Reactions of cod (Gadus morhua L.) in the pre-vessel zone to an approaching trawler under different light conditions - Preliminary results

The behaviour of cod (Gadus morhua L.) towards an approaching vessel during trawling was studied by means of a stationary positioning system. Three cod were tagged by allowing them to voluntarily swallow transmitters wrapped in bait. The behaviour was recorded before, during and after trawling, which was carried out during high and low light level. Nine hauls were carried out, but only six hauls are included in the analysis, because of technical problems during three hauls. Only in two out of six hauls all three cod were observed. Behaviour towards the approaching trawler differed from tow to tow. Only three out of thirteen cases were classified as reaction; all three occurred during daylight. In two of the three cases a change in the swimming direction away from the approaching vessel was observed, whereas a reduction in swimming speed was recorded in the third case. The reaction distance from the vessel, between 470 and 1470 m, corresponds to a sound level between 82 and 92 dB re 1 mu Pa per Hz. There were indications of a lower level of sensory awareness during reduced light level.