Bioenergetic Assessment of Temporal Food Supply and Consumption Demand by Salmonids in the Strawberry Reservoir Food Web

This study quantified temporal, spatial, and ontogenetic trophic interactions in Strawberry Reservoir to determine whether salmonid sport fish production was limited by food supply or predation. We combined field data on growth, diet composition, distribution, abundance, survival, and thermal experience with species-specific bioenergetics models for salmonids in Strawberry Reservoir to quantify monthly consumption of all prey fish, Daphnia, and macroinvertebrates. Fish prey represented a minor fraction of the annual diet of large cutthroat Oncorhynchus clarki and rainbow trout O. mykiss, but acute episodic piscivory associated with stocking of juvenile cutthroat trout in autumn could account for the low (1%) initial survival of these hatchery cohorts in recent years. Daphnia and other macroinvertebrates were important to all species and size-classes of salmonids. Model simulations suggested that salmonid consumption represented 5–30% of the Daphnia biomass during the June–October growing season in 1996–1997, but planktivorous demand exceeded Daphnia biomass in late winter. The bottleneck in prey supply during late winter and early spring suggests that Daphnia may not be able to support community consumption during this period if planktivore populations increase, either as a result of the expanding cyprinid populations or as a result of increased salmonid stocking or recruitment. Defining the role and impact of piscivorous and planktivorous trophic interactions provided information for management decisions regarding stocking strategies, recruitment, carrying capacity, and other food web processes.