I studied winter ecology of kokanee salmon (Oncorhynchus nerka) in high-mountain, oligotrophic lakes in the Sawtooth Valley, Idaho, USA. Sockeye salmon, also O. nerka, once abundant in these lakes, are now listed as an endangered species. Efforts are underway to restore sockeye populations, but recovery resources are limited. I hypothesized that winter may be crucial to juvenile O. nerka in the lakes, and that management options must address the potentially limiting conditions during winter. Specifically, I examined diet, growth, and diel migrations of kokanee during winter, and combined the results in a bioenergetic model to calculate winter carrying capacities of the lakes.

Kokanee salmon ate mostly zooplankton during winter, but their diet did shift. Gut fullness was low, but 98% of stomachs contained food. This suggests that during winter kokanee must feed continually to meet metabolic demands. Kokanee experienced a loss of energy density over the winter, even though length and weight sometimes increased. Lipid content dropped up to 50% from November to May.

I used a bioenergetic model to predict winter zooplankton consumption of O. nerka populations in the Sawtooth Valley lakes and to model three recovery strategies, including: (1) stocking additional sockeye; (2) the effects of whole-lake fertilization; and (3) reducing the number of kokanee competitors. My results showed that current winter consumption by O. nerka meets or exceeds our estimate of zooplankton production in three of four lakes. Therefore, increasing zooplankton populations by whole-lake fertilization could increase carrying capacity.