The effective swept volume of a bottom trawl

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The reaction of individual fish towards an approaching trawling vessel has previously been measured using target-tracking methods. In this paper these measured velocities are fitted to a deterministic mean behaviour model and a diffusion-advection model taking the random component into account. The output of the diffusion-advection model is used to estimate the probability of a fish to be caught by the bottom trawl. For typical depths in the Barents Sea, 200 - 350 m, and for immature Cod feeding on capelin, a mean fishing height of about 10 metres is found, and the effect of horizontal herding from the warp is shown. The effect of random fish movement is visualised by the different results from the two models.