Diel feeding patterns and the development of food webs in pelagic 0-group cod (Gadus morhuaL.), haddock (Melanogrammus aeglefinusL.), whiting (MerlangiusmerlangusL.), saithe (Pollachius virensL.), and Norway pout (Trisopterus esmarkiiNilsson) in the northern North Sea

Abstract

Stomach content analysis was used to investigate feeding interactions between pelagic 0-group gadoids in the northern North Sea. The species studied were cod (Gadus morhua L.), haddock (Melanogrammus aeglefinus L.), whiting (Merlangius merlangus L.), saithe (Pollachius virens L.) and Norway pout (Trisopterus esmarkii Nilsson). The fish were caught at a site south-east of Shetland in late spring during 24 h depth-stratified fishing using a mid-water trawl. There was overlap in the range of prey consumed by the various species. Fish of 1–3 cm in length fed mainly on copepods and other invertebrates. Larger Norway pout continued to feed almost entirely on copepods, but the other species diversified as they grew to include fish in their diet, cod and whiting becoming almost entirely piscivorous. Haddock and saithe ate moderate amounts of fish combined with crustacea in roughly equal proportions. The depth at which the fish were caught did not appear to influence the range of prey consumed, but fish was slightly more prevalent in the diet of 0-groups caught near the surface. The results indicate that a food web of increasing complexity can develop during a relatively short time scale in late spring when the 0-group gadoids are pelagic. Generalised linear modelling revealed diel feeding patterns which were size-dependent and species-specific. In 2–3 cm cod, for example, peak feeding was during the period 1200–2000 h and high numbers of prey (mainly invertebrates) were consumed. The stomachs of the larger cod (3–5 cm) contained smaller numbers of mainly fish prey and peak feeding switched to 0000–0800 h. The piscivorous 0-groups were capable of eating relatively large prey items, providing the opportunity for larger individuals to eat younger or slower-growing individuals from the same year class. Whiting in particular were subject to substantial levels of cannibalism and inter-specific predation. Feeding interactions of this sort might be of sufficient magnitude to influence significantly the survival and recruitment of gadoids. High fecundity in whiting, coupled with multiple spawnings over a protracted period of time means that offspring from late spawnings could act as a food reserve for siblings from earlier spawnings. Such prey provide substantially more energy than the female invested in producing the original egg from which the prey was derived. This mechanism could prove to be an energy-efficient way of providing food reserves for the early offspring. It is also an efficient way of governing population size through density-dependent mortality.