Effects of Hydroelectric Operations on the Kokanee Population in the Flathead River System, Montana

Abstract

Kokanee (Oncorhynchus nerka) were introduced into Flathead Lake in 1916 and developed both lakeshore and inlet river system spawning populations. River system spawning, including that in the main stem of the Flathead River and in McDonald Creek, was a major source of kokanee recruitment in the 1960s and 1970s. Changes in the operation of Hungry Horse Dam on the South Fork of the Flathead River in the early 1970s, coupled with angler harvest, resulted in a rapid 90% decline of main-stem kokanee spawning escapement in the late 1970s. Strong relationships existed between kokanee year-class strength and river flow conditions during 1966-1984 (r = -0.93). Hungry Horse Dam operations caused dewatering of kokanee spawning beds and high winter incubation mortality. This resulted in a rapid decline of the spawning ran. Management of flow levels in the Flathead River is critical to the recovery of the kokanee run. The Montana Department of Fish, Wildlife and Parks recommended a stable flow release of 3,500-4,500 ft 3/s during the fall spawning period, and a minimum flow of 3,500 ft 3/s during the winter incubation period to eliminate dewatering of spawning areas. Managing flows and angler harvest in the system could result in recovery of the population to optimum levels by the late 1990s. Timing of the recovery could vary depending on natural fluctuations in kokanee survival rates from egg to fry and fry to adult, and changes in the carrying capacity of Flathead Lake.