The feeding ecology and behaviour of whiting (Merlangius merlangus L.)

The development of multispecies fisheries models has led to a need for improved information on the diet composition and consumption rates of fish. This study was an attempt to investigate the feeding ecology of whiting, with an emphasis on how the fishes' behaviour might influence its diet. Analysing the stomach contents of North Sea whiting, found that as whiting increased in size they switched from a crustacean, to a fish dominated diet, and the average size of prey eaten increased. Smaller trawl caught whiting were found to have a higher instance of regurgitation. The whiting appeared to be exploiting one prey species in an area. The majority of fish stomachs sampled at a station contained the same prey type. Studies showed that sandeels were evacuated from the stomachs of whiting at a higher rate than prawns. When fed in combination the evacuation rate of sandeels increased, and the rate of prawns decreased. It was postulated that these evacuation rate changes could be because of stomach packing and/or the prawns' abrasive exoskeleton. Video analysis of whiting feeding on shrimp studied the effects of changing the predator to prey size ratio, stomach fullness and experience. As the predator to prey size ratio increased the handling time decreased, and probability of successfully eating a shrimp increased. As the fish gut filled up the probability of a strike being successful decreased and handling time increased. With increasing experience the hunting sequence of the fish became simplified and the probability of a strike being successful increased. Results from the video analysis were used to develop a stochastic dynamic program of whiting feeding. The program was used to model the optimal prey choices of whiting feeding on two sizes of shrimp. In the model the probability of success was found to be the most important factor in determining the optimal shrimp size. The time spent in handling the shrimp was also found to be important. It was concluded that a more behavioural approach could help in understanding why fish eat what they do and aid investigations into the problems of fish species interactions.