Seasonal and interannual variability in grey seal diets on Sable Island, eastern Scotian Shelf

W D Bowen,G Harrison

doi:10.7557/3.2728

Abstract

We studied seasonal and interannual variability in the diet of grey seals (Halichoerus grypus) using faecal samples collected from Sable Island, Nova Scotia between 1991 and 1998. More than 28,000 prey from at least 28 taxa were identified from 1,245 faecal samples collect mainly in spring, fall and winter. Sand lance (Ammodytes dubius) dominated the diet in all seasons and years, but the importance of this and other species varied over time. There was also evidence of seasonal and interannual variation in the size of prey consumed both within and among species. We compared diet composition with estimates of prey numbers and biomass from annual researchtrawl surveys conducted in March and July. Species-specific numerical corrections were applied to otolith counts to account for the complete digestion of otoliths, and fish catchability correction factors applied to trawl survey catches to account for trawl selectivity. Based on an odds ratio index of prey selectivity, grey seals positively selected sand lance in both seasons. Other species were either relatively avoided or eaten roughly in proportion to their estimated abundance.

Highlights

Both intrinsic and environmental factors such as sex, age, season, geographic location, and prey abundance and distribution are known to affect the diets of pinnipeds
We described the diet of grey seals near Sable Island based on faecal samples collected between 1991 and early 1993 (Bowen and Harrison 1994)
The present study investigates the extent to which the diet of grey seals may have changed during an 8 year period, the extent to which inter-annual changes in diets near Sable Island can be understood in terms of changes in the abundance and distribution of major prey species and evidence of prey selection by grey seals

Summary

Introduction

Both intrinsic and environmental factors such as sex, age, season, geographic location, and prey abundance and distribution are known to affect the diets of pinnipeds (reviewed in Bowen and Siniff 1999, Bowen et al 2002). Seasonal and geographic variability in pinniped diets are thought to be driven mainly by changes in the relative abundance and distribution of prey (Sinclair et al 1994, Thompson et al 1996, Bowen and Harrison 1993, 1996), but demographic factors such as sex and age can underlie differences in diets in addition to the effects of prey characteristics (e.g., Lawson et al 1995, Beck et al 2005). Interannual variability in prey abundance and distribution can affect the composition of pinniped diets over longer time scales (Sinclair et al 1994, Bowen and Harrison 1996, Lawson et al 1997). Recent data from satellite-tagged adults indicates extensive foraging on offshore banks on the Scotian Shelf, Sable/Western Bank (Bowen et al 2005), reflecting the importance of this offshore area

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