Feeding ecology and prey resource partitioning of lenok (Brachymystax lenok) and Baikal grayling (Thymallus arcticus baicalensis) in the Eg and Uur rivers, Mongolia

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.