Centennial Valley Arctic Grayling Adaptive Management Project Annual Report, 2016

The Arctic Grayling Adaptive Management Project is focused on identifying the limiting factor, or factors, for Arctic grayling in the upper Centennial Valley of southwestern Montana - one of the last populations of endemic adfluvial grayling remaining in the Upper Missouri River drainage. Arctic grayling (Thymallus arcticus) are a freshwater holarctic salmonid that were once widespread throughout the Upper Missouri River drainage as a glacial relict population. Red Rock Lakes National Wildlife Refuge encompasses nearly all of the currently occupied grayling spawning habitat within the Centennial Valley.

The estimated number of grayling in the 2016 Red Rock Creek spawning population was 214 (95% CI = 161–321), nearly unchanged from the previous year (Nˆ = 1131, 95% CI = 1069–1210; Figure 1). Suitable winter habitat 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 8 ha. This represented the second lowest area of suitable winter habitat measured since 1995 (n = 7). Suitable spawning habitat was most recently quantified in 2014–2015, with an estimated total area of suitable spawning habitat (Ats) of 6.5 ha, and weighted area of suitable habitat (Atw) of 11.9 ha, in Red Rock and Elk Springs creeks (see METHODS below for description of variables). The amount of suitable spawning habitat that was available has yet to be calculated; Ats and Aws will be corrected for 1) availability based on fish passage as a function of barrier (i.e., beaver dam) characteristics and 2) and backwatered areas. Hydrology during the 2016 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) and temperature (◦C) – mean daily discharge (mdd = 59.6 cfs), cumulative degree days from peak emergence to five weeks post-emergence (cdd = 454.5), days above 1 bankfull discharge (cbf = 0), and days above 67% of bankfull discharge (c67bf = 3). These values represent 74%, 110%, 0%, and 16% of the long-term mean values for mdd, cdd, cbf, and c67bf, respectively.