AFSC/REFM: Acoustic trawl cooperative survey near Shumagin Islands 2007-2013

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.