(2001 Stock Asessment Summary: part 8)
ABC (t) for 2002
for GOA Flatfish Group
The flatfish group is subdivided
into deep-water flatfish, rex sole, shallow-water
flatfish, and flathead sole. The 2002
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 and 2001 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.
The 2001 GOA bottom trawl survey biomass was
used as the current biomass for calculation of ABC.
The overfishing levels for the flatfish groups are
determined by the fishing mortality rates determined
from the tier structure and the exploitable biomass
estimates of the individual species. The catch
levels associated with those fishing mortality rates
GOA Arrowtooth Flounder
The 2002 estimated female spawning biomass of
arrowtooth flounder was 1,098,130 t. This
estimate was based on abundance estimates derived
from an age-structured model developed with AD Model
Builder software. 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 mortality rate than females.
Exploitable biomass in 2002 is estimated to be
greater than B40%,
and ABC was determined to be 146,264 t based on Tier
3a calculations (F40%
= 0.134). Using Tier 3a criteria, the
overfishing level based on F35%
= 0.159 is estimated at 171,057 t.
GOA Atka Mackerel
Prior to 1997, exploitable biomass and ABC for
Atka mackerel were based on triennial bottom trawl
survey estimates. However, schooling behavior,
patchy distribution, and habitat preference make
this species difficult to sample with standard trawl
survey gear. Atka mackerel are also poor
targets for hydroacoustic surveys because they lack
swim bladders. Re-evaluation of historical
survey data indicated abundance estimates prior to
1997 were also compromised by high variability.
Thus, existing GOA bottom trawl survey data
has limited utility for either absolute abundance
estimates or indices for Atka mackerel.
The Plan Team continues to support a bycatch only
fishery as a conservative harvest policy for Atka
mackerel because: 1) there is no reliable
biomass estimate; 2) localized depletion may occur;
and 3) this species has previously exhibited a
particular vulnerability to fishing pressure in the
GOA. The Team recommends an ABC of 600 t in
2002 to satisfy bycatch needs in other fisheries.
In 2001, the thornyhead assessment was updated
with several new types of data including: 1999 -
2000 harvest levels by gear, biomass estimates from
the 2001 GOA bottom trawl survey, and relative
population numbers from the 2001 sablefish longline
survey. Alternate models examined assumptions
regarding natural mortality and length at age.
The recommended ABC and spawning stock biomass
levels were 2,494 t and 23,235 t, respectively.
By Anne Hollowed.
quarterly Oct-Dec 2001 sidebar
Auke Bay Lab