Population viability of Arctic grayling (Thymallus arcticus) in the Gibbon River, Yellowstone National Park

Abstract

The fluvial Arctic grayling Thymallus arcticus is restricted to less than 5% of its native range in the contiguous United States and was listed as Category 3 under the Endangered Species Act (ESA) until a 2007 ruling removed its protection. Fluvial grayling were thought to be restricted to the Big Hole River, Montana, where abundances were declining. Although fluvial grayling of the lower Gibbon River, Yellowstone National Park (YNP) were considered extirpated by 1935, anglers frequently reported catching grayling throughout the river since 1980. My goal was to determine if a viable population of fluvial grayling persisted in the Gibbon River, or if fish caught in the river were downstream emigrants from lacustrine populations in headwater lakes. I developed three objectives to address this goal: 1) determine grayling abundances in the Gibbon River, 2) determine the source of grayling in the Gibbon River detected downstream of headwater lakes (occupied by lacustrine populations), and 3) determine if grayling are successfully spawning in the Gibbon River.

In 2005 and 2006, estimated abundances ranged from 0 to 95 and 0 to 109, respectively. Larger estimated abundances resulted when the Chapman modification was incorporated into electrofishing estimates, increasing ranges to vary from 0 to 421 and 0 to 506 in 2005 and 2006, respectively. However, both methods of estimation were accompanied by large standard errors reflecting the few grayling detected in the river. Relatively few grayling were caught by anglers in comparison to other systems. Genetic analyses indicated that grayling from throughout the Gibbon system likely belonged to the same population, most notably supported by very low genetic differentiation (FST = 0.0021 ± 0.002) between headwater lake and river fish. Lengths at most ages were similar among all Gibbon system fish and successful river spawning was not documented below Little Gibbon Falls (the first barrier to upstream movement downstream of headwater lakes). Few grayling adults and no fry were detected in the Gibbon River, implying that a reproducing fluvial population does not likely exist. These findings may affect future ESA considerations of fluvial grayling while providing data for management within and outside of YNP.