Acoustic Vessel-of-Opportunity (AVO) index for midwater Bering Sea walleye pollock, 2018-2019

An acoustic vessel of opportunity (AVO) index for age 1+ midwater walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea has been estimated since 2006 using backscatter information collected during the annual Alaska Fisheries Science Center bottom trawl (BT) survey. AVO index estimates for summer 2018 and 2019 are reported here. The 2018 AVO index decreased 8% and 13.5% from the 2017 and 2016 index values, respectively. The 2019 AVO index increased slightly (1%) from 2018. Both estimates (2018, 2019) were similar to a number of previous years in the time series (2007, 2010, 2012-2013, 2016-2017) based on overlapping 95% confidence intervals. Most pollock backscatter appeared to be distributed broadly across the shelf between 50 and 200 m isobaths in 2018 and 2019. The percentage of pollock backscatter east of the Pribilof Islands (east of 170○ W longitude) in the AVO index was 15% in 2018 and 24% in 2019. This is much greater than the percentage in summers 2010-2012 (range 4-9%), 2018 was less than that observed in 2016 (22%) and 2017 (19%), but 2019 was similar tothat observed in 2013-2015 (ca. 25%). The relationship between the AVO index and AT survey time series was slightly reduced with the addition of 2018 results (R2 = 0.74, p = 0.003). The presence of fish backscatter in the upper 30 m of the water column in some areas throughout the AVO regions during both 2018 and 2019 was assumed to be young-of-the-year (age-0) pollock, and it was therefore excluded from the 2018 and 2019 AVO indices. The uncertainty associated with the correct classification of this shallow backscatter was minimal in 2018 (3% of the 2018 index) but more significant in 2019 (13% of the 2019 index). 2020 NMFS (National Marine Fisheries Service) AFSC (Alaska Fisheries Science Center) Submitted https://doi.org/10.25923/5pf5-5707 Public Domain 1930