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Status of Stocks & Multispecies Assessment Program

Scientific Acoustic Data from Commercial Fishing Vessels: Eastern Bering Sea Walleye Pollock (Theragra chalcogramma)

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Apr-May-June 2012
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figure 1, walleye pollock drawing

The International Council on the Exploration of the Sea (ICES) Working Group on Fisheries Acoustics, Science and Technology has provided guidance on using commercial fishing vessels for collecting opportunistic acoustic data (OAD). However, an explicit approach for working with these non-traditional datasets has not been addressed.

A scientific study was designed and conducted to test the feasibility of collecting and utilizing acoustic data from commercial fishing vessels fishing walleye pollock (Theragra chalcogramma) in the eastern Bering Sea as part of a PhD dissertation project. This study demonstrated methods for processing and analyzing acoustic data collected from commercial fishing vessels to investigate current issues in fisheries management. Although the opportunistic acoustic data in this study were uncalibrated and, therefore, could not be used for biomass estimates, it was found to be suited to investigating fisheries issues where an index of abundance proportional to biomass could be substituted.

This study demonstrates the scientific application of opportunistically collected acoustic data from commercial fishing vessels operating in the winter eastern Bering Sea (EBS) walleye pollock fishery (Fig. 1; pdf, 1.2MB).

Due to their high resolution and wide spatial and temporal range, opportunistic acoustic data provided an excellent data source for investigating population spatial and temporal dynamics during the winter. These data were used to identify spatio-temporal dynamics of pollock aggregations over scales ranging from hundreds of meters to hundreds of kilometers and from minutes to months.

Spatial analyses identified three levels of pollock aggregation (Fig. 2; pdf, 1.2MB), and time series analysis identified diel changes in pollock distribution in both the vertical and horizontal planes and an overall decline in pollock density over the study period.

Questions on the intensity of the EBS pollock fishery arising from the decline of Steller sea lion (Eumetopias jubatus) populations have been a focus of many studies, but a lack of informative data on winter pollock distributions has hindered these efforts. This data provided an abundance index that was used in a spatially-explicit depletion model to examine possible differences in fishery exploitation rates inside and outside of Steller sea lion critical habitat areas. Findings suggested that exploitation rates were relatively low (~6%), but that fishing inside SSL critical habitat was more than twice that observed outside critical habitat (Fig. 3, Fig. 4, and Table 1; pdf, 1.2MB).

The lack of comprehensive survey data on pollock distribution in the EBS during the spawning season is problematic for predicting possible effects of climate change in the EBS. The opportunistic acoustic data were used as an index of abundance to develop and evaluate a generalized additive model for projecting winter EBS pollock distributions using available data and oceanographic conditions.

Sea surface temperature, bottom depth, distance to the ice edge, and cumulative catch were all found to be significant predictors of pollock density in the winter (Fig. 5, Fig. 6, and Fig. 7; pdf, 1.2MB). Functional relationships between temperature, bottom depth, and pollock density were also found to overlap between summer and winter.

These studies show the utility of opportunistic acoustic data for gaining insight into the spatial and temporal dynamics of winter pollock aggregations which have previously not been examined by fisheries scientists due to the lack of scientific quality data at the appropriate resolutions and ranges.

By Steve Barbeaux
 

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