Auke Bay Laboratory  (cont.)

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 NPFMC’s 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 year’s 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 (B_35%) 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 F_40% strategy is much higher, 21,300 t.  In contrast to a yield of 17,300 t, the F_40% 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.

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AFSC Quarterly
Research Reports
Oct-Dec 2001