Diet of the harbour seal (Phoca vitulina vitulina) in the west and south-west of Ireland

Abstract

Although the harbour seal (Phoca vitulina vitulina) is relatively common in coastal waters of the Republic of Ireland, it remains largely unstudied. Issues including potential interactions with coastal fisheries and the occurrence of phocine distemper virus outbreaks in 1988 and 2002 have highlighted the need to increase our knowledge of the ecology of these animals. The diet of harbour seals was investigated using a combination of approaches: hard-part analysis of scat contents and fatty-acid analysis. A total of 102 scat and 13 blubber samples were collected from two locations in Ireland and were analysed following standard methodologies. Results from scat analysis indicate that the seals are opportunistic, generalist feeders, and probably consume prey in relation to its availability. Eighteen prey species were identified, with sandeels (Ammodytidae spp.) constituting 55% of the prey items by number. Biomass of prey was reconstructed using otolith-length regression equations, with and without the application of digestion correction factors and sole (Solea solea) (16.1%), sandeels (15.2%), andTrisopterusspecies (12.4%) were found to be the most important species by weight. Rarefraction curves were used to determine prey species diversity on a sample site basis and we used accumulation curves to ascertain whether sampling was sufficient on a site basis. Sixty-four fatty acids were isolated from the blubber and analysis of the fatty-acid profiles revealed the possible influence of season, seal weight and sex on the seals' long-term diet. Some of the prey species recorded in the diet overlap with commercially and recreationally caught fish species in Ireland; however, there was little overlap with marketed size-ranges.

Grey seal diet analysis in Ireland highlights the importance of using multiple diagnostic features

“…Unfortunately, research conducted on grey seal diet in Ireland is limited to a few studies in the 1990s . Furthermore, while both the Irish salmon aquaculture industry as well as rod and line fisheries in certain rivers/estuaries report substantial seal-related losses, particularly on the west coast , no evidence of salmonid remains have been identified in the diet of grey seals prior to this study, although salmonids have been recorded in the diet of harbour seals (Kavanagh et al 2010).…”

“…For example, studies in the northeast of Scotland have found very little evidence of salmonids in the diet (McConnell et al 1984, Prime & Hammond 1985. Kavanagh et al (2010) reported salmonids occurring in 3% of scat samples collected from harbour seal Phoca vitulina vitulina haul-out sites on the west coast of Ireland, but until now, no studies have found evidence of salmonids in the diet of grey seals in Ireland. Using bony material in addition to otoliths, this study has demonstrated that grey seals in southwest Ireland are consuming salmonids.…”

“…%N is the number of times a prey species occurs expressed as a percentage of the total number of times all prey species occurred. %F is the number of samples containing a certain prey species expressed as a percentage of all samples containing prey (Kavanagh et al 2010).…”

Effects of seal predation on a modelled marine fish community and consequences for a commercial fishery

“…We calculated length and species preferences separately for grey seals ( and ) and harbour seals ( and ) using seal species‐specific diet data (Kavanagh et al . ; Gosch et al . ) and catchability‐corrected Irish Groundfish Survey data for the prey field; see Appendix S2.…”

The conservation of seals in Irish waters: How research informs policy

“…Research on harbour seal diet in Ireland has been limited to studies conducted in Co. Down, Northern Ireland in the late 1990s [31], in Galway Bay in western Ireland 2001-2004, and in Bantry Bay and the Kenmare River in southwest Ireland since 2006 [32]. The studies suggest that harbour seals are opportunistic feeders as a wide variety of prey species were found in the diet of seals on both the west and southwest coasts, including 16 teleost species and two species of cephalopod, and there was seasonal and geographical variation in the diet evident.…”