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.
