The effect of pot design on behaviour and catch efficiency of gadoids
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The behavioural response of fish to pots is poorly understood but is a vital component of the fish capture process. Here, the behaviour of gadoids prior to and after capture in different baited fish pot designs was observed using in situ video footage. Bottom set and floated/lifted versions, respectively, of a collapsible and of a rigid pot were compared. A low entrance and high escape rate limited cod (Gadus morhua) catches, whilst a low encounter rate limited saithe (Pollachius virens) catches. Both species approached pots by swimming upstream. Cod tended to encounter and inspect pots more than saithe, which showed more cautious responses, characterised by a reluctance to inspect the pots at close range. Cod were thus more likely to enter the pots, and these differences in behaviour explain the observed differences in capture efficiency between the species. Once inside the pot, cod showed slow swimming (milling) and tended to search the pot walls in attempts to escape. Saithe tended to hang still and were less likely to escape than cod. With regards to the effect of pot design on behaviour, cod encountered pots less when they were floated/lifted above the seabed whilst saithe encountered floated and bottom set pots at the same rate. Entrance and escape rates for both species were not affected by floating collapsible pots above the seabed. The probability of capture for a fish was dependant on species and fish size, as well as social attraction and repulsion effects from other fish already caught. The findings of this study have important implications for future pot design and optimisation and contribute towards efforts to establish a pot fishery for gadoids in Norway.