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.

cooperative research, outreach

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multi year calibrations indicate a stable acoustic system for 38kHz, 120kHz

exploratory survey, some areas with multi year coverage.

relational db, acoustic calibration, noise removal, bottom editing

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Archival site needed for fish abundance acoustic data.

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