Length dependant corrections of survey estimates of cod and haddock in the Barents Sea
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This paper describes an attempt to utilize new information on gear selectivity and fish target strength to improve time series of survey estimates. Swept area estimates and acoustic estimates of cod and haddock in the Barents Sea have been obtained from combined bottom trawl acoustic surveys each winter since 1981. A part of the swept area time series have earlier been corrected for a change from bobbins to rockhopper ground gear (Godø and Sunnanå 1992). Here corrections for the remaining time series are presented. In addition a new time series is presented taking account of the fish size dependant trawl efficiency reported by Dickson (1993 a and b). The general effect of both corrections is an increase in the estimates of the youngest age groups relative to adult fish. As the size (and age) distribution of the acoustic estimates directly depends on the size composition in trawl hauls, similar corrections were also made for the acoustic estimates. These revisions also took account of new target strength estimates which are considered more reliable than those used earlier. The relative age distribution of the old acoustic estimate was modified by applying the swept area conversion factor by age group, while the absolute level of the estimate was determined by keeping unchanged the total acoustic value (s_A). Hence the product of average acoustic cross-section and total estimated number of all age groups of cod and haddock should be unchanged. The change in average acoustic cross-section was estimated from the change in age distribution and the change in target strength function. Ideally all corrections should have been made station by station. Some of the raw data were, however, not available in a standardized format. Mean length at age were available for nearly all years, and the corrections were therefore done on the total survey estimate by age group. For the conversion from bobbins to rockhopper the two procedures were compared, and they showed good agreement.
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