Baikal grayling (Thymallus arcticus baicalensis) and lenok (Brachymystax lenok) are two salmonids which co‐occur in lakes and rivers of the Selenga River and Lake Baikal drainage in northern Mongolia and Siberia. Populations of both species have declined due to habitat loss and overfishing. Previous studies have established that diets of both species are comprised of aquatic and terrestrial invertebrates, but none have examined how prey resources are partitioned between the species. We explored resource partitioning between these species using information from stomach content analysis and stable isotope analysis of carbon and nitrogen. Stomach content data were also compared to invertebrate prey availability estimated from drift and benthic samples. Stomach content analysis indicated that lenok were benthic specialists, while Baikal grayling exhibited a more generalised and surface‐oriented diet, preying upon both terrestrial and aquatic invertebrates. In addition, drifting invertebrate prey availability was positively related to diet overlap based on stomach content analysis, suggesting that competition was involved in the prey resource partitioning we observed. Our analysis of assimilated diet using stable isotopes was generally consistent with stomach contents, indicating that prey partitioning was sustained over a period of several months, but also revealed a greater importance of fish prey to lenok diets. This study provides a baseline description of prey utilisation and prey resource partitioning between lenok and Baikal grayling, which may be used to guide management and future research of these threatened species.

Resource polymorphism is a widespread phenomenon in post-glacial fishes where multiple morphotypes of a species occur sympatrically and exhibit distinct resource use. Availability of open niches and high levels of within and among species competition are thought to drive differences in morphology and may provide insights into early stages of speciation. Hovsgol grayling (Thymallus nigrescens) are endemic to Lake Hovsgol, a lake colonised by fish following the Pleistocene, and are threatened with habitat loss due to climate change and illegal harvest. Previous analysis of Hovsgol grayling diet inferred through C and N stable isotopes suggested the presence of littoral and pelagic foraging groups. We hypothesised that morphology of the two foraging groups would reflect predictions of functional morphology, indicating the presence of resource polymorphism. To test this hypothesis, we evaluated evidence from C and N stable isotopes, stomach contents, fish ages, capture location and morphology. Two foraging groups of Hovsgol grayling were identified through stomach content and C and N stable isotope analysis. Individuals with greater zooplankton consumption were more frequently captured in the pelagic zone, exhibited higher gill raker counts, larger orbit (eye) sizes, longer paired fins, narrower head width, larger maxilla and smaller size at age than the group with greater reliance on benthic invertebrate prey. These differences were generally consistent with those described in other fish species exhibiting littoral-pelagic resource polymorphism.Our study provides the first example of littoral-pelagic resource polymorphism in the salmonid subfamily Thymallinae and highlights the potential influence of competition on fish evolution. K E Y W O R D Sfisheries conservation, functional morphology, Lake Hovsgol, resource polymorphism ACK N OWLED G EM ENTSWe are very grateful to Mfor their assistance with this research. We also thank J. Hoffman for providing his invaluable expertise in stable isotope analysis.

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