Spatial and Temporal Trends in the Abundance and Distribution of Forage Fish in Pelagic Waters of the Eastern Bering Sea During Late Summer, 2002–2016
Pelagic fish and jellyfish were sampled using a trawl net towed in the upper 20 m of the eastern Bering Sea during the Alaska Fisheries Science Centers’ Bering Arctic Subarctic Integrated Surveys (BASIS) during late summer, 2002–2016. Stations were approximately 30 nautical miles apart and a trawl was towed for approximately 30 minutes. Area swept was estimated from horizontal net opening and distance towed.
Fish catch was estimated in kilograms. Surveys were not conducted in the south (<60°N) during 2013 and 2015 and north (>60°N) during 2008 but fish densities in these areas were estimated using geostatistical modeling methods (Thorson et al. 2015). Four forage fish commonly captured in the trawl: capelin (Mallotus villosus), herring (Clupea pallasii), sand lance (Ammodytes hexapterus), and sandfish (Trichodon trichodon).
Abundance and distribution (center of gravity and area occupied) were estimated for using the VAST package for multispecies version 1.1.0 (Thorson 2015; Thorson et al. 2016a, b, c) in RStudio version 1.0.136 and R software version 3.3.0 (R Core Team 2016). The abundance index is a standardized geostatistical index developed by Thorson et al. (2015, 2016) to estimate indices of abundance for stock assessments. We specified a gamma distribution and estimated spatial and spatio-temporal variation for both encounter probability and positive catch rate components at a spatial resolution of 100 knots. Parameter estimates were within the upper and lower bounds and final gradients were less than 0.0005.