The measurement error of marine survey catches: the bottom trawl case
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We have formulated a model for analyzing the measurement error of marine survey abundance estimates using data from parallel surveys (trawl haul or acoustic measurement). The measurement error is defined as the component of the variability that cannot be explained by covariates such as temperature, depth, bottom type etc. The method presented is general, but we concentrate on bottom trawl catches of cod (Gadus morhua). Catches of cod from 10 parallel trawling experiments in the Barents Sea with a total of 130 paired hauls were used to estimate the measurement error of trawl hauls. Based on the experimental data, the measurement error is fairly constant in size on the logarithmic scale and is independent of location, time and fish density. Compared with the total variability of the winter- and autumn surveys in the Barents Sea, the measurement error is small (approximately 2-5%, on the log scale, in terms of variance of catch per towed distance). Thus, the cod catch rate is fairly precise measure of fish density at a given site at a given time.