DOI: 10.5063/aa/mbdecker.3.12
title: Bering Sea Bottom Trawl Survey 1979-2009

description: The Bering Sea Bottom Trawl Survey covers a standard study area, surveyed each year since 1979. The survey encompasses a major portion of the eastern Bering Sea shelf between the 20-m and the 200-m isobaths and from the Alaska Peninsula north to approximately the latitude of St. Matthew Island (60 degrees 50 minutes N). These jellyfish data came from quantitative bottom trawl surveys of the EBS shelf conducted by the Resource Assessment and Conservation Engineering (RACE) division of the Alaska Fisheries Science Center (AFSC), NOAA (data owner: Robert Lauth). Since 1979, jellyfish have been collected annually during the period May 22 - Aug 19. The survey of 356-376 stations arranged in a grid-like pattern takes approximately 2 months for 2 fishing vessels to complete. A ~17-m wide, ~2.6 m high trawl is towed on the bottom for 30 min. Because jellyfish are mostly distributed throughout the water column, their capture in the bottom trawl gear is likely occurring during the net retrieval operation. We assume that these records, though not representative of the actual jellyfish abundance, can accurately represent the temporal and spatial variation throughout the study region. Although taxonomic data were not collected on the jellyfish caught, species composition data from recent years suggest that most of the biomass (&gt;85%) consisted of one species, Chrysaora melanaster (Brodeur et al. 2002; Acuna, unpublished data). Catches less than about 1150 kg were sorted and weighed completely. Larger catches were subsampled before processing. Biomass density for each taxon was expressed as kg per ha of area swept by the trawl (distance fished x net width). Catches of all large jellyfish (medusae) are weighed and standardized to catch per unit effort (CPUE in kg per ha, where 1 ha = 10,000 square meters) (see Brodeur et al., 1999, for details). Station-by-station CPUE data can be provided upon request. The annual data provided for the JEDI database are in the form of a biomass index (units: 1000s of metric tons). This biomass index was calculated for the survey area, adjusting for the area within each of six geographic areas (i.e., strata). These jellyfish biomass estimates were made by expanding the mean density within each of six strata to the total area of the strata and summing over all areas. Even though medusae are most likely to be captured during deployment or retrieval (as expressed above), the use of density is believed to be appropriate for this biomass index. Most references to density of pelagic zooplankton are expressed in weight per 1000 cubic meters of water filtered. For purposes of comparison an average tow in this survey filters approximately 124,000 cubic meters. However, the actual water filtered during deployment and retrieval is unknown and therefore the index concept is more appropriate for purposes of annual comparisons. Temperature profiles were taken at each station using an expendable bathy-thermograph (XBT) or a Sea-Bird micro-bathythermograph (MBT) attached to the headrope of the net. Depth to bottom was obtained by adding net height to headrope depth.