Feeding preferences of Celtic Sea fishes were investigated using a database of stomach content records, collected between 1977 and 1994. The diet of cod Gadus morhua, hake Merluccius merluccius, megrim Lepidorhombus whiffiagonis, whiting Merlangius merlangus and saithe Pollachius virens changed markedly as the animals grew larger, and although large predators generally chose larger bodied prey, the variability of prey sizes consumed also increased. Large predators continued to select small, low value, benthic prey (e.g. Callionymus spp. and Trisopterus spp.) which were easier to catch, rather than larger, more energy lucrative pelagic prey (e.g. mackerel Scomber scombrus), even though these pelagic prey-fishes were nearly always available and were often very abundant. Stock estimates of the International Council for the Exploration of the Sea and U.K. groundfish survey catches were used as indices of prey abundance. Blue-whiting Micromesistius poutassou and other small pelagic fishes (Argentina spp. and clupeoids) were identified as being particularly important, and were consumed by some predators more often than would be expected given the abundance of these prey in the environment. There was no evidence for density-dependent feeding by predators on mackerel and only hake exhibited densitydependent feeding on horse-mackerel. Hake, cod and megrim consumed more blue-whiting when this prey was at higher abundance in the environment. In choosing what prey to consume, predators must balance costs and benefits, considering the quality of prey and the energy expended during search, capture and handling.

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The spatial and temporal structure of predator-prey relationships in the Celtic Sea was investigated for 4 commercially important predator species (cod Gadus morhua, hake Merluccius merluccius, megrim Lepidorhombus whiffiagonis, and whiting Merlangius merlangus) using stomachcontent and bottom-trawl survey data for the period 1982 to 1995. Blue whiting Micromesistius poutassou were consumed more often during the summer months, whereas mackerel Scomber scombrus and Trisopterus spp. (poor cod T. minutus, Norway pout T. esmarkii, and bib T. luscus) were found more often in predator stomachs during the winter half-year. On a spatial scale, blue whiting was consumed over the shelf edge, in accordance with their higher densities in the environment, while mackerel, horse mackerel Trachurus trachurus and Trisopterus spp. were eaten more often on the continental shelf, again in agreement with their depth-related density-distribution patterns. The spatial distribution of whiting closely matched that of a key prey, Trisopterus spp. This might indicate an 'aggregative response' by the predator. The results of this study suggest spatial and seasonal prey-switching behaviour by cod, hake and whiting. Overall, the Celtic Sea fish community is characterised by opportunistic predators and general independence between predator and prey distributions. Interspecific predator interactions are reduced by size-, space-and time-dependent feeding behaviours.

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INTRODUCTIONTwo groups of mackerel (Scomber scombrus L.) are recognised by the International Council for the Exploration of the Sea (I.C.E.S.) for stock assessment purposes in the north-east Atlantic. The North Sea ‘stock’, which overwinters along the edge of the Norwegian Trench and spawns off the south coast of Norway, in the Skagerrak, Kattegat and the central North Sea (Hamre, 1980), and the Western ‘stock’, which overwinters and spawns along the edge of the continental shelf from the west of Ireland to the Bay of Biscay (Lockwood, Nichols & Dawson, 1981).