Research Document - 2000/012

The potential impact of seal and seabird predation on North American Atlantic salmon.

By D.K. Cairns, and D.G. Reddin

Abstract

This paper seeks to determine whether seal and seabird predation could have caused the major decline in pre-fishery abundance of North American Atlantic salmon. Growth rates of marine-phase salmon were modeled as a series of logistic curves. Numbers of salmon alive were modeled by applying a size-based mortality function to estimates of North American smolt runs and subsequent adult pre-fishery abundance. The biomass of North American post-smolts estimated for the 1990s increased sharply during the summer to peak at 1,400-1,800 t in late fall.

Windows of predation vulnerability were taken as periods when salmon and seals or seabirds spatially co-exist, and salmon are of a size taken by the predator. All seals and seabirds considered in this paper take smolt-size prey. Seals take marine-phase salmon of all sizes, but birds are constrained to smaller prey, and post-smolts outgrow predator capability by mid-June to fall, depending on the species. Vulnerability windows extend year-round for harbour and harp seals, in spring-summer-fall for grey seals, and in winter-spring for hooded seals. Vulnerability windows to birds run from river exit to the time when post-smolts are too large to eat.

Despite large sampling effort, salmon remains have been found in only two harp seal stomachs, two grey seal stomachs, one harbour seal stomach, and one common murre stomach in the Northwest Atlantic. Four other records from grey seals may reflect fish stolen from nets.

Potential impact of seal and seabird predation on marine-phase salmon was modeled under scenarios where predators took given percentages of prey biomass. If all seal and seabird predators combined remove 100% of a post-smolt cohort, the post-smolts would constitute 0.04% of their diet. The predator with the largest consumption is the harp seal. If harp seals take 100% of a post-smolt cohort, then post-smolts would be 0.09% of their diet.

Seals and seabirds in the Northwest Atlantic could harvest a high percentage of marine-phase salmon, even though salmon is a rare item in their diets. Given the rising populations of seals and some seabirds it is plausible that seal and seabird predation could have caused the decline in pre-fishery salmon numbers. However, present data are insufficient to determine whether this in fact occurred.

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