Density dependent catchability in bottom trawl surveys
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Fish form schools, layer or patches in which the individual fish's behaviour is not independent of its neighbours movements. On the other hand, at low densities fish may have the freedom to act as single individuals independently of what other fish are doing. Potentially, if these contrasts occur in nature, they may give rise to behavioural differences of fish in front of the trawl at high and low densities with successive effects on catchability and bottom trawl indices of stock abundance. We explore in this paper the hypothesis that the density of fish has a significant effect on catchability of the survey trawl. Data from Norwegian and Canadian video observation recorded during trawling are studied, and related to bag trawl experiments which measured escapement underneath the survey trawls in both regions.