Interannual variation in the summer diets of harbour seals Phocavitulina at Mousa, Shetland (UK)

Abstract

The main prey species in the summer (July–September) diets of harbour seals (Phoca vitulina) on the Island of Mousa (Shetland, UK), 1994–1997, were whiting (Merlangius merlangus), herring (Clupea harengus), sandeel (Ammodytidae) and garfish (Belone belone). Norway pout (Trisopterus esmarkii) was numerically important but comprised no more than 11% of prey weight in any year. There were marked between-year fluctuations in the relative importance of these prey, with whiting comprising 16–34% (by weight) of the diet, herring 12–28%, sandeels 7–18% and garfish 7–22%. Additional data on spring (April–June) diet were available for 1995–1997: sandeels were the most important prey (% weight) in all three years (51–60% of the diet), while herring (8–48%) and gadids (2–22%) varied in importance. The average size of fish eaten was larger than that reported in comparable studies from other areas. Harbour seals appear to have selected larger sandeels, whiting and Norway pout than the average size available in the area, as indicated by survey trawls, although between-year changes in the size of Norway pout in the diet did to some extent reflect availability. Interannual variation in the importance of Norway pout in the diet appeared to track trends in abundance, although the short time series precluded detection of a statistically significant correlation. Thus, some of our results are consistent with harbour seals feeding opportunistically while others point to selectivity, particularly for prey size. Estimated consumption of fish by the 500 harbour seals on Mousa during summer ranged from 152–195 tonnes.

Intraspecific dietary variation in the short-beaked common dolphin Delphinus delphis in the Bay of Biscay: importance of fat fish

“…In the northeast Atlantic, dietary variation has been assessed and included in food consumption models for the most common pinnipeds (e.g. Hammond et al 1994, Brown et al 2001, but for cetaceans, this has only been assessed in common dolphins and porpoises in Galician and Scottish waters, respectively (Santos et al 2004a,b).…”

“…Young & Cockcroft 1994, Santos et al 2004a,b for cetaceans; Brown et al 2001, Page et al 2005 for pinnipeds). One may expect some degree of ontogeny in foraging abilities as a consequence of the development of sensorial, swimming and diving performances.…”

“…In the northeast Atlantic, dietary variation has been assessed and included in food consumption models for the most common pinnipeds (e.g. Hammond et al 1994, Brown et al 2001, but for cetaceans, this has only been assessed in common dolphins and porpoises in Galician and Scottish waters, respectively (Santos et al 2004a,b).Concerning the common dolphin, our results and those of previous studies (Young & Cockcroft 1994, Santos et al 2004a) showed temporal and sex-/maturity-related changes in the diet, which could potentially affect significantly the estimation of daily food intake as a result of variation in average energy content of the diet. The main result of the present study was that despite extensive changes in the proportion of most prey species according to years, seasons or sex-/ maturity-categories, the overall daily food intake changed little; changes in dietary composition tended to operate at a fairly constant and high rate of fat fish in the diet.…”

Seals, sandeels and salmon: diet of harbour seals in St. Andrews Bay and the Tay Estuary, southeast Scotland

“…In the North Sea, there is good recent information on the diet of grey seals and some relatively recent information on cetacean diet , Windsland et al 2008. However, most information on harbour seal diet in the North Sea is at least 10 yr old (Tollit & Thompson 1996, Tollit et al 1997, Brown & Pierce 1998, Hall et al 1998, Brown et al 2001, Middlemas et al 2006.…”

“…In the North Sea, there is good recent information on the diet of grey seals and some relatively recent information on cetacean diet , Windsland et al 2008. However, most information on harbour seal diet in the North Sea is at least 10 yr old (Tollit & Thompson 1996, Tollit et al 1997, Brown & Pierce 1998, Hall et al 1998, Brown et al 2001, Middlemas et al 2006.One heavily exploited fish that is important to many predator species in the North Sea is the lesser sandeel Ammodytes marinus (Furness & Tasker 2000). North Sea landings of sandeels fluctuated between 700 000 and 1 110 000 tonnes (t) from 1994 to 2002 but declined to around 200 000 t in 2007 (ICES 2007).…”