Estimating Confidence in Trawl Efficiency and Catch Quantification for the Eastern Bering Sea Shelf Survey
April 24, 2018
There are multiple sources of variability in the sampling efficiency of the trawl gear used for the survey, both species- and size-specific, that may affect inferences about the nature of the overall catches and relative animal densities. Furthermore, the quality and specificity of catch quantification (weighing and counting) at sea of many taxa can vary depending on the overall size of the catch and the species composition of the sample, particularly for non-commercial taxa of small size. This document is an assessment of the relative reliability of survey catch weights, numbers, and catch per unit effort estimates derived from the eastern Bering Sea shelf bottom trawl survey and is intended to serve as a guideline for researchers interested in using the survey data and results. Included within is a confidence matrix of sampling efficiency and catch quantification for all fish and invertebrates identified during the shelf survey.
The eastern Bering Sea (EBS) continental shelf supports an abundance of fish and invertebrate taxa. Many of these maintain the high biomass levels required to support large commercial fisheries. The Alaska Fisheries Science Center’s (AFSC) Resource Assessment and Conservation Engineering (RACE) Division has been monitoring these resources with annual summer bottom trawl surveys employing a standardized 83-112 Eastern bottom trawl and standardized sampling procedures (Stauffer 2004, Lauth and Conner 2014). The stratified random design of the EBS shelf survey consists of a grid with stations placed at the center of each 20 × 20 nmi square (Fig. 1). Each year, beginning in 1982, the same 356 stations have been sampled. An additional 20 stations were added to the northwest sector in 1987, making a survey-wide total of 376 stations. Information gathered from these surveys provides fisheries independent population trends that are invaluable for stock assessments and the development of management strategies for commercially exploited fish and invertebrate species in the region. In addition to their utility as a management tool, data and results from these surveys have been used by a variety of constituents to examine spatial and temporal trends in the fish and invertebrate fauna of the EBS (e.g., Brodeur et al. 2008, Mueter and Litzow 2008, Pinsky et al. 2013).