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

Abstract

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 a proxy for 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 some evidence for ontogenetic shifts in habitat‐use (δ13C) for thorny and little skate and diet (δ15N) of 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 expected spatial variability in the 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 values, suggesting individuals may use 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 a degree of 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.

Niche Partitioning among Native Ciscoes and Nonnative Rainbow Smelt in Lake Superior

“…; Shipley et al. ); however, this cutoff has not been rigorously tested. For small fish, the probability of overlap was highest for Rainbow Smelt being found in the Cisco niche (100%) and for Bloater being found in the Kiyi niche (57%).…”

“…The habitat overlap probabilities ranged from near zero to 100% for small fish and up to 86% for large fish ( Figure 5). Prior studies have suggested that overlap probabilities that are >60% indicate significant niche overlap (Guzzo et al 2016;Shipley et al 2019); however, this cutoff has not been rigorously tested. For small fish, the probability of overlap was highest for Rainbow Smelt being found in the Cisco niche (100%) and for Bloater being found in the Kiyi niche (57%).…”

First Observations of Long‐Distance Migration in a Large Skate Species, the Winter Skate: Implications for Population Connectivity, Ecosystem Dynamics, and Management

“…, ). A recent study utilizing carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N) suggested that Winter Skate individuals may predominantly forage within a specific latitudinal range (Shipley et al, in press), a pattern that has also been hypothesized for oceanic sharks (Bird et al. ).…”

“…Although the functional significance of these movements is difficult to ascertain without complementary approaches (e.g., ecogeochemical tracers, direct observation, or additional biotelemetry devices), complex movements may reduce resource competition with sympatric species (sensu Bizzarro et al. ), like Little Skate, which are assumed to exhibit lesser dispersal (Shipley et al., in press). This observation also underscores a limitation of passive acoustic telemetry networks that generally remain close to the shore and do not provide adequate coverage in offshore regions.…”

Separation of realized ecological niche axes among sympatric tilefishes provides insight into potential drivers of co‐occurrence in the NW Atlantic

“…chamaeleonticeps and C . microps would (a) undergo enrichment of 15 N and 13 C with body size, indicative of ontogenetic shifts in diet and habitat use (Sánchez‐Hernández et al., 2019), (b) exhibit spatially distinct isotopic composition aligned with regional isotopic baselines (Oczkowski, Kreakie, McKinney, & Prezioso, 2016; Shipley, Olin, Power, Cerrato, & Frisk, 2019), and (c) exhibit differentiation in one or more of the measured niche axes. The coupling of isotopically derived ecological niche information with spatial and habitat characteristics offers insights into the potential drives of co‐occurrence among these understudied deep‐water species and provides a framework for future studies describing drivers of niche dynamics.…”