Abundance, spatial distribution, and diet of endangered Hovsgol grayling (Thymallus nigrescens)

Abstract

The Hovsgol grayling, endemic to Lake Hovsgol, Mongolia, is considered endangered, but published descriptions of the species abundance, distribution, or behavior do not exist. We used hydroacoustics, vertical and horizontal gillnetting, zooplankton sampling, and stomach content analysis to characterize abundance, distribution, prey availability, and diet of Hovsgol grayling. Pelagic Hovsgol grayling densities averaged 34.2±6.8 individuals·ha −1 (biomass of 4.4±0.9 kg·ha −1 ) and were concentrated along the western and northern areas of the lake. Gillnet catch rates were 7.5 times higher for littoral sets than pelagic sets. Pelagic vertical distributions of zooplankton and Hovsgol grayling were concentrated above 50 m, with grayling exhibiting diel vertical migrations from 15 m at night to 30 m during the day. Smaller Hovsgol grayling fed primarily on zooplankton while larger individuals fed more heavily on benthic prey in littoral and pelagic areas. The results from this study may be used to guide conservation management and monitoring strategies for Hovsgol grayling, and provide a conservation reference point as human population growth and environmental change continues in the Lake Hovsgol catchment.

Pelagic–littoral resource polymorphism in Hovsgol grayling Thymallus nigrescens from Lake Hovsgol, Mongolia

“…In late July and early August of 2011, 2012 and 2013, vertical and horizontal gill nets were set throughout the lake, following longterm sampling protocols established by Ahrenstorff et al (2012).…”

“…Though Lake Hovsgol is believed to have formed 2-5 million years ago , recent fossil diatom evidence has revealed that the lake's ecosystem collapsed during the last glacial period (Karabanov et al, 2004). All but Hovsgol grayling exclusively inhabit the littoral zone (Ahrenstorff et al, 2012) and exhibit some degree of diet overlap (Olson, 2014;Young et al, 2015). All but Hovsgol grayling exclusively inhabit the littoral zone (Ahrenstorff et al, 2012) and exhibit some degree of diet overlap (Olson, 2014;Young et al, 2015).…”

“…All but Hovsgol grayling exclusively inhabit the littoral zone (Ahrenstorff et al, 2012) and exhibit some degree of diet overlap (Olson, 2014;Young et al, 2015). In our analysis, we attempted to account for these assumptions by utilising stomach content information from this study and others (i.e., Ahrenstorff et al, 2012, Olson, 2014 to verify that prey types included in our stable isotope analysis were the primary prey types consumed by Hovsgol grayling. As with any study inferring diet through stable isotope analysis, two key assumptions are that all prey sources and any spatial and temporal isotopic variation in prey sources is accounted for (Phillips et al, 2014).…”

“…2-5 mya.) Within Lake Hovsgol, Hovsgol grayling is the only species which is currently found in both the littoral and pelagic zone (Ahrenstorff, Jensen, Weidel, Mendsaikhan, & Hrabik, 2012) and a recent analysis of diet inferred from stable isotopes, suggested the presence of littoral and pelagic foraging groups (Olson, 2014). Lake Hovsgol's ecosystem likely collapsed during the Last Glacial Maximum and the current fish community is hypothesised to have colonised the lake sometime in the last ~13,000 years (Karabanov et al, 2004;Roman et al, 2018).…”

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

“…Size‐based diet differences have also been observed in a closely related lake‐dwelling grayling (Hovsgol grayling Thymallus nigrescens ) higher in the Eg River watershed, which shift from a diet of zooplankton at smaller sizes to benthic invertebrates at larger sizes (Ahrenstorff et al. ).…”