Summary

Description

In May of 2006, scientists from the Alaska Fisheries Science Center participated in a pilot outreach program in Sand Point, Alaska. At the meetings, fishermen raised a number of concerns about the adequacy of walleye pollock (Theragra chalcogramma) assessment. The issues appeared to be related to the contrasting observational scales of fishermen and National Marine Fisheries Service (NMFS) scientists responsible for conducting assessment surveys and population modeling. NMFS resource assessment surveys have a large spatial scale (i.e., the entire Gulf of Alaska) and occur infrequently as they are biennial summer bottom trawl and acoustic-trawl surveys, with annual or biennial winter acoustic-trawl surveys targeting key spawning areas(von Szalay, et al. 2010; Guttormsen and Yasenak, 2007; Guttormsen and Jones, 2010). The stock assessment model for pollock estimates stock abundance for the central and western Gulf of Alaska (Dorn et al., 2011). In contrast, the observational scale of fishermen is spatially restricted but temporally extensive, since they are on the water fishing throughout the year. Fishermen questioned whether NMFS assessment activities were appropriately designed to monitor the status of pollock in their area. They noted that additional surveys in different seasons and a more comprehensive acoustic-trawl survey effort in winter could help address seasonal issues such as movement between spawning and feeding areas. Cooperative research projects with the Sand Point fishing community were identified as a potential approach for addressing some of these issues, but fishermen wanted the assurance that their investment of time and effort will lead to improvements in stock assessment. The acoustic-trawl surveys described in this report resulted from collaboration between NMFS scientists, the Aleutian East Borough, and local fishermen to address issues raised during the outreach program. Acoustic-trawl surveys were conducted from a local fishing vessel in the western Gulf of Alaska during 2007-2013. To some extent, the surveys carried out during this period reflected shifting program objectives. The initial objective was simply to demonstrate the feasibility of using local fishing vessels to conduct acoustic-trawl surveys and to collect acoustic and biological information sufficient to estimate walleye pollock biomass and spatial pattern. Once this was demonstrated, the focus of the project expanded to include other objectives. Specific objectives of the project were the following: Evaluate the feasibility of conducting acoustic-trawl surveys of pollock using local fishing vessels. Evaluate the quality of acoustic data collected from the Simrad ES60 echosounder. Evaluate the appropriateness of the design of the current NMFS acoustic-trawl surveys in the western Gulf of Alaska. Specific issues to be addressed included 1) transect density in bathymetrically complex regions in the western Gulf of Alaska, 2) day/night differences in pollock density, and 3) temporal variability of pollock density during replicate transects. Evaluate the timing of the NMFS survey (mid-February) in Sanak Trough by conducting a survey in January prior to the NMFS survey. Evaluate relationships between adult pollock density, young-of-the-year pollock density, euphausiid density, and the distribution of foraging humpback whales (Megaptera novaeangliae). The estimates of abundance and distribution of walleye pollock were developed for areas covered by cooperative acoustic-trawl surveys conducted aboard a fishing vessel during alternate Septembers and Januarys between 2007-2013. Physical oceanographic and biological composition observations, initial findings from ancillary data collections of marine mammal observations, and dual-frequency differencing techniques to discriminate different types of acoustic backscatter were also developed.