2003 Stock Assessment Summary (cont.)
GOA Pacific Cod
The 2002 Pacific cod assessment incorporated several new types of data
including size composition from 2002 and 2003 commercial fisheries, size
composition from the 2003 GOA bottom trawl survey, the 2003 survey biomass
estimate, and catch from the 2003 fisheries. The 2003 GOA bottom trawl
survey biomass estimate was 297,361 t, up 6% from 2001.
No changes were made to the structure of the assessment model. For the
past several years, the BSAI and GOA Pacific cod assessments have advocated
a harvest strategy that formally addresses uncertainty surrounding natural
mortality and catchability. The Bayesian meta-analysis, which has formed
the basis for risk-adverse ABC recommendations in the 1996-99 assessments,
was not performed for the 2003 assessment. Similar to procedures used
in 2001, the ratio between the recommended FABC and F40%
estimate given in the 1999 assessment (0.87) was assumed to be an appropriate factor by
which to multiply the 2003 maximum permissible FABC to obtain a recommended
The estimated 2003 spawning biomass for the GOA stock was 103,000 t, up
about 17% from last years estimate for 2003. The recommended 2004 ABC
for the GOA stock was 62,800 t, up about 19% from last years recommendation
for 2003. The OFL for the GOA Pacific cod stock in 2004 was 102,000 t,
up about 46% from last years estimate for 2003.
The flatfish group is subdivided into deep-water flatfish, rex sole, shallow-water
flatfish, and flathead sole. An age structured assessment was developed
for flathead sole and results of this modeling exercise were treated separately
from the GOA flatfish chapter. In addition, a preliminary age structured
assessment for Dover sole was developed and presented as an appendix to the SAFE.
The projected 2004 exploitable biomass for each category is based on a
delay difference model that includes estimates of growth, natural mortality,
and recruitment, as well as biomass estimates from the 1996, 2001 and 2003
bottom trawl surveys. ABC and OFL were calculated by species, with individual
species identified as Tier 4, 5, or 6 depending upon the available data.
Greenland turbot and deep-sea sole ABCs and OFLs were based on a Tier
6 calculation. The ABCs for northern and southern rock soles were based
on Tier 4 and the ABCS for other flatfish were based on a Tier 5 calculation.
The 2003 GOA bottom trawl survey biomass was used as the current biomass
for calculation of ABC for stocks in Tiers 4 and 5 as shown below.
GOA Arrowtooth Flounder
The 2003 stock assessment for arrowtooth flounder was updated with new
catch, size composition, and survey biomass information. The 2003 NMFS
bottom-trawl survey biomass estimate for arrowtooth flounder was 2,822,830
t, up 74% from 2001.
An age-structured model developed with AD Model Builder software was utilized
for this assessment. Similar to the previous assessment, the model accommodated
a higher proportion of females in the larger size intervals of both survey
and fishery data by giving males a higher natural mortality rate than females.
Reliable estimates of biomass, B40%, F40%,
and F35% exist, placing arrowtooth
flounder in Tier 3. Spawning biomass in 2004 was 1,306,640 t. This estimate
was greater than B40% (620,336 t) so harvest recommendations were based
on sub-Tier 3a. Under sub-Tier 3a, the 2004 ABC of 194,934 t was derived
by applying a F40% (0.142) fishing mortality rate.
GOA Atka Mackerel
In 2003, the GOA Atka mackerel assessment was updated with catch data,
and length and age data from the 2003 bottom trawl survey. There are no
reliable estimates of current biomass from the Gulf of Alaska bottom trawl
survey. An increase in adult Atka mackerel in the Western and Central
regions of the GOA was noted in 2003 fishery data. Two strong back-to-back
year classes (1998 and 1999) have shown up prominently in the Aleutian
Islands, and the Gulf of Alaska Atka mackerel may be 4-year olds from the
1999 year class as indicated by age data from the 2003 Gulf of Alaska survey.
The ABC and OFL are based on a Tier 6 estimate. The maximum permissible
values for ABC and OFL are 4,700 t and 6,200 t, respectively. The recommended
ABC for 2004 was set at 600 t, a level sufficient to allow for bycatch.
In 2003, the thornyhead assessment was updated with 2002 harvest levels
by gear, biomass estimates from the 2003 bottom trawl survey, and relative
population numbers from the 2003 sablefish longline survey. The 2003 survey
biomass estimate of 102,000 t was the highest ever estimated since regular
surveys began in 1984. This compares with the average over the period
1984-2003 of 53,000 t. While there are differences in depth strata covered
in different years (and the assessment model attempts to account for these),
the overall pattern suggests that the stock is healthy. Consequently,
model results suggest a moderate increase in abundance over that estimated
last year. The model-based ABC, OFL, and spawning stock biomass levels
were 2,945 t, 3,510 t and 26,202 t, respectively. However, the author
suggested an alternative method for calculating ABC and OFL based on Tier
5 criteria which acknowledged the data-poor status of this stock at this
time; the Plan Team concurred and adopted and ABC of 1,940 t and an OFL
of 2,586 t.
GOA Skates (Rajidae)
In 2003, a directed fishery for the two most abundant skate species developed
in the Gulf of Alaska. Skates are currently managed as part of the other
species category. However, in 2004 skates will be managed separately
to better account for this development. A new assessment was provided
to review the available biological and fishery information on skates in
the GOA. Given available information, the authors recommended establishing
area-specific ABCs and OFLs for big skate and longnose skate, and an areawide
ABC and OFL for all other Bathyraja species.
GOA Forage Fish
A forage fish assessment was developed as an appendix to the GOA SAFE.
This assessment provides a compilation of available information on forage
species in the GOA. An evaluation of current exploitation rates on forage
species suggests that bycatch of forage fish in commercial fisheries is
well below the overfishing level.
By Anne Hollowed.
Atka Mackerel Tag Recovery Cruise Near Amchitka Island and Seguam Pass
The first objective of our tag release-recovery study is to determine the
efficacy of trawl exclusion zones as a management tool to maintain prey
abundance/availability for Steller sea lions at local scales. Trawl exclusion
zones were established around sea lion rookeries as a precautionary measure
to protect critical sea lion habitat, including local populations of prey
such as Atka mackerel. Localized fishing may affect Atka mackerel abundance
and distribution near sea lion rookeries. Tagging experiments are being
used to estimate abundance and movement between areas open and closed to
the Atka mackerel fishery.
During the years 1999-2002, NMFS released roughly 32,000 tags in Seguam
Pass and 14,520 in Tanaga Pass areas. During 2003 roughly 14,750 tags
were released near Amchitka Island. This recovery charter recovered tagged
fish in the Seguam Pass and Amchitka Island areas.
The cruise lasted for 27 days, started in Dutch Harbor on 5 October and
ended in Adak on 31 October. A total of 54 tows were conducted near Amchitka
Island, and 41 tows were conducted in Seguam Pass. A total of 794 metric
tons (t) of Atka mackerel were caught near Amchitka Island, and 983 t were
caught in Seguam Pass. During the cruise, 87 tagged Atka mackerel were
recovered near Amchitka Island, and 17 were recovered in Seguam Pass.
Tag recovery rates were estimated by seeding the catch with tagged fish.
Recovery rates were high, ranging from 90%-100% depending on area and
stratum. In addition to tag recoveries, catch composition was determined,
and sexed length frequencies and biological samples were collected for
every haul. Biological samples consisted of gonads, stomachs, and age
structures from randomly selected fish. During the course of the cruise,
775 biological samples and 12,485 sexed length data were collected.
By Susanne McDermott.
quarterly Oct-Dec 2003 sidebar
Auke Bay Lab