Abstract
Understanding species interactions among top marine predators and interactions with their prey can provide important insight into community-level responses to changing prey availability and the role of apex predators as indicators of ecosystem change. On the northeast Newfoundland coast, marine predators rely on capelin (Mallotus villosus), a dominant forage fish, as a food source. Capelin migrate into coastal regions to spawn during July, essentially transforming food supply from low during early summer (i.e. pre-spawning) to high later in the summer (i.e. spawning). During July-August, 2016, we used stable isotopic ratios of nitrogen (δ15N) and carbon (δ13C) to investigate shifts in dietary niche metrics at the predator group-level (trophic position, dietary niche breadth) and community-level (niche overlap, trophic diversity) for multiple marine predators under varying capelin availability. Predator groups included non-breeding shearwaters (great shearwater Ardenna gravis, sooty shearwater A. grisea), humpback whales (Megaptera novaeangliae), and gull chicks (herring gull Larus argentatus, great black-backed gull L. marinus). We also tested the sensitivity of community-level dietary metrics to a variety of published trophic discrimination factors. Tissue samples from shearwaters (blood cellular component), gull chicks (whole blood), and whales (skin), representing average diets over 2-3 weeks, were collected during three periods (early, mid, late) corresponding to increasing capelin availability. Isotopic niche breadth (Standard Ellipse Area, SEAb) narrowed and trophic position shifted toward higher δ15N for all predator groups as capelin availability increased, suggesting a higher reliance on capelin. Trophic diversity (distance to centroid) decreased with increased capelin availability, while pairwise niche overlap between predator groups was highly variable and sensitive to varying trophic discrimination factors. Findings suggest that although capelin is the dominant forage fish during the summer, predators rely on capelin as prey to varying degrees. Combining species- and community-level metrics of dietary niche and trophic diversity may provide a more complete picture of predator responses to prey availability and, thus, may be important monitoring tools to indicate changes in the food supply of marine predators.
Highlights
Understanding the complexities underlying the structure and function of marine ecosystems and the impacts of anthropogenic factors, fisheries activities, are key challenges in marine ecology (Moreno et al, 2016)
Our study focused on dominant marine predators on the northeast Newfoundland coast (Davoren, 2013a), including three non-breeding species and two breeding species
When capelin availability in the study area increased from early to mid and late, all predator groups exhibited a narrowing of dietary niche breadth, indicating reduced prey diversity (Layman et al, 2007a)
Summary
Understanding the complexities underlying the structure and function of marine ecosystems and the impacts of anthropogenic factors, fisheries activities, are key challenges in marine ecology (Moreno et al, 2016). The impact on higher trophic levels of marine food webs under wasp-waist control are of particular concern, as significant amounts of energy are funneled through only a few forage fish species (Smith et al, 2011; Pikitch et al, 2012). As predators are sensitive to changes in prey availability (Piatt et al, 2007; Cury et al, 2011), they may act as sentinels or indicators of ecosystem change (Cairns, 1987; Diamond and Devlin, 2003). Despite a growing literature of seabird responses to changing prey regimes, many challenges remain before using predators as indicators. Suggested approaches include incorporating multiple species and trophic levels along with multivariate approaches (Piatt et al, 2007)
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