Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

21 December 2018

Shipley, Oliver; Olin, Jill; Power, Mike; Cerrato, Robert; Frisk, Michael

We evaluated whether existing assumptions regarding the trophic ecology of a poorly-studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ13C) and nitrogen (δ15N) stable isotope values as metrics of foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in 13C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e., from smaller to larger prey items) and become enriched in 15N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found someweak evidence for ontogenetic shifts in habitat-use (δ13C) for thorny and little skate and diet (δ15N) for thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ15N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate, highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked the expected spatial variability in isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ13C and δ15N, suggesting individuals may pool resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g., little and thorny skate), sympatric species (e.g., little and winter skate) exhibited potential trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly studied, vulnerable group of predators, and highlight a need to re-examine assumptions pertaining to aspects of their ecology.