Acoustic vessel-of-opportunity (AVO) index for midwater Bering Sea Walleye pollock, 2014-2015
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Acoustic vessel-of-opportunity (AVO) index for midwater Bering Sea Walleye pollock, 2014-2015
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    An acoustic vessel of opportunity (AVO) index for midwater walleye pollock (Gadus chalcogrammus) has been estimated since 2006 using backscatter information collected during the annual bottom trawl (BT) survey. AVO index estimates for summer 2014 and 2015 are reported here. The 2014 AVO index increased slightly (29%) from the 2013 index value, and 36% from 2012. The 2015 AVO index increased slightly (6%) from 2014. Both estimates (2014, 2015) exceeded all earlier time series estimates (2016-2013) based on non-overlapping 95% confidence intervals. Most pollock backscatter appeared to be distributed broadly across the shelf between 50 and 200 m isobaths in 2014 and 2015. The percentage of pollock backscatter east of the Pribilof Islands (east of 170°W longitude) in the AVO index was 24% in 2014 and 25% in 2015. This was similar to the percentage in 2013 (6%), but much greater than reported for summers 2010-2013 (range 4-9%). This implies that there has been more midwater pollock biomass east of the Pribilof Islands in recent years. Comparison of the AVO index and AT survey time series continues to show a strong correlation (r²=0.90, p=0.0011). Midwater hauls were conducted to sample midwater pollock aggregations during the 2014 (n=31) and 2015 (n=32) BT surveys to investigate the feasibility of using these hauls to convert the AVO backscatter index to abundance at length or age. Some portions of the AVO index area were not sampled by these hauls in both year. Preliminary analyses of these haul data (ability to target and catch pollock, catch composition, and length-frequency comparisons) showed 1) hauls targeted appropriate fish layers and were dominated by pollock, 2) bottom trawls and midwater trawls caught pollock of different length compositions and 3) length modes in midwater hauls from BT and AT surveys were similar, but occurred in different proportions even when restricted to the same subarea. Due to a number of factors including logistical and staffing concerns, full evaluation of how well BT survey haul data could be used to convert AVO backscatter to number of fish at length or age was deferred to a later time. Some guidance on what would be required for this analysis is outlined in the Appendix.
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