Functional and aggregative responses of harbour seals to changes in salmonid abundance

Abstract

There is intense debate over the potential impact of seal predation on declining salmon stocks in both the Pacific and Atlantic oceans. However, efforts to model such interactions have been constrained by a lack of data on the functional and numerical responses of these predators. Based upon theory, and data from small-scale terrestrial and freshwater systems, a type 3 functional response is expected to best describe predation by generalist pinnipeds. Similarly, theory also predicts that seal numbers should increase with salmon density in rivers following an aggregative response of predator to prey. We tested these predictions by studying the diet and local density of harbour seals in relation to seasonal variations in the abundance of salmonid in a Scottish river system. As predicted, the abundance of seals in the river was directly related to the abundance of returning salmon, and dietary data supported the type 3 functional response to changes in salmonid abundance. These studies provide empirical support for the use of type 3 response in modelling studies.

Figure 1

The functional relationship between the abundance of salmonids in the environment and the percentage by weight of salmonids in the diet of harbour seals using the Cromarty Firth during 2000. The fitted Michaelis–Menton function suggests a type 3 functional response.

Figure 2

The aggregative response of harbour seals in the mouth of the River Conon to changes in the abundance of adult salmon. Data points represent observed monthly sighting probabilities and the line shows sighting probabilities (p(s)) Predicted by the logistic equation: $p(\text{s})=(\text{exp}(-2.9+(0.05\times \text{salmon})))/(1+\text{exp}(-2.9+(0.05\times \text{salmon}\text{catch}\text{index})))$.