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 the 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 (δ 15 N) and carbon (δ 13 C) 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 Larus 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, SEA b ) narrowed and trophic position shifted toward higher δ 15 N 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 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.

Spatial patterns of breeding seabirds are influenced by the distribution of resources in relation to the colony and the density of conspecifics from the same or adjacent colonies. We conducted an inter-colony comparison of foraging space use and behavior, diet, and reproductive success of common murres Uria aalge breeding at a large offshore and a small inshore colony on the northeastern coast of Newfoundland (Canada) during 2016-2018 under varying prey (capelin Mallotus villosus) biomass. Murres from the large offshore colony foraged over a greater area, with greater individual foraging distances, indicative of higher commuting costs compared to the smaller inshore colony. Although this pattern might reflect prey depletion near the offshore colony due to higher conspecific densities, it likely also reflects the greater distance to predictable, high-abundance prey aggregations. This is supported by high spatial overlap of foraging areas from both colonies near coastal, annually persistent capelin spawning sites. Adult diet was similar between colonies during incubation, but diverged during chick-rearing, with offshore murres consuming a higher proportion of alternative prey, while inshore murres consumed more capelin. These differences did not affect fledging success, although hatching success was lower in the larger colony, suggesting that divergent factors (e.g. predation, nest attendance) influence colony-specific population dynamics. Overall, our findings suggest that abundant local prey is key in shaping spatial patterns of breeding common murres in northeastern Newfoundland and results in apparently minimal intraspecific competition. As anthropogenic pressures on resource availability heighten, insight into factors influencing intraspecific foraging niche dynamics will be critical to inform management.

While foraging, a predator can feed solitarily or in a group. The net energy gain of joining a group is predicted to vary with prey patch quality, species-specific prey capture behavior, and the size and species composition of the predator group. In coastal Newfoundland, Canada, capelin (Mallotus villosus), a key forage fish, migrates inshore to spawn during the summer, resulting in a dramatic shift in prey availability. During July–August 2015–2017, we examined the numerical and behavioral responses of procellarid (Great Shearwater [Ardenna gravis], Sooty Shearwater [A. grisea], Northern Fulmar [Fulmarus glacialis]), and gull species (Herring Gull [Larus argentatus], Great Black-backed Gull [L. marinus]) to fish offal under varying capelin availability as well as flock size and composition using an at-sea experiment on the northeast Newfoundland coast. The experiment consisted of providing offal every 30 s (10-min experimental period), along with 10-min control periods before and after. We recorded the species-specific number of birds on the water, the number of birds simultaneously attempting to capture offal, and the number of successful attempts (“foraging success”). The number of birds on the water was lower during high capelin availability for all species, except for Northern Fulmar. The number of conspecifics simultaneously attempting to capture offal increased with the number of conspecifics on the water, but plateaued at different numbers (4–17) for most species. The species-specific proportion of successful attempts (i.e. foraging success) varied with flock size and composition (i.e. number of conspecifics, heterospecifics, species). Foraging success of Herring Gulls and fulmars were moderately affected by flock size and composition, suggesting that they may be dominant competitors. Findings suggest that seabirds rely more heavily on supplemental food sources, such as fisheries discards and offal, when natural prey availability declines, potentially resulting in a higher risk of by-catch during fisheries activities as forage fish stocks decline.

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To grow, survive and reproduce under anthropogenic-induced changes, individuals must respond quickly and favourably to the surrounding environment. A species that feeds on a wide variety of prey types (i.e. generalist diet) may be comprised of generalist individuals, specialist individuals that feed on different prey types, or a combination of the two. If individuals within a population respond differently to an environmental change, population-level responses may not be detectable. By tracking foraging movements of great black-backed gulls (Larus marinus), a generalist species, we compared group-level and individual-level responses to an increase in prey biomass (capelin; Mallotus villosus) during the breeding season in coastal Newfoundland, Canada. As hypothesized, shifts in prey availability resulted in significantly different individual responses in foraging behaviour and space use, which was not detectable when data from individuals were combined. Some individuals maintained similar foraging areas, foraging trip characteristics (e.g., trip length, duration) and habitat use with increased capelin availability, while others shifted foraging areas and habitats resulting in either increased or decreased trip characteristics. We show that individual specialization can be non-contextual in some gulls, whereby these individuals continuously use the same feeding strategy despite significant change in prey availability conditions. Findings also indicate high response diversity among individuals to shifting prey conditions that a population- or group-level study would not have detected, emphasizing the importance of examining individual-level strategies for future diet and foraging studies on generalist species.