Northeast Fisheries Science Center Reference Document 07-13
Monkfish Assessment Summary for 2007
by Northeast Data Poor Stocks Working Group
SAW
Chair/Contact: Dr. James Weinberg
SAW Homepage
National Marine Fisheries Serv., Woods
Hole Lab., 166 Water St., Woods Hole MA 02543
Print
publication date August 2007;
web version posted August 15, 2007
Citation: Northeast Data Poor Stocks Working Group. 2007. Monkfish assessment summary for 2007. US Dep Commer, Northeast Fish Sci Cent Ref Doc. 07-13;
12 p.
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State of Stock
Based on existing biomass
reference points in the Monkfish Fishery Management Plan, the resource would be
considered overfished in both the northern and southern stock management areas
(Figure 1). In the northern area, the most recent biomass index, based on
the 2004-2006 NEFSC fall survey 3-yr average, is 1.1 kg per tow. This is lower than the current Bthreshold value for the northern management area (1.30 kg/tow), and also lower than Btarget (2.60 kg/tow). In the southern
area, the most recent biomass index, based on the 2004-2006 NEFSC fall survey
3-yr average, is 0.87 kg per tow. This is
lower than the Bthreshold (0.92 kg/tow) and Btarget (1.84
kg/tow) for the southern area.
New reference points
were developed as part of the 2007 assessment, based on a revised yield-per-recruit
analysis (using a revised value of M) and results of a length-tuned model that
incorporates multiple survey indices and catch data. Based on these new reference
points, monkfish in both management regions are not overfished and overfishing
is not occurring (Figure
2). New estimates
of Bthreshold are 65,200 mt of total biomass in the north and 96,400
mt of total biomass in the south. Estimates of Btarget are 92,200 mt in the north and 122,500
mt in the south. Estimates of total biomass for 2006 are 118,700 mt in the
north and 135,500 mt in the south, both of which are greater than their respective
biomass targets. The existing overfishing threshold is based on Fmax,
and this was retained, although new values were estimated. The new, updated estimates of Fmax are 0.31 per year in the north and 0.40 per year in the south. Estimates of current F (2006) are 0.09 per
year in the north and 0.12 per year in the south, both of which are lower than
their respective overfishing thresholds.
The development of a new
analytic model ("SCALE") for monkfish is a significant advance. However, the new assessment results are accompanied
by substantial uncertainty, and therefore need to be viewed with caution. Reservations stem from: (a) input
uncertainties (under-reported landings and unknown discards during the 1980s
and incomplete understanding of key biological parameters such as age and
growth, longevity, natural mortality and stock structure); (b) the shorter assessment
time frame (1980-2006) than in previous assessments (1963-2006); and (c) the
relatively recent development of the assessment model. Compared to the previous monkfish assessment
approach, the new model integrates more types of information and incorporates
temporal variation in fishery selectivity patterns. It was not possible to utilize all sources of
information with the previous approach. (See "Special Comments" section below.)
As indicated by NEFSC
survey recruit abundance indices for approximate ages 1 and 2 (inferred from
lengths, Figure
3), the frequency of better than average recruitment events increased
since the late 1980s in the northern area. Relatively strong year classes were produced in 1993, 1999 and 2001. In
the south, recruitment has varied without trend during 1963-2006; however, a
relatively strong 2001 year class is apparent in the south (Figure
3).
The median size of
monkfish in both regions declined as landings increased in the 1980s (Figure
4). Maximum sizes have also declined, from about
110 cm during the 1960s to 90 cm since the early 1990s in the north, and from about
100 cm in the 1960s to 75 cm since the 1990s in the south.
Projections
The SCALE (Statistical
Catch-at-Length) assessment model was used to evaluate the impacts of TACs proposed
in Framework 4 (5,000 mt in the north and 5,100 mt in the south), assuming long-term
average recruitment. The results
indicate that total biomass in both regions would continue to increase through 2009
and remain above Btarget (Figure
5). These results did not incorporate any uncertainty
associated with the stock size estimates for 2006. Further work is necessary to develop a
complete forecasting approach.
Catches
Reported total landings
(live weight) increased from an annual average of 2,500 mt in the 1970s to
8,700 mt in the 1980s, 23,000 mt in the 1990s, and 22,000 mt during
2000-2005. Total landings in 2006
declined to 14,500 mt, the lowest level since 1990, due to management
regulations (Figure
6). Landings in the early part of the time series are
thought to be under-reported. The accuracy of landings data has likely improved
with mandatory reporting, which began in 1994. In the
northern area, landings peaked in 2003 (15,000 mt), and have since declined to 6,700
mt in 2006. In the southern area, landings
peaked in 1998 (19,300 mt), and declined to 7,800 mt in 2006.
During 1990-1999, 53% of
USA monkfish landings were taken in otter trawls, 28% in sea scallop dredges, and
18% in gillnets. During 2000-2006, 53%
of USA monkfish landings were taken in otter trawls, 7% in sea scallop dredges,
35% in gillnets, and 6% other gear. While
trawl gear accounts for most of the landings in the northern area (75% during
2000-2006, Figure
7), gillnets now account for the majority of the landings in
the southern area (54% during 2000-2006, Figure
7).
Estimated total discards
of monkfish have ranged between 1,600 mt (1992) and 7,500 mt (2001) per year,
with a long-term discard/kept ratio of 0.15 (1989-2006, north and south
combined). Discard rates have been
highest in the sea scallop dredge fisheries in the southern area, particularly
since 2000, and lowest in the gillnet fisheries. Discard ratios and discard levels (mt)
increased in the southern area after 2000 (overall discard/kept ratio for
2001-2006 =0.34).
Click here for Table 1. Catch and status table (weights in '000 mt): monkfish.
Stock
Distribution and Identification
The monkfish resource in
US waters is distributed from the Gulf of Maine through Cape Hatteras, NC. Current management practice divides US waters
into two regions north and south of Georges Bank to accommodate differences in
fishery practices; however, there is no strong biological evidence (growth,
maturity, and genetic information) of separate stocks.
Data and
Assessment
Monkfish were last
assessed at SAW-40 in November 2004. Data used in the current assessment include NEFSC research survey data,
data from cooperative monkfish surveys conducted in 2001 and 2004, and
commercial fishery data from (a) vessel trip reports, (b) dealer landings
records, and (c) on-board fishery observers. The assessment assumed a natural mortality rate (M) = 0.3;
previous assessments used M=0.2. Fishing mortality rates were estimated from survey
catch-per-tow-at-age from NEFSC research surveys, and using several
length-based approaches (catch-survey analysis, statistical catch-at-length analysis
(SCALE), length-based mortality, stage-based mortality). Although these methods were useful for exploratory
data analysis, the only method deemed adequate for assessment was the SCALE
model. The model could only be applied to the period from 1980 to the present,
because the early (pre-1980) commercial catch data were too uncertain.
Biological
Reference Points
Existing biological
reference points (BRPs) for monkfish are from Framework 2 of the Fishery
Management Plan for Monkfish (2003). For
both management areas, the existing Btarget was established as the
median of the 3-year moving average of NEFSC fall survey biomass indices during
1965-1981. Fthreshold was set equal to Fmax (F=0.2 per
year). The Framework 2 overfishing definition
did not include an Ftarget reference point.
New biomass reference
points were developed as part of the new assessment, based on an updated age-based
yield-per-recruit analysis, and results of the SCALE model, both of which
assumed M=0.3 (previous assessments used M=0.2). The new Btarget is
the average of total biomass during the 1980 – 2006 period, estimated as 92,200
mt in the north and 122,500 mt in the south. The new Bthreshold is defined
as the lowest value of total biomass in the assessment time series (1980 - 2006)
from which the stock subsequently increased (termed "BLoss"), estimated
as 65,200 mt in the north and 96,400 mt in the south.
The existing overfishing
threshold is based on Fmax, and this was retained in the new
assessment, although the value was updated. The revised estimates of Fmax are 0.31 per year in the north
and 0.40 per year in the south. The
recommended Ftarget is F at 40% of maximum spawning potential (F40%),
estimated to be 0.18 per year in the north and 0.31 per year in the south. F40% was chosen to ensure some
adequacy in spawning potential and because it has been used in managing other
fisheries. The differences between areas in the F40% estimates are due
to different selectivity patterns of the predominant gears in the two regions (ottertrawls
in the north, large mesh gillnets in the south).
Monkfish is a data-poor
species, and there are significant uncertainties associated with the assessment
results. This should be considered when
developing management measures.
Fishing
Mortality
Previous assessment reviews
(SAWs -31, -34 and -40) concluded that instantaneous fishing mortality rates
(F) estimated from NEFSC research survey length frequency distributions were
not sufficiently reliable to allow evaluation of current F with respect to
reference points.
In the current assessment,
fishing mortality in 2006, estimated using the SCALE assessment model (assuming
M=0.3 per year), was F=0.09 per year in the north, and F=0.12 per year in the
south. Fishing mortality has declined in
both regions since 2003 (Figure
2).
Recruitment
Size-based indices of
abundance indicate strong recruitment in the northern area in 1993, 1999 and
2001 (Figure
3). The strong recruitment in 1999 and 2001 led to rebuilding of
stock biomass in the north. Recruitment
has been stable in the south, with a strong year class produced in 2001 (Figure
3).
Stock Biomass
Total biomass in the
northern region declined steadily from the early 1980s through the early 1990s,
remained at a relatively low level during the 1990s, and then increased after
1999, reflecting strong recruitment and management efforts from 2000 onwards
(Figure
2). Biomass in the north was estimated to be 118,700 mt in 2006. In the south, total biomass increased until
the late 1980s and then declined during the 1990s. Since 2000, biomass has increased in the
south, and was estimated to be 135,500 mt in 2006 (Figure
2).
Median body size of
monkfish, in fall NEFSC bottom trawl surveys of the northern area, declined
rapidly during the 1980s, but since 1990, has stabilized at a relatively small
body size (20-40 cm recently, compared to 60-80 cm before 1982) (Figure
4). Maximum size has also declined, from approximately
100-120 cm to 80-100 cm. In the southern
area, median size has been more variable, but shows a gradual decline over time
(Figure
4), and maximum size has declined from around 100 cm before 1982 to
60-80 cm since 1990.
Special Comments
This assessment is
uncertain for a number of reasons, including poor quality of some data and
uncertainties in life history parameters. The assessment hinges critically on assumptions regarding growth,
longevity, and natural mortality of monkfish, all of which are poorly known. In
addition, commercial catches prior to 1993 are not well characterized. Model results are sensitive to the assumed
value of natural mortality, revised in this assessment from 0.2 to 0.3 per year. This decision was based on the observed
longevity of male and female fish in the resource; however, the actual lifespan
of monkfish may be greater than that which has been thus far observed. Uncertainties in key life history parameters
and historical catches are unlikely to be resolved in the short term.
In developing management alternatives, it should
be recognized that monkfish is a "data-poor" species and this assessment has significant
uncertainty. Landings on the order of
5,000 mt in each management area (roughly the proposed TACs
in FMP Framework Adjustment 4) are unlikely to result in a change in stock
status, and should allow monkfish resources in both regions to increase.
The SCALE model used for
assessment could only be applied to the period from 1980 to the present. Monkfish
biomass indices in NEFSC surveys were approximately twice as high prior to 1980
than after this time. As such, the
productivity of the resource may be higher than reflected in this assessment
and thus, the possibility of attaining higher biomass levels in the future
should not be discounted. Reconsideration
of the newly proposed biomass reference points might thus be justified in the
future.
Sources of
Information
Chikarmane HM., Kuzirian
AM, Kozlowksi R, Kuzirian M, Lee M, Lee
T. 2000. Population genetic structure of the goosefish, Lophius americanus. Biol Bull. 199:227-228.
Northeast Fisheries
Science Center (NEFSC). 1997. Report of
the 23rd Northeast Regional Stock Assessment Workshop (23rd SAW). NEFSC Ref Doc. 97-05; 191 p.
NEFSC. 2000. Report of the 31st Northeast Regional Stock Assessment Workshop
(31st SAW). NEFSC Ref Doc.
00-15; 400 p.
NEFSC. 2002. Report of the 34th Northeast Regional Stock Assessment
Workshop (34th SAW): SARC Consensus Summary of Assessments. NEFSC Ref Doc. 02-06; 346 p.
NEFSC. 2005. 40th Northeast Regional Stock
Assessment Workshop (40th SAW) Assessment Report. NEFSC Ref Doc. 05-04; 146 p.
