Trophic relationships among capelin (Mallotus villosus) and seabirds in a changing ecosystem

Abstract

Trophic interactions among seabirds and capelin (Mallotus villosus), a key forage species off Newfoundland and Labrador, are examined. During the 1990s, estimates of relative year-class strength of capelin were similar to estimates in the 1980s, capelin spawned later and matured younger, mean fish size was smaller, and there were large-scale distributional shifts of capelin. Most of these changes were linked to below-normal sea temperatures during the early 1990s, but the changes have persisted even though temperatures have returned to normal. Seabirds bred later in the 1990s and changed diets and foraging strategies. Off eastern Newfoundland, the breeding success of black-legged kittiwakes declined during the early 1990s owing to a suite of factors, including their inability to dive to capture capelin, the late arrival of capelin inshore and predation by gulls. Common murres and puffins did not suffer breeding failures because they could dive and catch capelin at depth. Off Labrador, black-legged kittiwakes experienced breeding failures because of the lack of capelin while common murres were able to find alternate prey and bred successfully. The diets of gannets in the 1990s contained a higher proportion of cold-water prey than in earlier periods when warm-water prey predominated. The population sizes of all seabird species remained stable or increased, with the exception of gulls. Increasing populations of seabirds are related in part to their lessened mortality from gillnets since the closures of the eastern Canadian groundfishery in 1992, whereas declining populations of gulls can be related to reduced food availability from fishery discards and offal. This food limitation has led gulls to switch much of their foraging effort from scavenging to predation on seabird adults and chicks before capelin arrive inshore.