Studies of bait preference in the cod, Gadus morhua L.: characterisation of feeding stimulants using an operant conditioning technique

The ability of cod (Gadus morhua L.) to discriminate between a vertical and a horizontal pattern of lights, when these were associated with unflavoured food or synthetic squid mixture (SSM)-flavoured food, was established by means of an operant conditioning technique. A demand feeder with two submerged feeding points was used in experiments conducted in aquarium tanks. In a second series of experiments cod demonstrated a consistent preference for one of two differently flavoured foods when presented at feeding points which both displayed the same light pattern. The results are in general agreement with those obtained in earlier feeding trials based on appetite where only a single food type was presented at a time. The only difference observed was with food flavoured with SSM minus betaine, trimethylamine oxide and trimethylamine. In the previous feeding trials, this omission slightly reduced palatability, whereas in the present preference trials, palatability increased. The method developed in the present experiments, which requires less time than the previous feeding trials, is particularly suitable for choice experiments aimed at characterising feeding stimulants. A considerable number of foods can be tested quickly, and the final results, indicating the preferred food, are not influenced by changes in appetite with time.