Estimated Prey Consumption by Harp seals (Phoca groenlandica), Hooded seals (Cystophora cristata), Grey seals (Halichoerus grypus) and Harbour seals (Phoca vitulina) in Atlantic Canada

Abstract

Consumption of prey by harp (Phoca groenlandica), hooded (Cystophora cristata), grey (Halichoerus grypus), and harbour (Phoca vitulina) seals in Atlantic Canada was estimated for the period 1990-96 by synthesizing and integrating information on individual energy requirements, population size, distribution and diet composition. Total annual consumption by these four species increased from 3.1 million to 4.0 million tons over the seven-year period. Seventy-seven percent (by weight) of the total prey consumption consisted of fish, of which capelin and sand lance were the dominant species accounting for 49% (by weight) of the total fish consumed. The majority (74%) of total prey consumption occurred off southern Labrador and Newfoundland (Div. 2J and 3KL), followed by the northern Gulf of St Lawrence (Div. 4RS) (18%), and the eastern Scotian Shelf (Div. 4VsW) (4%). Harp seals were the most important predator, accounting for 82% of total consumption, followed by hooded seals (10% of total prey consumption), grey seals (7.8%), and harbour seals (0.2%). Regional differences existed in predation impacts of the four pinnipeds; harp seals were most important in Div. 2J and 3KL and in Div. 4RS, hooded seals were most important in Div. 2J and 3KL and Div. 3M (Flemish Cap), while grey seal predation predominated in the southern Gulf of St. Lawrence (Div. 4T) and on the Scotian Shelf (Div. 4VsW). Of the 3.1 million tons of fish consumed by the four species of seals in 1996, only about 20% accounted for commercial species such as Greenland halibut (7%), Atlantic cod (6%), redfish (4%), and Atlantic herring (3%). Most of the consumption of these commercial species consisted of juveniles.

Intersexual differences in the postbreeding foraging behaviour of the Northwest Atlantic hooded seal

“…Separation between sexes in mean dive depths would reduce intraspecific competition at the vertical scale (Baird et al 2005). Detailed information on the diet as well as on the distribution and abundance of potential prey of hooded seals in our study area are lacking, but in other areas they feed on capelin Mallotus villosus and Greenland halibut Reinhardtius hippoglossoides, which are known to be abundant during summer in the area (Gascon 1995, Hammill & Stenson 2000, Bourdages et al 2002, Haug et al 2007). Since larger fish generally occur at deeper depths than smaller fish (Macpherson & Duarte 1991, Chouinard & Swain 2002, larger males may feed on larger prey than females.…”

“…Many large phocid species are sexually dimorphic (Weckerly 1998, Hammill & Stenson 2000, Lindenfors et al 2002. Furthermore, while phocids are characterized by foraging at sea, they need to return to a solid substrate for reproduction and moulting, where they rely on energy reserves to sustain either lactation (females) or competition for access to females (males) (Boness & Bowen 1996.…”

Decline and Recovery of Atlantic Cod (Gadus morhua) Stocks throughout the North Atlantic

“…One of the paradoxes associated with these analyses is that elevated mortality appears in not only juvenile cod but also adult cod, whereas diet data generally reveal seal feeding only on small cod (Hammill and Stenson 2000). It has been suggested by some (e.g., Chouinard et al 2005) that the high mortality of adult cod might be a consequence of belly-feeding (a manner of feeding whereby the seal takes a bite from the cod's belly, leaving the head and muscle mass).…”

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

“…When modelling annual takes of resources by a population of marine mammals, the composition by mass of the diet is often considered to be constant over time and among the different segments of the population (e.g. Hammill & Stenson 2000, Bjørge et al 2002.…”