Spatial and Temporal Trends in the Abundance and Distribution of Jellyfish in the Eastern Bering Sea During Late Summer, 2002-2016
Pelagic 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, 2004-2016. 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.
Jellyfish 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 jellyfish densities in these areas were estimated using geostatistical modeling methods (Thorson et al. 2015). All jellyfish medusae caught in the surface trawl (top 18-20 m of the water column) are sorted by species and subsampled for bell diameter and wet weight. Six species are commonly caught with the surface trawl: Aequorea sp., Chrysaora melanaster, Cyanea capillata, Aurelia labiata, Phacellocephora camtschatica, and Staurophora mertensi. Biomass is calculated for each species and compared across species, and oceanographic domains on the Bering Sea shelf.
Abundance and distribution (center of gravity and area occupied) were estimated for each jellyfish species using the VAST package for multispecies version 1.1.0 (Thorson 2015; Thorson et al. 2016a, b, c) in RStudio version 0.99.896 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, 2016a, 2016b, 2016c) 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 50 knots. Parameter estimates were within the upper and lower bounds.