Energy densities and percent dry matter of zooplankton were estimated from both field collections and from stomach contents of rainbow trout Oncorhynchus mykiss. Dry‐mass energy densities were similar for zooplankton collected from the field and from fish stomachs, averaging 22.7 kJ/g. The percent dry matter of zooplankton from fish stomachs was approximately twice that of zooplankton from field collections, although species composition was similar. Energy density of wet zooplankton was consequently twice as high in fish stomachs as it was in field collections. These results indicate that the wet‐mass energy density of zooplankton prey used in fish bioenergetics models should be increased over values that have been reported in the literature.

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Daphnia are a keystone species in lentic systems worldwide and an important prey source for planktivorous fishes, such as kokanee (lacustrine Oncorhynchus nerka). Managers have added nutrients, nitrogen (N) and/or phosphorus (P), in an attempt to improve the growth or carrying capacity for fish by increasing available prey, such as Daphnia. To be successful, this strategy requires that fish growth be prey limited, and prey availability be nutrient limited. In this study, kokanee in Dworshak Reservoir, Idaho fed preferentially on Daphnia ≥ 0.8 mm TL. The biomass of consumable Daphnia decreased with increasing kokanee abundance, indicating top-down control. However, the biomass of consumable Daphnia was greater at any given level of kokanee abundance when N was added to the reservoir, indicating bottom-up control. These results provide an example where Daphnia biomass was simultaneously controlled by both bottom-up and top-down forces, and that N addition alone can be an effective tool for increasing Daphnia biomass.