The effects of seasonal and annual differences in prey and environmental conditions on a colonial seabird at the southern limit of its breeding range

d'Entremont, Kyle James Nestor (2022) The effects of seasonal and annual differences in prey and environmental conditions on a colonial seabird at the southern limit of its breeding range. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Northern Gannet (Morus bassanus) is the largest seabird breeding in the North Atlantic Ocean. Their breeding colonies stretch across a broad geographical range, and colonies near the southern extent of their breeding range have recently exhibited poor productivity. Gannets are opportunistic, generalist foragers and consume a wide range of prey including Atlantic mackerel (Scomber scombrus), capelin (Mallotus villosus), Atlantic herring (Clupea harengus) and Atlantic saury (Scomberesox saurus). The distribution and availability of these pelagic fishes depend on ocean temperature. In this thesis, I fill in critical knowledge gaps on how gannets nesting at their southernmost colony at Cape St. Mary’s, NL respond to differences in prey availability and sea surface temperature. In Chapter 2, I examine the effects of prey availability and sea surface temperature during the breeding season on gannet productivity over a multi-decadal timespan. I found that declines in mackerel availability and warming waters in late chick-rearing were associated with depressed productivity. In Chapter 3, I examine the foraging behaviour of gannets and how it differs between and within years. Gannets switched their foraging tactics from preying on capelin in early chick-rearing to larger pelagics in late chick-rearing. Gannets exhibited greater foraging effort during early chick-rearing in 2020 than 2019 which was associated with lowered capelin availability. Overall, gannets were negatively impacted by lowered prey availability and warming sea surface temperatures.

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