The diet of the grey seal [Halichoerus grypus (Fabricius, 1791)] in Ireland and potential interactions with commercial fisheries

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Gosch, Martha
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University College Cork
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Interactions between grey seals and fisheries have seemingly increased in recent decades, with high levels of depredation reported in set-net fisheries. There exists a perception that grey seals are competing for commercial fish stocks, however, the extent to which commercial species contribute to diet composition has not been recently quantified. This study aimed to investigate prey species assemblages occurring within grey seal diet in Ireland, particularly with regards to the presence of commercial species, and provide current baseline data on diet composition. The traditional method for diet reconstruction was used with a number of approaches tested. Prey detection increased substantially when “all structures” were included, while a deficit of correction factors tended to under-estimate the biomass contribution of certain prey species. To assess whether temporal and regional variation in diet occurred, faecal samples were collected from two colonies of national importance on the southwest and southeast coast of Ireland. Results highlighted significant seasonal and inter-annual variations within diet samples collected from the southwest site, while significant regional differences occurred between geographic locations. Differences in prey assemblages are assumed to be related to their seasonal abundance and the foraging habitat type surrounding each haul-out site. While commercial species were found in relatively low abundances, haddock/pollock (Melanogrammus aeglefinus/Pollachius pollachius/ P. virens) species were substantial biomass contributors to the diet in Ireland. Low quantities of fishery target species were recovered from juvenile bycaught seal stomachs obtained from the west and south coast of Ireland. Lengths from a total of four prey fish overlapping with sizes targeted by the fisheries. Results suggest this cohort is not primarily responsible for the reported levels of depredation. The findings from this study are discussed in relation to results from other geographical areas of the grey seals distribution, with potential mitigation measures and future recommendations considered.
Grey seal , Fisheries interactions , Traditional diet analysis , Temporal variation , Regional variation , Trophic interactions , Foraging habitat , Depredation , Bycatch , Mitigation
Gosch, M. 2017. The diet of the grey seal [Halichoerus grypus (Fabricius, 1791)] in Ireland and potential interactions with commercial fisheries. PhD Thesis, University College Cork.