 |
0.17 |
0.12 |
0.14 |
0.16 |
0.19 |
0.61 |
0.11 |
0.34 |
| Exploitation rate |
31% |
27% |
12% |
14% |
10% |
12% |
13% |
16% |
42% |
9% |
26% |
1Over period 1963-2000. 2Data 1994 through 2000 are
provisional. 3Over period 1962-1976. 4At beginning
of the spawning season
(April 1). 5Geometric mean.
Stock Identification and Distribution: Georges Bank haddock
are distributed from the Northeast Peak to Nantucket Shoals, with minor occurrence
in the Southern New England and Mid-Atlantic regions. Highest concentrations
are currently found along the Northern Edge and Northeast Peak of Georges Bank.
Historically, haddock were also abundant in the Great South Channel area of
Georges Bank, and recent commercial landings and research vessel surveys suggest
that abundance in this area has increased significantly.
Catches: Total commercial landings increased sharply in 1965
and 1966 as a result of strong recruitment and increased exploitation by distant
water fleets commencing in the early 1960s. Landings declined to less than
6,000 mt between 1972 and 1976, but increased in the late 1970s to 27,500 mt
in 1980. Total landings then declined to only 2,300 mt in 1995, but have since
increased to 8,800 mt in 2000 (Figure B1). Discards have been periodically
estimated and added to the catch when levels were significant. Estimates of
regulatory discarding occurring from 1994-1998 in response to U.S. trip limits
are included in the current assessment. Landings by U.S. vessels are almost
exclusively by otter trawl while Canadian landings are taken by otter trawl
and longline gear. Recreational landings from this stock have been negligible.
Data and Assessment: An analytical assessment (VPA) was conducted
incorporating 1931-2000 commercial catch-at-age data, assuming natural mortality
(M = 0.2). The VPA was calibrated using the Adapt method using Canadian spring,
and U.S. spring and autumn survey numbers at age. The precision and
uncertainty associated with the estimates of fishing mortality and spawning
stock biomass in 2000 were quantitatively evaluated (Figures B5-B6).
Biological Reference Points: Yield and spawning stock biomass
per recruit analyses, performed with an assumed M of 0.20, indicated that F0.1 =
0.26 (21% exploitation) and the corresponding equilibrium SSB per recruit is
40.9% of maximum (Figure B8). F0.1 is used as a proxy for FMSY and
a spawning stock biomass level of 105,000 mt is used as a proxy for BMSY.
SFA Control Rule: The SFA harvest control rule established
SSB as a proxy for BMSY, F0.1 as a proxy for FMSY,
and fully recruited F (ages 4-7, unweighted) as the metric for fishing mortality
(Figure B4). When SSB is greater than 105,000 mt (SSBMSY), the threshold
fishing mortality rate is F0.1 (0.26, 21% exploitation) and the
target fishing mortality rate is 75% of FMSY proxy (0.20, 16% exploitation).
As SSB declines below SSBMSY, the threshold fishing mortality rate
declines linearly from 0.26 to 0.00 at 1/2 SSBMSY (52,500 mt), and
the target fishing mortality rate declines linearly from 0.20 to 0.00 at 68,000
mt.
Fishing Mortality: Fully recruited fishing mortality remained
between 0.28 and 0.45 (22% - 33% exploitation) during most of the 1980s and
early 1990s before declining to 0.14 (12% exploitation) in 1995 (Figure B1).
Fully recruited fishing mortality has remained at low levels ranging between
0.12 (10% exploitation) and 0.19 (16% exploitation) since 1995. There is an
80% probability that fishing mortality in 2000 is between 0.17 (14% exploitation)
and 0.22 (18% exploitation) (Figure B5).
Recruitment: While recruitment has improved in the 1990s,
recruitment since 1979 has been far below the historical average recruitment
estimated for 1931 to 1960, when healthy stock conditions were observed. The
1998 (39.6 million) and 2000 (57.3 million) year classes are currently estimated
to be the strongest since 1978; however, these year classes are only average
to slightly below average relative to 1931-1960 levels. (See Special Comment
#3). There is an 80% probability that age 1 stock size in 2001 (2000 year class)
is between 36,900 and 88,000 fish (Figure B7).
Spawning Stock Biomass: SSB declined by 84% between 1978
and 1993 (69,700 mt to 11,300 mt). SSB began to increase in 1994 with higher
survivorship and improved recruitment, and reached 64,100 mt by 2000 (Figure B2). There is an 80% probability that SSB was between 58,000 mt and 72,500
mt (Figure B6). Historically, SSB during the 1935-1960 period, when landings
were sustained between 40,000 and 60,000 mt, was estimated to average 105,000
mt. The probability of strong recruitment is significantly greater when SSB
exceeds 80,000 mt (Figure B3).
Special Comments:
- Lack of paired market category samples from U.S. commercial landed trips
and biological samples from eastern Georges Bank contribute to uncertainty
in estimates of the size and age composition of the U.S. catch. The contribution
of the 1996 year class to the 2000 U.S. catch at age may be underestimated
due to pooling of samples from U.S. ports with different market category
culling practices.
- Vessel trip reports (logbooks) and at-sea observations are inadequate
to reliably estimate the quantity of haddock discards or to characterize
their size and age composition. Monitoring of discards in 2001 is particularly
important because of the recruitment of the 1998 year class.
- There is uncertainty about the size of the 1998-2000 year classes. In
the 1999 and 2000 assessments, there were significant downward revisions
to estimates for the 1998 and 1999 year classes. The terminal year estimates
of age 1 stock size are less certain in general due to variability in the
research vessel survey data used to estimate these stock sizes (Figure B7).
Since these age 1 stock sizes are incorporated into projections, forecasts
of stock biomass are less certain, particularly when terminal year age 1
recruitment is estimated to be large, as it was for the 2000 year class.
Source of Information: Brown, R.W. 2001. Assessment of the
5Z Georges Bank haddock stock, 1931-2000. NEFSC Ref. Doc. 2001-xx.
C. GEORGES BANK YELLOWTAIL FLOUNDER ADVISORY REPORT
Figure C: HTML Format - PDF
Format
State of Stock: Overfishing is not occurring, and the
stock is recovering from an overfished state, according to the Sustainable
Fisheries Act (SFA) status determination criteria. In 2000, mean biomass
was 56,000 mt, above the Amendment 9 rebuilding target (BMSY =
49,000 mt). Although biomass increased, the age structure has also improved
but remains truncated and dominated by younger ages. Fishing mortality
was low (fully recruited F = 0.14, biomass weighted F = 0.12, which is
well below the Fthreshold = FMSY = 0.30, Figure
C1). However, the exploitation rate on ages 2 and 3 has not declined
proportionally and the partial recruitment to the fishery for these ages
has increased. Spawning stock biomass (43,100 mt in 2000, Figure C2)
has been increasing since 1995. Recruitment is strong, with an outstanding
1997 year class and above average 1996, 1998 and 1999 cohorts (Figure
C2).
Catch and Status Table (weights in '000 mt, recruitment in millions):
Georges Bank Yellowtail Flounder
| Year |
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000 |
Max1 |
Min1 |
Mean1 |
| US |
Landings2 |
2.1 |
1.6 |
0.3 |
0.8 |
1.0 |
1.8 |
2.0 |
3.7 |
15.9 |
0.3 |
7.3 |
Discards |
1.1 |
0.1 |
0.0 |
0.0 |
0.1 |
0.1 |
0.5 |
0.4 |
5.6 |
0.0 |
1.5 |
| Canada |
Landings |
0.7 |
2.1 |
0.5 |
0.5 |
0.8 |
1.2 |
2.0 |
2.9 |
2.9 |
0.0 |
0.3 |
Discards3 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
0.0 |
| Total catch |
3.9 |
3.9 |
0.8 |
1.3 |
1.8 |
3.1 |
4.4 |
6.9 |
21.3 |
0.8 |
9.3 |
| Mean Biomass4 |
6.7 |
4.8 |
5.8 |
9.9 |
18.4 |
35.3 |
52.1 |
56.0 |
56.0 |
3.3 |
15.0 |
| SSB4 |
4.4 |
2.5 |
2.5 |
5.1 |
8.5 |
16.1 |
32.3 |
43.1 |
43.1 |
2.2 |
9.5 |
| Recruitment (age 1)4 |
12.2 |
11.3 |
15.8 |
23.4 |
50.0 |
77.0 |
45.1 |
33.8 |
77.0 |
5.8 |
26.9 |
| F (ages4+, fully recruited,unweighted)4 |
1.11 |
2.41 |
0.86 |
0.48 |
0.44 |
0.33 |
0.18 |
0.14 |
2.41 |
0.14 |
1.05 |
| Exploitation rate (4+)4 |
62% |
86% |
53% |
35% |
33% |
26% |
15% |
12% |
86% |
12% |
65% |
| F(age1+,biomass weighted )4 |
0.50 |
0.90 |
0.14 |
0.13 |
0.10 |
0.09 |
0.09 |
0.12 |
1.25 |
0.09 |
0.54 |
1Over period 1963-2000, except as otherwise noted. 2U.S.
landings for 1994-2000 are provisional. 3Canadian discards previous
to 1996 are unknown, but considered to be small. 4From VPA,
1973-2000
Stock Distribution and Identification: Yellowtail flounder
inhabit relatively shallow waters (20-100 m) of the continental shelf of the
northwest Atlantic from Labrador to Chesapeake Bay. Tagging observations, larval
distribution, and geographic patterns of landings and survey data indicate
that yellowtail flounder on Georges Bank comprise a relatively discrete stock.
The Georges Bank yellowtail stock is defined as the entire Bank, east of the
Great South Channel (U.S. statistical reporting areas 522, 525, 551, 552, 561,
and 562).
Catches: Landings, which were predominantly taken by the
U.S. fleet, averaged 14,200 mt during 1963-1976, with some taken by distant
water fleets. Since 1975, only Canada and the U.S. have landed yellowtail (the
Canadian fishery was initiated in 1993). U.S. landings declined to approximately
6,000 mt between 1978 and 1981. With strong recruitment and intense fishing
effort landings rose to over 10,500 mt in 1982 and 1983. U.S. landings fell
to a low of 1,100 mt in 1989, averaged 2,200 from 1990 to 1994 and dropped
to record lows of 300 and 800 mt in 1995 and 1996. For the first time on record,
the majority of Georges Bank yellowtail yield was landed by Canadian fishermen
in 1995. In the late 1990s, total landings steadily increased, to 6,500 mt
in 2000.
Discarding of small yellowtail has been an important source of mortality in
the Georges Bank yellowtail population. The magnitude of discarded catch has
generally been related to the size of recruiting cohorts, except in recent
years, when increased mesh sizes appear to have reduced trawl discards. Beginning
in 1999, discards increased when a U.S. scallop fishery was allowed in closed
Area II under an exemption program with an allocated TAC of 387 mt in 1999
and 250 mt in 2000. Estimated bycatch from the exemption program was approximately
395 mt 1999 and 238 mt in 2000, nearly all of which was discarded. Total discards,
including those under the exemption program, were estimated to be 484 mt in
1999 and 358 mt in 2000.
Data and Assessment: A virtual population analysis (VPA)
of 1973-2000 commercial landings and discards at age was completed (assuming
natural mortality, M = 0.2). Information on recruitment and stock abundance
was obtained from DFO and NEFSC spring surveys, NEFSC autumn bottom trawl surveys,
and NEFSC scallop surveys.
A non-equilibrium surplus production model was also used to assess the stock
due to uncertainties in the age composition in recent years (see Special Comment
#2). Input data included commercial landings and discards and data from the
three bottom trawl surveys used in the VPA. Unlike the VPA, no information
on age structure is required.
Biological Reference Points: F0.1 has been estimated
to be 0.25 (20% exploitation, Figure
C3). The surplus production model estimated
MSY to be 14,140 mt (see Special Comment #2) at a total stock biomass of 43,470
mt (BMSY); FMSY is 0.33 (which is F biomass weighted
, ages1+). Amendment 9 states that MSY = 14,500 mt at BMSY = 49,000
mt, and FMSY is 0.30.
SFA Control Rule: The SFA control rule specifies a biomass
target Bmsy, a biomass threshold ( 25% BMSY), a F threshold ( FMSY),
and F on biomass (1+,wb) as the metric for fishing mortality . When biomass
exceeds BMSY, target F is the tenth percentile of the FMSY estimate.
When biomass is between BMSY and 1/2BMSY, threshold F
is the F that allows rebuilding to BMSY in 10 years at the estimated
intrinsic rate of increase. When biomass is between ½ BMSY and 1/4
BMSY, F is the F that allows rebuilding to BMSY in 5
years. When biomass is below 1/4 Bmsy, threshold F = 0 (Figure
C4).
Fishing Mortality: The VPA biomass weighted F and the surplus
production model F produced similar trends in exploitation rates. Fully-recruited
F (ages 4-5, unweighted) was very high (averaged 1.2, 65% exploitation) during
the 1973-1994 period, but declined in the late 1990s to 0.14 (12% exploitation)
in 2000 (Figure
C1). Exploitation rate on ages 2 and 3 has not declined proportionally
and the partial recruitment to the fishery for these ages has increased. There
is an 80% probability that fully-recruited F in 2000 was between 0.12 and 0.17
(10% to 14% exploitation, Figure
C5). Biomass weighted F (1+,wb) generally
exceeded FMSY from the 1970s to 1994 but has sharply decreased to
approximately 0.1 since 1996.
Recruitment: Age 1 abundance estimates from VPA indicate
that four dominant year classes of at least 50 million were produced during
1973-1980. Abundance of all cohorts produced from 1981 to 1995 was less than
25 million at age 1. Recruitment increased in the late 1990s. The 1997 yearclass
appears to be the strongest in the time series at 77 million age-1 abundance,
and the 1996, 1998, and 1999 year classes are also above average (Figure
C2).
Stock Biomass: Spawning biomass was 21,000 mt in 1973, declined
to less than 4,000 mt from 1984-1988, fluctuated below 6,000 mt from 1989 to
1996, and then steadily increased to 43,000 mt in 2000. Biomass has increased
and the age structure has also improved but remains truncated and dominated
by younger ages. There is an 80% probability that SSB in 2000 was between 37,000
mt and 50,500 mt (Figure
C6). Trends in mean biomass (ages 1+) are similar
to those for SSB. There is an 80% probability that mean biomass in 2000 was
between 48,000 mt and 66,500 mt (Figure
C7).
Special Comments:
- Inadequate sampling of U.S. landings, the lack of sufficient discard samples
in the U.S. fishery, and the absence of age determinations from the Canadian
fishery contribute to uncertainty in estimates of size and age composition
of the catch. and raise concerns about the reliability of VPA results. Retrospective
analysis indicates a pattern of inconsistencies in which estimates of F in
the last year of the VPA are less than revised estimates of F and estimates
of SSB in the last year are more than revised estimates of SSB.
- While the historical population reconstruction from the VPA and the surplus
production model show concurrence, projections from the two models diverge
significantly. The projection results from the surplus production model imply
high equilibrium recruitment levels that are not consistent with historical
estimates. Accordingly, only the VPA projections should be considered.
Source of Information: Stone, H.H., C.M. Legault, S.X. Cadrin,
S. Gavaris, J.D. Neilson,, and P. Perley. 2001. Stock assessment of Georges
Bank (5Zjmnh) yellowtail flounder for 2001. CSAS Res. Doc. 2001/068. 87 p.
