Acoustic vessel-of-opportunity (AVO) index for midwater Bering Sea Walleye pollock, 2012-2013
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Acoustic vessel-of-opportunity (AVO) index for midwater Bering Sea Walleye pollock, 2012-2013
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    "Surveys of the semi-demersal gadoid walleye pollock (Gadus chalcogrammus) are conducted in summer on the eastern Bering Sea shelf by the Alaska Fisheries Science Center (AFSC). The demersal portion of the stock is surveyed annually as part of a multispecies bottom trawl (BT) survey of groundfish and crab, while the midwater portion of the stock is assessed biennially with an acoustic-trawl (AT) survey. The acoustic vessel-of-opportunity (AVO) index of midwater walleye pollock biomass uses acoustic data collected by BT survey vessels to provide information on the biomass of midwater walleye pollock during years in which AT survey data are not available. The AVO index time series began in 2006, and this information has been used in the Bering Sea walleye pollock stock assessment since 2010 as an additional fishery-independent index of walleye pollock abundance. Index estimates for summers 2012 and 2013 are reported here. Differences were not detectable between the 2012 and 2013 estimates or among them and most other estimates in the time series based on overlapping confidence bounds. Exceptions were that 2012 and 2013 were different from the relatively low 2008 and 2009 values, and 2013 was different from 2006 and 2011. Most walleye pollock were distributed north and west of the Pribilofs in both years, although in 2013 the center of gravity of the population shifted to the south and east. The 2012 AVO index compared well with the 2012 AT survey walleye pollock biomass estimate to confirm that the index remains a good proxy for midwater walleye pollock abundance and distribution. Several sources of uncertainty affecting the AVO index and its relationship to the AT survey biomass estimate are discussed, including presence of larger than usual amounts of non-pollock backscatter in the index area, potential inclusion of small amounts of suspected age-0 walleye pollock, and different potential ways of scaling the index."
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