AFSC/RACE/GAP/McConnaughey: Pribilof Hydro-2009-From Archiver-Catchability Experiment, Bering Sea 38 KHz Acoustic Sediment Classification

In stock assessment models, catchability is the link between an index of relative abundance from a fishery-independent survey and the modeled population size. For bottom trawl surveys that estimate the population size using swept-area methods, catchability can be estimated because it is largely determined by sampling efficiency (i.e., the proportion of animals within the sampled area that is caught) which can be experimentally measured. However, estimating survey catchability is complicated because trawl efficiency has been shown to vary over a survey area in response to variation in bottom sediment type. Catchability experiments have been conducted on the bottom trawl used for the annual Eastern Bering Sea (EBS) survey, resulting in a survey-wide estimate of catchability for snow crab (Chionoecetes opilio) which, when included in the stock assessment model, produced significant changes in the Allowable Catch Limit. This catchability model accounted for spatial variation in trawl efficiency as a function of crab size, sex, depth, and sediment type. Unfortunately, sediment data over the geographic distribution of snow crab are quite fragmentary due to the remoteness of the area, and direct estimates of sediment properties such as grain size are generally unavailable at the trawl-sampling locations. In some cases, estimates were based on sediments collected over 60 miles away. The option to collect sediment data at all 270 trawl-sampling stations included in the snow crab distribution is prohibitively expensive considering the additional ship time required and the sample processing costs.

This project examined whether indices of bottom type, derived from calibrated ES-60 acoustic data collected at each snow crab sampling station and processed with IMPACT software produced by the Quester Tangent Corporation, are more informative in the snow crab bottom trawl catchability model than measured values of sediment type that were broadly extrapolated. This determination was based solely on the amount of spatial variation in the snow crab efficiency model that is explained by the two kinds of sediment information. While the currently used data are based on a directly measurable attribute of the sediment (mean grain diameter), the acoustically derived index is related to this attribute but also to a variety of previously unmeasured variables affecting the time-dependent shape of the bottom echo.

The purpose of this record is to document the main entities and attributes of the ES-60 acoustic data after processing with IMPACT software produced by the Quester Tangent Corporation.

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Microsoft Windows 7 Version 6.1 (Build 7601) Service Pack 1; Esri ArcGIS 10.2.2.3552;

The commercial software we used to process the ES60 data is IMPACT12.3.2. The processing algorithms used in the software are based on the following reference: Biffard, B. R. 2011. Seabed remote sensing by single-beam echosounder: models, methods and applications. Ph.D. dissertation. School of Earth and Ocean Sciences, University of Victoria. 356 p.

Only 38 kHz data were processed from the R/V Arcturus and R/V Aldeberan 2009 Bering Sea Surveys.

Data were collected over short period of time - one summer, using a calibrated instrument with consistent settings.

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