Predation on marine-phase Atlantic salmon (Salmo salar) by gannets (Morus bassanus) in the Northwest Atlantic

Abstract

Predation on Atlantic salmon (Salmo salar) has been recorded in rivers and estuaries, but there is little documentation of predation at sea. Prey landed by gannets (Morus bassanus) over 24 years in a large colony off northeast Newfoundland included small proportions of post-smolt Atlantic salmon. Before 1990, when shifts in oceanographic conditions and pelagic food webs occurred in the Labrador Sea, post-smolts, on average, made up 0.29% of estimated intake by gannets during August 1977–1989. In contrast, during the 1990s, this estimate increased to 2.53%, peaking at 6.37% in 1993. Model estimates with wide error margins projected that gannets consumed a mean of 1.6 t and 19.2 t of post-smolts during August 1977–1989 and 1990–2000, respectively, making up 0.22% and 2.70% of estimated North American post-smolt biomass during these periods. The migratory routes of post-smolt Atlantic salmon pass through the foraging ranges of gannet colonies, but limited sampling at colonies other than Funk has not revealed salmon in gannet diets. Sampling seabird diets is an economic, biological means of investigating the ecology and natural mortality of Atlantic salmon. Spatial and temporal expansion of this sampling would enhance its oceanographic context and reduce uncertainty associated with estimates of predation by seabirds.