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The behaviour of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) towards baited hooks was observed in the sea with underwater television. Both species exhibited a diel rhythm of feeding activity most pronounced in cod, with an increase at dawn and a decrease at dusk. The activity rhythm of cod changed from bimodal with peaks in the morning and afternoon in September, to unimodal in December. Haddock showed a less clear activity rhythm during daytime. Activity of fish decreased at high current velocities. Both cod and haddock swam upstream towards baited hooks. More haddock responded to the bait than cod, but haddock showed a less intense response characterized by biting repeatedly at parts of the bait, whereas cod completely ingested the bait and thereby increased the probability of getting hooked. The observed difference in hooking behaviour between cod and haddock explains differences between the species found in longline fishing trials with different bait sizes and hook designs.

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