Measuring Total Catch at Sea: Use of a Motion-Compensated Flow Scale to Evaluate Observer Volumetric Methods

Abstract

We evaluated procedures used by North Pacific groundfish observers to estimate total catch aboard a factory trawler in the Bering Sea during 1997. A motion-compensated flow scale was tested for precision and bias by using known weights of fish. The flow scale performed within an error limit of 3% in daily tests, although a positive bias of +1% was detected. We used flow scale weights to evaluate volumetric methods of haul weight estimation: codend volume and bin volume measurements by observers and bin volume measurements by electronic bin sensors. Codend volume estimates were most variable; slight volume overestimation was detected for codends larger than the trawl alley, and significant between-observer differences were detected. Bin volume estimates were more precise than codend volume estimates and showed no differences between observers. Bin sensor volume estimates agreed closely with observer visual estimates when fish level was at least 1 m below the transducers but gave inaccurate readings when bins were full or nearly full. All volumetric methods depend on the correct volume-to-weight conversion (density) factor. Several statistical approaches to estimating density indicated that the mean density of catch in the fishery for walleye pollock Theragra chalcogramma is 1.02 metric tons (mt)/m³ in codends, while the mean density in bins is 0.98 mt/m³. Evaluation of density estimates made with sampling baskets, the current procedure used in many North Pacific groundfish fisheries, revealed several problems, especially a tendency to underestimate density. Density estimates obtained by using a specially designed density sampler were consistent between observers and similar to the density of catch in bins. We recommend further research to obtain prescribed densities for volumetric haul weight estimation in other groundfish fisheries.

Notes

¹ Reference to trade names does not imply endorsement by the National Marine Fisheries Service, National Oceanic and Atmospheric Administration.