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Groundfish Assessment

Bottom Trawl Survey of the Eastern Bering Sea Completed

The 2006 annual crab and groundfish bottom trawl survey of the eastern Bering Sea (EBS) continental shelf was completed on 28 July. Two chartered commercial fishing vessels, operated 60 days each, were used to deploy National Marine Fisheries Service (NMFS) research nets at 405 stations over a survey area that represents approximately 500,000 square kilometers.

Derived estimates of fish biomass, population size, and size and age composition are key inputs to stock assessment analyses used by the North Pacific Fishery Management Council (NPFMC) in the annual setting of allowable biological catch (ABC) and total allowable catch (TAC) for commercial groundfish in the EBS such as walleye pollock, Pacific cod, and various flatfish species. Current biomass and population estimates from the 2006 EBS crab and groundfish bottom trawl survey were presented at the Crab Plan Team Meeting, 13-15 September, and at the Groundfish Plan Team Meeting, 19-22 September.

Groundfish Assessment Program Summer Interns

Sam Galle, NOAA Hollings Scholar, recently finished a summer internship with Lyle Britt investigating how flatfish locate, track, and capture prey. By analyzing high-speed video of feeding events and calculating the visual acuity, ocular movement ranges, and retinal mosaic patterns within the eyes of English sole (Parophrys vetulus) and starry flounder (Platichthys stellatus), Sam found that the dorsal and ventral eyes of these species had very different morphological arrangements and were used for different roles in prey capture. This discovery may be the first instance in which a vertebrate has been found to possess asymmetrical morphological constructs within the visual system. After additional analysis is completed, Sam hopes to publish his findings.

Cara Murphy, Western Washington University student, participated in an AFSC paid internship during summer 2006. Cara worked with RACE groundfish scientist Stan Kotwicki on the design and development of a new Oracle database for storage of light intensity data collected during RACE groundfish surveys. Her work was very successful and the database is in final stages of development.

Cara also created 3-dimensional (3D) maps of light intensity data from EBS trawl surveys using ArcScene – ArcGIS software for presenting 3D data. She will continue to cooperate with the AFSC during her senior year in college. She will use light intensity data to describe the relationship between satellite data (chlorophyll a, primary productivity, diffuse attenuation coefficient) and “in situ” observations of light levels in the Bering Sea. We wish her best of luck in her future endeavors.

By Robert Lauth

Correlating Trawl Catch and Acoustic Data in the Eastern Bering Sea

The commercial fishing vessels chartered for the annual EBS bottom trawl survey in recent years have installed Simrad ES-60 echosounders, which are capable of collecting scientific-quality acoustic backscatter data. Because the operation of these echosounders during the survey provides a large amount of inexpensive and continuous backscatter data between trawl stations, it is of interest to investigate whether these acoustic data could be used to improve the precision of the walleye pollock (Theragra chalcogramma) trawl index of abundance estimates through incorporation of acoustic data in the estimation methodology.

Catch and acoustic backscatter data collected from over 400 stations executed during the 2005 field season were analyzed to estimate the correlation between trawl-based and acoustic-based estimates of fish abundance in various layers above the seafloor. Initial comparisons of the trawl- and acoustic-derived values revealed good correlations, with the best correlation evident for the walleye pollock layer observed between the bottom and the headrope (R2 = 0.61).

The pattern of correlation for those layers above the headrope was characterized by a monotonic decline with increasing height, indicative of a lack of vertical herding response among pollock. By comparison, there was no observed correlation (R2 = 0.02) between trawl and acoustic data for Pacific cod (Gadus macrocephalus), the only other significant source of fish acoustic backscatter. Further work will focus on the potential reasons for the discrepancy in the strength of the trawl-acoustic relationship between pollock and cod as well as the factors that may adversely affect the correlation between trawl and acoustic data in general.

By Paul von Szalay


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