Trophic Niche Separation between Sympatric Sibling Flatfishes in Relation to Gill Raker Morphology

Abstract

AbstractArrowtooth Flounder Atheresthes stomias and Kamchatka Flounder A. evermanni are upper‐trophic‐level predators that have been considered trophically equivalent in their sympatric range. Here, we present evidence against trophic equivalence of these sibling species based on flounder size and space‐stratified analyses of stomach content data from 5,724 flounder sampled from the eastern Bering Sea (EBS) during summer 2007–2016. We found that interspecific trophic niche separation occurred at sizes up to 39 cm FL before convergence at larger size‐classes. Our findings are consistent with the hypothesis that foraging efficiency differences arise due to divergence in gill raker counts. Arrowtooth Flounder, which have more gill rakers than Kamchatka Flounder, consistently consumed more zooplankton than Kamchatka Flounder, while Kamchatka Flounder typically consumed more benthic fishes. Although a benthivory–zooplanktivory axis of resource partitioning is common among sympatric trophic polymorphs in freshwater ecosystems, our findings are novel for marine sibling species and flatfishes (Pleuronectiformes). Abundance estimates for both Arrowtooth Flounder and Kamchatka Flounder have substantially increased in Alaska during recent years, and we suggest that trophic niche separation alleviates interspecific competition, which may buffer sympatric carrying capacities for these two flatfishes.