Southeast Alaska Sea Lion Prey Study
The third cruise in Frederick Sound for the Southeast Alaska Steller sea
lion prey study was conducted 27 November - 10 December 2001 using the
chartered fishing vessel Solstice. The purpose of the study is to test
the hypothesis that juvenile Steller sea lion prey diversity and seasonality
are related to Steller sea lion population trends. It is a comparison
study to a similar one being conducted around Kodiak Island by the University
of Alaska. During the Frederick Sound winter cruise, prey abundance was
measured using echo-integration and midwater trawl. Sea lion scat was
collected to infer diet. Fish also were collected for proximate and free
fatty acid analysis.
During previous cruises in May and September, herring formed scattered
schools near the surface. In this cruise, herring were found concentrated
at the bottom in water depths of 80-100 m, forming large, distinctly bordered,
cohesive schools. Some of the concentrations were found at known herring
overwintering sites, such as Port Houghton (Will Bergman, Alaska Department
of Fish and Game (ADF&G) personal communication). Scientists participating
in the cruise were Mike Sigler, Johanna Vollenweider, and Dave Csepp of
ABL and Lara Dzinich of the University of Alaska. Quarterly sampling is
planned for the Frederick Sound study area with the next cruise scheduled
for March 2001.
By Michael Sigler.
Stock Assessment of Gulf of Alaska Slope Rockfish and Pelagic Shelf Rockfish
Updated stock assessments of slope rockfish and pelagic shelf rockfish
in the Gulf of Alaska were completed in November 2001 by ABL staff. The
assessments for Pacific ocean perch (POP) and northern rockfish and members
of the slope rockfish assemblage used age-structured models. The estimated
exploitable biomass was 293,240 metric tons (t) for POP and 94,350 t for
northern rockfish. The POP stock is increasing. The northern rockfish
stock is decreasing because of recent weak recruitment.
The assessments of other species of slope rockfish and pelagic shelf rockfish
in the Gulf of Alaska rely exclusively on biomass estimates provided by
trawl surveys. The updated assessments indicate the following stock levels
and stock trends: shortraker and rougheye rockfish, exploitable biomass
66,830 t, trend unknown; other slope rockfish, exploitable biomass 107,960
t, trend unknown; and pelagic shelf rockfish, exploitable biomass 62,490
t, trend unknown.
The recommended Acceptable Biological Catches (ABC) for 2002 were the following:
13,190 t for POP; 1,610 t for shortraker and rougheye rockfish; 4,980 t
for northern rockfish; 5,040 for other slope rockfish; and 5,490 t for
pelagic shelf rockfish. Compared with 2001, the 2002 ABC for POP decreased
approximately 300 t, slightly increased for northern and other slope rockfish,
slightly decreased for shortraker and rougheye rockfish, and decreased
approximately 500 t for pelagic shelf rockfish. These ABC values were
all accepted by the North Pacific Fishery Management Council (NPFMC) at
its December 2001 meeting. For further information on Alaska groundfish
stock assessments for 2002 see the
REFM Division report in this issue.
By Jon Heifetz.
Alaska Sablefish Assessment
The final sablefish assessment for the combined Gulf of Alaska and Bering
Sea/Aleutian Islands region stock of sablefish was prepared during the
quarter and presented to the NPFMCs Groundfish Plan Teams and Scientific
and Statistical Committee. The assessment shows that sablefish abundance
increased during the mid-1960s due to strong year classes from the late
1950s and 1960s. Abundance subsequently dropped during the 1970s due to
heavy fishing; catches peaked at 56,988 t in 1972. The population recovered
due to exceptional year classes from the late 1970s; spawning abundance
peaked again in 1987. The population then decreased as these exceptional
year classes died off.
The survey abundance index for sablefish (based on the annual NMFS longline
survey) increased 16% in numbers and 13% in weight from 2000 to 2001.
These increases follow decreases from 1999 to 2000 in the survey abundance
index of 10% in numbers and 8% in weight, so that relative abundance in
2001 is slightly higher than in 1999. Although the fishery abundance index
decreased 5% in weight between 1999 and 2000, changes in fishery abundance
data for 2001 are not available because the fishery was still open at the
time the assessment was prepared. Exploitable and spawning biomass are
projected to increase 4% and 2%, respectively, from 2001 to 2002. Alaska
sablefish abundance now appears to be low and slowly increasing. The slow
increase confirms the projection from last years assessment that abundance
will increase slowly due to the above-average 1995 and 1997 year classes;
the size of the increase depends on the actual strength of the above-average
1997 year class and another year class that likely is above average, 1998.
Spawning biomass is projected to increase to 35% of unfished spawning
biomass (B35%) in 2002, having been as low
as 33% of unfished spawning biomass during 1998 to 2000.
A decision analysis was completed to determine what catch levels will likely
result in stable or increasing spawning biomass. The decision analysis
indicates that a yield of 17,300 t will likely maintain spawning biomass.
The maximum permissible yield from an adjusted F40%
strategy is much higher, 21,300 t. In contrast to a yield of 17,300 t,
the F40% yield has a high
probability (>0.99, decision analysis) of decreasing 2006 abundance below
2002 abundance and a substantial probability (0.18) of decreasing 2006
abundance below 90% of 2002 abundance. Based on these results, a 2002
ABC of 17,300 t was recommended for the combined stock, similar to the
2001 ABC of 16,900 mt (a 2% increase). The 2002 ABC was accepted by the
NPFMC at its December 2001 meeting.
By Michael Sigler.
quarterly Oct-Dec 2001 sidebar
Auke Bay Lab