2003 Gulf of Alaska Biennial Groundfish Assessment Survey
The third in the series of biennial bottom trawl surveys of Gulf of Alaska
(GOA) groundfish resources was conducted from 20 May through 9 August 2003.
Prior to establishing a biennial schedule in 1999, the RACE Division had
surveyed groundfish resources in the GOA triennially in 1984, 1987, 1990,
1993, and 1996. The GOA triennial surveys covered the continental shelf
(out to 500 m depth) but only included portions of the continental slope
in 1984 (to 825 m) and 1987 (to 750 m). The biennial surveys since 1999
were designed to cover the continental shelf and slope between the Islands
of Four Mountains (long. 170 °W) and Dixon Entrance (U.S.-Canada border
in Southeast Alaska) out to the 1,000-m depth contour. While the 1999
survey succeeded in sampling the entire area, the 2001 survey area was
reduced due to the Divisions increased survey responsibility in other
areas under limited funding. The 2001 survey area did not include the
area east of long. 147°W, nor did it extend deeper than 500 m. This year
the survey covered the entire geographic extent, but the outermost depth
stratum (700-1,000 m) was omitted because the vessels were unable to fish
Sampling was conducted aboard three chartered commercial trawlers, Sea
Storm, Gladiator, and Northwest Explorer. The 75-day survey period was
divided into four legs of 18-19 days each. Sampling began near the Islands
of Four Mountains and progressed eastward on the continental shelf and
slope to the U.S.-Canada border in Southeast Alaska. Stations were allocated
among 54 depth and geographic strata and were preselected randomly from
a grid of potential sites overlaying the survey area. If rugged bottom
or heavy commercial fishing prevented sampling a station, a nearby alternate
station was selected. Of the 880 attempted standard survey tows, 809 were
successfully completed, ranging in depth from 13 m to 667 m.
The primary focus of the biennial groundfish surveys is to build a standardized
time series of data designed to assess, describe, and monitor the distribution,
abundance, and biological condition of various GOA groundfish stocks.
Specific objectives of the 2003 survey were to:
Define the distribution and relative abundance of the principal groundfish
and invertebrate species inhabiting the continental shelf and slope of the GOA
Collect data to define various biological characteristics of major groundfish species,
such as age, sex, size, growth rates, length-weight relationships, and feeding habits
Collect integrated fishing performance, net configuration, and position data for all trawl hauls
with which to derive precise effort estimates
Collect environmental data such as surface-to-bottom water column temperatures
Collect biological specimens and data requested by scientists from the AFSC or other
cooperating research groups.
Results from the survey are preliminary and will be finalized following
further examination of the effort data for each tow.
When looking at changes since the last survey in 2001, we can only compare
abundance estimates in the central and western subareas (west of long.
147 °W) shallower than 500 m. In that area, the most abundant species in
2003 were, in order, arrowtooth flounder, Pacific halibut, walleye pollock,
Pacific ocean perch, Pacific cod, and flathead sole. Since 2001, the estimated
abundances of all of these species except Pacific ocean perch increased:
arrowtooth flounder by 87% to 2,540,000 metric tons (t), halibut by 50%
to 518,000 t, pollock by 84% to 387,000 t, cod by 10% to 283,000 t, flathead
sole by 56% to 239,000 t. The abundance estimate of Pacific ocean perch
declined by 47% to 356,000 t.
We can compare abundance estimates from the entire survey area between
2003 and 1999 except for species that occur commonly in the deepest stratum
(700-1,000 m), such as grenadier. Over the entire GOA survey area, arrowtooth
flounder was by far the most abundant species with a total biomass estimate
of more than 2.8 million t, a 124% increase over the 1999 estimate. Nearly
78% of its biomass was from the central GOA survey subarea. The second
most abundant species was Pacific halibut with a biomass estimate of 634,000
t, an 8% increase since the 1999 survey, with 66% of its biomass coming
from the central subarea. Pacific ocean perch ranked third in abundance
at 457,000 t, down 37% from 1999 with 62% of its biomass occurring in the
central GOA. Walleye pollock ranked fourth in abundance at 425,000 t,
a 33% decrease since 1999, with 50% and 41% of its biomass coming from
the western and central GOA, respectively.
By Mark Wilkins
One of the larger survey catches (primarily walleye pollock) aboard the Arcturus during the 2003
eastern Bering Sea groundfish survey. Photo by Erika Acuna.
2003 Eastern Bering Sea Shelf Crab-Groundfish Bottom Trawl Survey
The Bering Sea Resource Assessment group completed their annual Bering
Sea shelf crab-groundfish bottom trawl survey from 2 June to 22 July 2003.
This years survey was performed aboard the chartered fishing vessels
Arcturus and Aldebaran. This marks the 11th consecutive survey using these vessels.
In general, temperatures were warmer than the long-term mean beginning
in 1982. Mean bottom temperature was 3.81° C, compared to the long-term
value of 2.44° C. Surface values were also higher at a mean of 7.79° C and
a long-term mean of 6.60° C.
A total of 399 stations were sampled including 18 special stations from
inner Bristol Bay to continue our evaluation of yellowfin sole spawning
populations during the sampling period.
Abundance estimates for the major species showed some unexpected changes
from 2002 (Table 1 below). Walleye pollock abundance was significantly higher
in 2003; increasing to 8.5 million t. Pacific cod decreased slightly from
616,900 t in 2002 to 605,700 t this year.
|Table 1. Bering
Sea resource assessment bottom trawl survey biomass estimates.
Among the flatfish species only flathead sole decreased slightly in abundance
this year at 529,200 t, down from 574,900 t in 2002. Yellowfin sole, rock
sole, Alaska plaice, arrowtooth flounder, and Greenland turbot all showed
increases in abundance with the most significant increase being in arrowtooth
flounder at 553,900 t this year, up from 355,100 t in 2002.
It should be noted that these abundance estimates are the bottom trawl
survey biomass estimates and NOT the status of stocks final model estimates.
After the standard survey was completed on 22 July, two important experiments
were performed during 23 July - 2 August 2003. The first was a 9-day special
project conducted in the southeast Bering Sea which examined trawl footrope
selectivity for skates by utilizing an auxiliary net beneath the survey
trawl to account for skates escaping under the footrope. The results of
this study will be available next year and will estimate capture probability
as a function of skate size.
An exploratory study was conducted in late July 2003 aboard the Aldebaran
in the Pribilof Canyon area focusing on northern rockfish Sebastes polyspinis.
The goal of the 4-day study was to evaluate the feasibility of sampling
patchily distributed rockfish with hydroacoustic and bottom trawl methods
in which the two methods would be combined to increase the precision of
the biomass estimates.
By Erika Acuna.
Bottom Trawl Survey Protocol Experiments
In order to better understand the factors affecting the performance of
our standard survey trawls, the Groundfish Assessment Program conducted
three trawl performance experiments in September aboard the chartered fishing
vessel Vesteraalen, using the standard RACE Bering Sea survey 83-112 bottom
trawl. The first experiment investigated the effect of varying trawl wire
offset on the trawl bridle and footrope contact with the seafloor. The
second experiment studied the effect of three different trawling modes
(locked winches, tension-controlled autotrawl, and symmetry-controlled
autotrawl) on both trawl footrope and bridle contact with the bottom.
The third experiment examined the effect of vessel heave, pitch, and roll
on the bridle and footrope contact with the seafloor. Analyses of the
data are currently in progress.
quarterly July-Sept 2003 sidebar
Auke Bay Lab