Dietary Niche Shifts of Multiple Marine Predators under Varying Prey Availability on the Northeast Newfoundland Coast

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 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.

Stable Isotope Analyses of Multiple Tissues of Great Shearwaters (Ardenna Gravis) Reveals Long-Term Dietary Stability, Short-Term Changes in Diet, and Can be Used as a Tool to Monitor Food Webs

“…Previous studies have found a negative relationship between latitude and δ 13 C values in the blood of Cory's shearwaters (Calonectris borealis) in the North Atlantic [48,49]. Therefore, we also compared the δ 13 C and δ 15 N values of great shearwater RBCs and feathers from our study with those from previous studies [34,35,37,46].…”

“…Three studies that have collected and analyzed data on the δ 13 C and δ 15 N values of RBCs (i.e., the cellular portion of blood) in the North Atlantic ( [34,35] and this study), providing enough data to look for a latitudinal gradient in δ 13 C ( Table 1). We found that δ 13 C decreases significantly with latitude (F 1,10 = 24.07, p = 0.006) and that the relationship explains a high proportion of the variance (R 2 = 0.71; Figure 4A).…”

“…This is a large expanse of water and therefore we chose the great shearwater (Ardenna gravis) as our indicator species for several reasons: Great shearwaters captured off Cape Cod and in Massachusetts Bay travel widely, averaging 515 km per week, but appear to spend most of their time in the Gulf of Maine before beginning their migration south in August and September [32,33]. They feed on a wide range of prey species and as top marine predators, great shearwaters are sensitive to changes in their prey populations, such as sand lance, herring, and mackerel [34][35][36], as well as abiotic factors such as currents, surface temperature, and salinity [28,36,37]. Great shearwater populations have experienced mass mortality events of emaciated birds, the causes of which are currently unknown, suggesting their sensitivity as bioindicators of important ecological events that bear further investigation [37,38].…”

Coral Reef Degradation Differentially Alters Feeding Ecology of Co-occurring Congeneric Spiny Lobsters

“…These differences arise due to preferential retention of heavier isotopes and excretion of lighter isotopes, leading to relative enrichment in 15 N and 13 C with increasing trophic level (discrimination factors). The amount of 15 N in tissues is indicative of an animal's trophic position, whereas 13 C content can be used to recognize primary sources of carbon (i.e., foraging habitat) (Post, 2002;Gulka et al, 2017). Samples of lobster abdominal (tail) muscle were rinsed with ultrapure MilliQ water, dried completely at 60 • C and ground to a fine powder with agate mortar and pestle; then ∼1 mg of powdered tissue was transferred into tin capsules.…”

“…We did this analysis with Bayesian mixing models (Parnell et al, 2010) using the SIAR package in R (Jackson et al, 2011). Mixing models require the use of discrimination factors, the best of which are those derived from the species under study or, failing that, from closely related species (Gulka et al, 2017). In the absence of specific discrimination factors for P. argus or P. guttatus, we used a range of discrimination factors estimated by Waddington and MacArthur (2008) from tail tissue of spiny lobsters P. cygnus fed various diets (δ 15 N range 1.67-2.97 ; mean 2.57 ; δ 13 C range 2.92-3.60 ; mean 3.20 ).…”