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

Abstract

Seals and humans are top predators in many marine ecosystems, often targeting the same food resource. With global declines in fish stocks, competition between these top predators is of increasing interest to scientists and resource managers. To understand and quantify this competition, robust data on the diet of seals are necessary. We present new information on the diet of grey seals Halichoerus grypus from a colony on Great Blasket Island, southwest Ireland, with particular reference to commercially important prey species such as salmonids. Inter-annual variation in diet was investigated based on the recovery and identification of sagittal fish otoliths, bones and cephalopod beaks from faecal samples collected in 2009 and 2010. A total of 939 prey items were recovered, representing a minimum of 41 prey species. Thirty species were identified from otoliths while a further 11 species, including salmonids, were identified from additional diagnostic structures. The diet of grey seals was largely dominated by Gadiformes (21% corrected biomass), particularly Trisopterus spp., which contributed 7% in terms of corrected biomass and 52% by frequency of occurrence, while salmonids comprised over 36% of diet by corrected biomass. Inter-annual variation in sandeel (Ammodytidae) and blue whiting Micromesistius poutassou abundance was apparent, while the former was an important contributor to the diet in terms of corrected biomass (11%). The results support the principle that an 'all structures' approach to pinniped diet estimation can significantly increase the rate of detection for most fish groups, and result in improved estimation of seal diet.

Seal bycatch in gillnet and entangling net fisheries in Irish waters

“…and spider crabs (Maja squinado) in the tangle net fishery and these results are supported by diet studies which show no evidence of seals targeting these species (Gosch et al, 2014). The risk of seal bycatch in fisheries which aim to catch skates and spider crabs is, therefore, likely to be lower than fisheries targeting crawfish or monkfish.…”

Molecular prey identification in Central European piscivores

“…In contrast, piscivores with high population densities such as cormorants, herons or pinnipeds have long been perceived as food competitors by humans (Duffy ; Gosch et al . ) resulting in shootings, culling and repellent measures (Boudewijn & Dirksen ; Bowen & Lidgard ). Any management of piscivores aiming at either protecting or regulating these animals depends on a sound understanding of their feeding ecology.…”

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

“…() and Gosch et al . () (Spearman's ρ = 0·659; P < 10 −4 for grey and ρ = 0·576; P = 0·0003 for harbour seals), detailed in Tables S2 and S4. Rank order of species preferences of seals was very different from ranking catch by biomass in the fishery (Spearman's ρ = −0·252; P = 0·144 for grey seal and ρ = −0·026; P = 0·88 for harbour seals).…”

“…; Gosch et al . ). Smaller size classes of fish tend to belong to lower trophic levels and therefore be more abundant and productive.…”

“…Diets of both grey and harbour seals were derived from published scat (defecation) analysis of otoliths and bones for grey (Gosch et al . ) and harbour (Kavanagh et al . ) seals in south‐west Ireland.…”