Centennial Valley Arctic Grayling Adaptive Management Project Annual Report, 2018

Arctic grayling (Thymallus arcticus) are a freshwater holarctic salmonid that were once widespread throughout the Upper Missouri River (UMR) drainage as a glacial relict population. One of the last endemic grayling populations remaining in the UMR drainage resides in the Centennial Valley (CV) of southwestern Montana. Spawning is largely limited to Red Rock, Corral, Elk Springs, and Odell creeks, with Red Rock Creek likely supporting 80-90% of annual spawning in the CV. It is presumed that most of the grayling population in the CV spends non-breeding portions of the year in Upper Red Rock Lake (Upper Lake). Red Rock Lakes National Wildlife Refuge (Refuge) encompasses Upper Lake, and nearly all of the currently occupied grayling spawning habitat within Red Rock, Elk Springs, and Odell creeks.

The estimated number of grayling in the 2018 Red Rock Creek spawning population was 387 (95% CI = 168–682), nearly unchanged from the previous year (ˆN = 176, 95% CI = 159–213). Suitable habitat the prior winter within Upper Lake (i.e., water depth below the ice ≥ 1 m and dissolved oxygen ≥ 4 ppm) reached a minimum during February sampling at an estimated 19 ha. This falls within a range of suitable winter habitat (i.e., 10–25 ha) previously hypothesized to lead to reduced grayling survival as documented in 2016. Area of winter habitat for the current year (2019) was estimated in January as 0 ha; 1995 was the only other year 0 ha of suitable winter habitat was estimated.

Suitable spawning habitat was quantified in 2017, with an estimated total area of suitable spawning habitat (Ats) of 0 ha, and weighted area of suitable habitat (Atw) of 4 ha, in Red Rock and Elk Springs creeks (see METHODS below for description of variables). Barriers precluding grayling from accessing suitable spawning habitat, i.e., beaver dams, were notched prior to spawning again in 2018 (similar to 2017) as part of the second hypothesis test of the Adaptive Management Plan. Hydrology during the 2018 breeding season critical period (i.e., peak spawning + 188 degree days + 5 weeks) was characterized using four variables related to stream flows (cubic feet second−1 [cfs]) and temperature (°C) – mean daily discharge (mdd = 111.4 cfs), cumulative degree days from peak fry emergence to five weeks post-emergence (cdd = 364.2), days above bankfull discharge (cbf = 15), and days above 67% of bankfull discharge (c67bf = 38). These values represent 133%, 89%, 213%, and 186% of the long-term mean values for mdd, cdd, cbf, and c67bf, respectively.