AUKE BAY LABORATORY (ABL) (cont.)

Alaska Sablefish Assessment

The 2003 sablefish assessment for the Gulf of Alaska and Bering Sea/Aleutian Islands stock of sablefish was presented to the North Pacific Fishery Management Council's (NPFMC) Groundfish Plan Teams and Scientific and Statistical Committee. The assessment showed 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 metric tons (t) in 1972. The population recovered due to exceptional year classes from the late 1970s, and spawning abundance peaked again in 1987. The population then decreased as these exceptional year classes died off.

The longline survey abundance index increased 5% in number and 7% in weight from 2001 to 2002. These increases follow increases from 2000 to 2001 and decreases from 1999 to 2000, so that relative abundance in 2002 is about 10% higher than in 1999. Fishery abundance data for 2002 were not analyzed because the fishery was still open as of the date of the assessment.

Exploitable biomass and spawning biomass are projected to increase 6% and 3%, respectively, from 2002 to 2003. Alaska sablefish abundance now appears to be moderate and increased from recent lows. Projected 2003 spawning biomass is 39% of unfished spawning biomass, after having been as low as 35% during the period from 1998 to 2000. The increase confirms the projection from last year's assessment that abundance would increase due to the above-average 1997 year class. The 1997 year class is an important part of the total biomass and is projected to account for 24% of 2003 spawning biomass. Another year class from 1998 is likely to be above average. Whether sablefish abundance falls after the 2003 peak depends on the actual strength of the 1998 year class.

Our previous approach for recommending acceptable biological catch (ABC) considered the abundance trend. We chose a catch level that avoided further abundance decreases because abundance was low. Abundance now has increased, so we changed our ABC recommendation method to adapt to the changed circumstance. In our new approach, we completed a decision analysis to determine what catch levels likely will avoid the historic low abundance observed in 1979. The decision analysis indicates that a yield of 18,400 t has only a 0.2 probability of reducing 2007 spawning biomass below the historic low. The maximum permissible yield from an adjusted F_40% strategy (the rate of fishing mortality that reduces the spawning potential per female to 40% of the level that would be observed in an unfished stock) is much higher (25,400 t), but with a higher probability (0.6) of decreasing 2007 spawning biomass below the historic low. We recommended a 2003 ABC of 18,400 t for the combined stock; this yield has a low probability of reducing spawning biomass below the historic low and is 6% higher than the 2002 ABC of 17,300 t, consistent with recently increased sablefish abundance. A 2003 ABC of 18,400 t was recommended by the NPFMC Groundfish Plan Teams. A higher 2003 ABC of 20,900 t was recommended by the NPFMC Scientific and Statistical Committee (SSC) because the sablefish population is thought to be in moderate condition, near the B_40% level (based on the maximum sustainable yield, usually assumed to be 40% of the unexploited stock size). The SSC believed that a smaller reduction from the maximum permissible ABC was warranted. A 2003 ABC of 20,900 t was accepted by the NPFMC at its December 2002 meeting.

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 2002. The assessments for Pacific ocean perch (Sebastes alutus) and northern rockfish (S. polyspinis), members of the slope rockfish assemblage, use age-structured models. The estimated exploitable biomass for 2003 is 298,820 t for Pacific ocean perch and 105,260 t for northern rockfish. The Pacific ocean perch stock is increasing, whereas the northern rockfish stock is decreasing because of recent weak recruitment. The assessments of the other species of slope rockfish and pelagic shelf rockfish in the Gulf of Alaska are not based on modeling, but instead rely exclusively on biomass estimates provided by trawl surveys. Because no trawl survey was conducted in 2002, estimates of exploitable biomass and recommendations of ABC for 2003 remain the same as for 2002.

Thus, the updated assessment indicates the following stock levels and stock trends: 1) shortraker rockfish (S. borealis) and rougheye rockfish (S. aleutianus), exploitable biomass 66,830 t, trend unknown; 2) other slope rockfish, exploitable biomass 107,960 t, trend unknown; and 3) pelagic shelf rockfish, exploitable biomass 62,490 t, trend unknown. The recommended ABCs for 2003 were 1) 13,660 t for Pacific ocean perch; 2) 5,530 t for northern rockfish; 3) 1,620 t for shortraker and rougheye rockfish; 4) 5,050 t for other slope rockfish; and 5) 5,490 t for pelagic shelf rockfish. Compared with 2002, the 2003 ABC increased approximately 4% for Pacific ocean perch and 11% for northern rockfish. All these ABC values were accepted by the NPFMC at its December 2002 meeting.

The assessments also included some new analyses. For slope rockfish, a preliminary evaluation of model uncertainties for Pacific ocean perch was presented. In this analysis, exploratory runs of the model using different values for some of the model parameters generally resulted in lower estimates of ABC than the 13,660 t computed in the base model and recommended for the 2003 fishery. This does not necessarily imply that the base model is overestimating ABC, but it does suggest that caution is needed until uncertainties in the model are better understood and quantified. For pelagic shelf rockfish, an age-structured model for dusky rockfish was presented for the first time. This model appeared to provide reasonable fits to the data but is considered preliminary, and more exploratory analysis will be necessary before it is used to recommend ABC.

By David Clausen.
 

Pacific Sleeper Shark Relative Abundance Trends in the Gulf of Alaska and Bering Sea

Pacific sleeper sharks (Somniosus pacificus) are deepwater sharks of the North Pacific Ocean that have been suggested as a source of Steller sea lion mortality. Some information also suggests that Pacific sleeper shark abundance is increasing. We analyzed existing Pacific sleeper shark data from fishery independent longline surveys in the Gulf of Alaska and the Bering Sea, to determine the trend in abundance and whether any change was statistically significant. Relative population numbers (RPNs) of Pacific sleeper sharks captured in the NMFS domestic sablefish longline surveys increased from a low of 79 in 1988 to a high of 1,779 in 2000. The most substantial increase in RPNs occurred between 1992 and 1993, and RPNs remained high from 1994 to 2000. For Pacific sleeper shark RPNs in the longline surveys between the years 1989 and 2000, 95% bootstrap confidence intervals did not overlap for all years. This suggests that there has been a significant increase (at the 95% confidence level) in the relative abundance of Pacific sleeper sharks captured in the Gulf of Alaska and Bering Sea for some years during this period. The increase in RPNs was driven largely by Pacific sleeper shark catches in Shelikof Trough, a deepwater Gulf of Alaska gully on the continental shelf between Kodiak Island and the Alaska Peninsula. Pacific sleeper sharks appeared to be relatively abundant in this area. Pacific sleeper sharks may be relatively abundant in other areas, such as on the continental shelf of the Gulf of Alaska and the Bering Sea at depths less than 200 m, but these areas are not routinely sampled by the sablefish longline survey.

By Dean Courtney and Michael Sigler.
 

Sablefish Product Recovery Rate

A cooperative project between the Alaska Longline Fishermen's Association and the ABL was conducted to estimate a Product Recovery Rate (PRR) for round, bled sablefish. Most sablefish longline fishermen in Alaska bleed their sablefish before delivering them to shoreside processors, where the catch is weighed. A PRR is used to convert the weight of the bled, landed fish to the original whole or round weight when the fish were caught. Thus, the PRR affects the weight subtracted from a fisherman's Individual Fishing Quota share and the estimated catch used in quota management and stock assessment. An accurate PRR is necessary for an accurate determination of round weight, but there has been little research to determine the best value of the PRR. To address this problem, an experiment was performed at the request of the NMFS Alaska Regional Office and the fishing industry. The experiment was conducted 25-26 July 2002 on the upper continental slope near Yakutat Bay in the Gulf of Alaska, during the annual sablefish longline survey aboard the chartered fishing vessel Alaskan Leader. First, selected sablefish were weighed in a closed plastic tube to still the fish. These fish were then marked with a unique tag, and the two most posterior gill rakers on the fish's right side were cut to bleed the fish. Two subsamples were collected to test the effect of industry practice on blood loss. One subsample of fish was placed in a tank filled with flowing seawater to bleed. Another subsample of fish was placed in a tub with no seawater to bleed. The ratios of the post-bleeding and live weights were computed for each sampled sablefish.

A total of 252 sablefish were bled in flowing seawater. Seventy-four sablefish were bled without flowing seawater. Our results indicate that the maximum blood loss of sablefish is different depending on bleeding method. Median blood loss was 1.6% of live body weight for fish bled in flowing seawater and 2.0% for fish bled without flowing seawater.

By Chris Lunsford.
 

Video Documents Aleutian Islands Coral and Sponge Habitat

A video documentary on Aleutian Islands coral habitat entitled "Coral Gardens of Aleutians" documents exploration of coral and sponge habitat in July 2002 in the vicinity of Adak Island using the manned submersible Delta. The video contains footage of habitat consisting of high density gardens of corals, sponges, and other sessile invertebrates. Highlighted are submersible dives that took place in the vicinity of an underwater volcano and in areas of fishing activities. Commentary is provided by ABL scientists that participated in the exploration. Copies of the video are available from Jon Heifetz at the ABL or on the ABL website

By Jon Heifetz.
 

Symposium on Effects of Fishing Activities on Benthic Habitats

In November, five ABL staff members attended the "Symposium on Effects of Fishing Activities on Benthic Habitats" in Tampa, Florida. The symposium was cosponsored by the Ecological Society of America, the American Fisheries Society, the U.S. Geological Survey, and NOAA. More than 100 posters and 40 oral presentations were submitted at the 3-day symposium, which was attended by more than 300 people from several countries. Robert Stone gave an oral presentation titled "Effects of Bottom Trawling on Soft-Sediment Epibenthic Communities in the Gulf of Alaska." The study used a submersible to make in situ observations of benthic communities and to make comparisons between areas open and closed to bottom trawling. Conclusions included that prey taxa and biogenic structures were more common in the closed area. Patrick Malecha submitted a poster presentation titled "Living Substrate in Alaska: Distribution, Abundance, and Species Associations." This retrospective study used historical trawl survey data to map the distribution of anemones, sea whips, sea pens, ascidians, bryozoans, and sponges in Alaskan waters. Both presentations, along with manuscripts, were submitted for publication in a forthcoming volume of symposium proceedings.

Phil Rigby cochaired an oral presentation session. Jon Heifetz submitted the poster "Benthic Habitat in the Gulf of Alaska: Biological Communities, Geological Habitat, and Fishing Intensity," in which the characteristics of Portlock Bank, an important fishing ground, were detailed from multi-beam mapping, backscatter interpretation, submersible ground truthing, and fishery effort data.

By Patrick Malecha.

AFSC Quarterly
Research Reports
Oct.-Dec. 2002