Monthly variation in the diet of harbour seals in inshore waters along the southeast Shetland (UK) coastline

Abstract

We studied the diets of harbour seals Phoca vitulina along the southeast Shetland (UK) coastline by analysing prey remains found in faeces (N = 733) at haul-out sites. A total of 44325 fish otoliths were recovered. Sandeel (Ammodytidae) otoliths were the most numerous (38704), followed by Gadidae (4707). Otoliths were measured and experimentally derived digestion coefficients were applied (correcting for digestion in the seal's gut) to estimate the size of ingested prey fishes. Gadids accounted for an estimated 53.4% of the annual dlet by weight, sandeels 28.5% and pelagic fishes 13.8%. The dominant gadld fishes were whiting Merlanglus rnerlangus (25.3%) and saithe Pollachlus virens ( l 1 1 % ) . The range of species observed in the diet was similar to that recorded in other areas of the UK. One exception to this was garfish Belone helone accounting for 34.1 % of the diet in September (1996), which is a species not previously reported for harbour seal diets in UK waters. There Were s t l u~~y bedsonai patterns in the contribution of sandeels and gadids, with sandeels being important in spring and early summer, and gadids in winter. Pelagic species (mainly herring Clupea harengus, garfish and mackerel Scomber scombrus) were important in late summer and autumn. Observed seasonal patterns are similar to those previously recorded for harbour seal diets In the Moray Firth area of Scotland and appear to coincide with changes in prey availability. A comparison of the utility of using only otoliths to estimate seal diet with all identifiable structures showed that using otoliths alone underestimated the contribution of pelagic fish and overestimated the importance of gadids and sandeels.

Autumn diet of harbour seals (Phoca vitulina) at Prins Karls Forland, Svalbard, assessed via scat and fatty-acid analyses

“…Herring (Clupea harengus L., 1758; Clupeidae) have been found to be important prey of harbour seals at more southerly locations (Olesiuk 1993;Olsen and Bjørge 1995;Tollit et al 1997), but these fish do not generally occur at the latitude of Svalbard. However, sand lance (Ammodytes marinus Raitt, 1934) are present at Svalbard and are a common prey species for harbour seals elsewhere in the northeast Atlantic, United Kingdom Brown and Pierce 1998), as well as on the eastern coast of the United States (Payne and Selzer 1989). However, they were found in only two of the scat samples and were not present in the stomach samples in this study.…”

“…Dominant prey types vary regionally (e.g., Bigg 1981;Bowen and Harrison 1996;Härkönen 1987;Olsen and Bjørge 1995;Tollit et al 1998) and seasonally (e.g., Tollit and Thompson 1996;Brown and Pierce 1998;Hall et al 1998), as well as interannually (Tollit and Thompson 1996). Harbour seal diet studies considered in combination with a specifically designed prey selection study by suggest that harbour seals feed on small, schooling, pelagic fishes when these are available, but switch to demersal (benthic) species when the former prey are not abundant in an appropriate form (species, size class, condition, etc.…”

The status of the harbour seal (<i>Phoca vitulina</i>) in Ireland

“…The studies suggest that harbour seals are opportunistic feeders as a wide variety of prey species was found in the diet of seals on both the west and southwest coasts, including 16 teleost species and two species of cephalopods, and there was evidence of seasonal and geographical variation in the diet. These findings concur with studies on the diet of harbour seals in other parts of their range (Rae 1973, Pierce et al 1991, Brown and Pierce 1998. Overall the most common species of prey consumed by harbour seals in Ireland were whiting (Trisopterus sp.…”

Evidence of bottom-up control of diet driven by top-down processes in a declining harbor seal Phoca vitulina richardsi population

“…This conclusion is further supported by the fact that we observed no differences between years in diets of seals in GB, whereas inter-annual variation was pronounced in seal diet in PWS. The inter-annual variation in PWS likely follows yearly changes of prey species in seal foraging areas (Gibson et al 1993, Brown & Pierce 1998, Robards et al 2003. That no such inter-annual change in diet occurs in GB suggests that productivity in the system is relatively constant, but that availability and accessibility of prey change seasonally with the arrival of whales.…”