Food availability has been considered one reason for low fish production in the Irish Sea. To examine food conditions for larval fish in the Irish Sea, I tested the hypothesis that food levels are sufficiently high to prevent starvation of larval fish in two nursery areas off the coast of Isle of Man, Irish Sea, as densities of the appropriate size of copepod prey may have been underestimated and fish larvae may feed on alternative (protozoan) prey. This hypothesis was tested by combining experiments in the laboratory and in the field. In the laboratory, feeding experiments showed that fish larvae ingested ciliates at the densities that these prey occur in the field. However, this ingestion rate was not sufficient to maintain larval survival, suggesting a supplementary role of naked protozoan in larval fish diet. Moreover, in the field, analyses of larval fish diet indicated that, even though protozoa may contribute, copepods were the dominant food. From gut contents analyses, copepods 0.1-0.24 mm wide represented the fraction of copepod standing stock accessible to larval fish. Protozoan prey only had a marked contribution to larval fish diet when the rapid digestibility of this prey was considered, and then protozoa contributed up to 70 % of the diet of the whole larval fish assemblage. Using standing-stock values of larval fish and their prey, i.e. fraction of copepods accessible to larval fish and protozoan prey, I developed simple diagrammatic food web models. These descriptive models indicated that prey levels in the water column were generally 1 to 4 orders of magnitude higher than the required daily food intake for larval fish. Moreover, based on prey level densities which were accessible to larval fish in the Irish Sea, the model showed that prey densities were sufficient to sustain larval fish growth. Thus, it is highly unlikely that the whole larval fish assemblage exerts any top-down control on their prey or suffers starvation. However, without the inclusion of the smaller size fraction of copepods, and to some extent the protozoa, the estimated availability of prey would fall near or below the threshold levels, particularly for herring larvae off the east coast of the Isle of Man.
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