Effects of stream and lake regulation on reproductive success of kokanee in the flathead river system, Montana, U.S.A



Kokanee (Oncorhynchus nerka) provided an important recreational fishery in the Flathead River-Lake system (Columbia River Basin, U.S.A.-Canada) until the late 1970s when populations declined dramatically. The declines coincided with peaking river discharge from Hungry Horse Dam during the autumn spawning period and an increase in the duration of lake drawdown caused by the operation of Kerr Dam on the outlet of Flathead Lake. Redds of both river and lakeshore spawners were, therefore, dewatered in winter, causing high freezing and desiccation mortality of eggs during the incubation period. It appeared that growth rates of kokanee were density dependent. Female kokanee from strong year classes (i.e., those produced during years of favourable water levels and flow conditions during the incubation period) were smaller than those from weak year classes when unfavourable conditions for incubation prevailed. A highly significant relationship (r2 = 0.929, p < 0.001) was demonstrated between female kokanee spawner length, river gauge heights, and lake levels during years which produced each year class. The close fit of the relationship was further validated by the strong correlation (r = 0.964) between actual kokanee lengths and predicted lengths. Other factors which could have influenced kokanee year class strength include hatchery plants of kokanee fry, harvest of kokanee by anglers, variations in the carrying capacity of Flathead Lake, and natural fluctuations in egg and fry mortality.