By Anna Rindorf
The whiting is a voracious fish eater and readily consumes fish prey even of a third its own length. It is abundant in the southern North Sea where it influences the stock size of commercial fish species by eating large quantities of juveniles. However, exactly how many fish is consumed by whiting is difficult to estimate. Previously, large-scale investigations of whiting stomach contents have been performed on five occasions and the results indicate that whiting do not simply consume each prey according to its relative abundance.
Rather, it seems to be conservative in its diet selection. When a prey declines in relative abundance in the environment its relative abundance in the whiting stomach does not decline correspondingly. A selection pattern of this type may severely destabilise the prey populations, as this corresponds to higher predation mortality at lower prey population size.
The purpose of the present study is to perform a more detailed investigation of the prey selection of North Sea whiting. The data consists of more than 8000 whiting stomachs caught around the clock at five locations in the North Sea. The analyses so far performed indicated that whiting became increasingly unwilling to ingest a new meal as the amount of food already present in the stomach increased. The level at which this saturation occurred was much lower in the wild than in aquarium experiments. There was further a substantial diel variation in food intake. Benthic prey such as crabs, burying invertebrates and flatfish were eaten mainly at night whereas highly mobile prey such as fish were eaten during the day. Future work includes the establishment of a link between food intake and food abundance at the five locations and estimation of the effect of saturation and diel variation on the total food intake of whiting.