Centennial Valley Arctic Grayling Adaptive Management Project Annual Report, 2017

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 2017 Red Rock Creek spawning population was 176 (95% CI = 159–213), nearly unchanged from the previous year (ˆN = 214, 95% CI = 161–321; Figure 1). 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 10 ha. This represented the fifth lowest area of suitable winter habitat measured since 1995 (n = 7 winters, 19 sampling occasions), with the three lowest measurements all occurring during the winter of 1995. Minimum area of winter habitat for the current year (2018) also occurred in February (19 ha). There was a five-fold reduction in suitable winter habitat between January and February this winter largely due to a mean decrease in water depths by 30 cm; oxygen concentrations were the same between samples. Suitable spawning habitat was quantified in 2017, with an estimated total area of suitable spawning habitat (Ats) of 0.5 ha, and weighted area of suitable habitat (Atw) of 4 ha, in Red Rock Creek (see METHODS below for description of variables). Barriers precluding grayling from accessing suitable spawning habitat, i.e., 1 beaver dams, were notched prior to spawning in 2017 as part of the second hypothesis test of the Adaptive Management Plan. Hydrology during the 2017 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 = 115.9 cfs), cumulative degree days from peak emergence to five weeks post-emergence (cdd = 366.3), days above bankfull discharge (cbf = 12), and days above 67% of bankfull discharge (c67bf = 38). These values represent 140%, 89%, 180%, and 195% of the long-term mean values for mdd, cdd, cbf, and c67bf, respectively.