Arrowtooth flounder (Atheresthes stomias) and Kamchatka flounder (Atheresthes evermanni) are morphologically similar high trophic level flatfishes with an overlapping range in the eastern Bering Sea. Both species have increased markedly in abundance over the last three decades, and their ranges are expected to expand due to climate change. Arrowtooth flounder and Kamchatka floudner have been considered to occupy the same trophic niche, although higher gill raker counts in arrowtooth flounder suggest potential differences in functional foraging efficiency. I examined extensive diet data collected from 2007-2016 to determine if (1) arrowtooth flounder and Kamchatka flounder occupy the same trophic niche, (2) if trophic niche separation occurs along a resource gradient hypothesized from differences in gill raker morphology, and (3) if patterns of trophic niche partitioning are spatially and ontogentically consistent.
Trophic niche separation was consistent with hypothesized interspecific differences in foraging efficiency due to gill raker functional morphology. Arrowtooth flounder have more gill rakers than Kamchatka flounder and consistently consumed more zooplankton than Kamchatka flounder, while Kamchatka flounder consistently consumed more benthic fishes. Interspecific trophic niche separation was pronounced but spatially variable in fishes up to 39 cm. Ontogenetic trophic niche converged in larger fish. Findings indicate sympatric arrowtooth flounder and Kamchatka flounder partition resources along a benthivory-planktivory gradient commonly observed between sympatric teleost ecotypes in freshwater ecosystems. Niche partitioning likely alleviates interspecific competition between smaller size classes, and may allow higher Atheresthes carrying capacity in the eastern Bering Sea.