Resource Assessment & Conservation Engineering (RACE) Division
Midwater Assessment & Conservation Engineering Program
Winter Surveys in the Gulf of Alaska
Figure 3. Click image to enlarge.
Figure 4. Click image to enlarge.
Figure 5. Click image to enlarge.
Scientists from the Midwater Assessment and Conservation Engineering (MACE) Program conducted echo integration-trawl (EIT) surveys in the Shumagin Islands and Sanak Trough during 5-16 February aboard the NOAA ship Miller Freeman (Fig. 3), and in the Shelikof Strait area and along the Gulf of Alaska (GOA) shelf-break area between Chirikof and Middleton Islands during 13-31 March aboard the NOAA ship Oscar Dyson (Figs. 4 and 5). This was the first comprehensive EIT survey of the shelf-break area between Barnabas Trough and Middleton Island. The surveys provided data on the abundance, distribution, and biological composition of prespawning walleye pollock (Theragra chalcogramma). All surveys were conducted 24 hours per day.
Survey results indicated that walleye pollock abundance increased in 2008 compared to 2007 in the Shumagin Islands and Shelikof Strait areas and decreased in Sanak Trough and along the southern portion of the GOA shelfbreak in the vicinity of Chirikof Island. The densest walleye pollock aggregations were located in the southern part of the Shumagin Trough, off Renshaw Point, in the northern part of Sanak Trough, and in the southern Shelikof Strait area (Figs. 3 and 4). Few pollock were located along the shelfbreak between Barnabas Trough and Middleton Island. Dense aggregations of Pacific ocean perch (Sebastes alutus), however, were detected near Middleton Island and in the mouth of Amatuli Trench.
In the Shumagin Islands area, 10 cm fork length (FL) mode (age-1), and 31 cm FL mode (age-3) fish were most abundant. In the Shelikof Strait area, the size composition was broadly distributed, with modes at 12 cm FL (age-1), 24 cm FL (age-2), 31 cm FL (age-3), and most adult lengths between 50 and 65 cm FL. Walleye pollock size compositions for Sanak Trough and the shelf-break region near Chirikof GOA surveys were unimodal, with most fish between 50 and 65 cm FL.
Preliminary analysis of maturity stages indicated that survey timing was appropriate for the Shumagin, Shelikof Strait, and Chirikof shelf-break but an earlier survey would be better for Sanak Trough efforts.
Miller Freeman and Oscar Dyson Vessel Comparison
MACE scientists continued work on vessel-comparison studies to determine whether walleye pollock differentially avoid the NOAA ships Miller Freeman and Oscar Dyson. This latest vessel-comparison work was conducted during the Shumagin Island and Sanak Trough surveys. The vessel-comparison work is needed because the Oscar Dyson was designed to meet the International Council for the Exploration of the Sea (ICES) specification for underwater radiated noise to minimize vessel avoidance during fish abundance surveys, whereas the Miller Freeman is a conventionally built vessel which exceeds this specification. Thus, fish could potentially respond differently to the vessels due to the different auditory stimuli produced by each vessel. If this were the case, differential vessel avoidance reactions could influence biomass estimates derived from standard survey methods with the two ships. This is important because the Oscar Dyson will be routinely used for walleye pollock acoustic-trawl surveys, which have historically been conducted with the Miller Freeman.
Both vessels continuously collected acoustic backscatter at 18, 38, 120 and 200 kHz while traveling in close proximity to one another. The two-part experimental design consisted of a side-by-side vessel configuration where the ships traveled beside one another at a distance of 0.5 nmi along survey tracklines, and a follow-the-leader vessel configuration where one vessel followed the other at a distance of 1 nmi. The side-by-side configuration allowed for standard survey operations without compromising the data for stock assessment purposes. Acoustic data from both vessels were collected over a wide range of densities of adult walleye pollock and conditions typical of acoustic surveys in these areas. Analyses of these data are under way. Additional vessel-comparison work with these two ships is planned during the summer 2008 eastern Bering shelf EIT survey.
Research projects addressing selectivity of the midwater trawl used during the MACE EIT surveys were conducted during the Shelikof Strait survey. Eight trawl hauls were conducted with the trawl outfitted with small recapture bags or pocket nets attached to the outside of the trawl to sample escaping fish. The pocket net catches will be compared with the size and species composition of the catch retained in the codend. In addition, fish behavior in the trawl was observed using an underwater stereo-camera. The stereo-camera images are used to determine fish length. These observations will aid in the understanding of size-dependent behavior of pollock in the trawl.
By Mike Guttormsen and Chris Wilson