Importance and Predictability of Cannibalism in Rainbow Smelt

Cannibalism is a key interaction between young of year (age‐0) and older fish in many freshwater ecosystems. Density and spatial overlap between age‐groups often drive cannibalism. Because both density and overlap can be quantified, the magnitude of cannibalism may be predictable. Our study considered cannibalism in rainbow smelt Osmerus mordax in Lake Champlain (New York–Vermont, United States, and Quebec, Canada). We used acoustic estimates of the density and distribution of age‐0 and yearling‐and‐older (age‐1+) rainbow smelt to predict cannibalism in the diets of age‐1+ fish during 2001 and 2002. Experienced density, a measure combining density and spatial overlap, was the strongest predictor (R2 = 0.89) of the proportion of cannibals in the age‐1+ population. Neither spatial niche overlap (R2 = 0.04) nor age‐0 density (R2 = 0.30) alone was a good predictor of cannibalism. Cannibalism among age‐1+ rainbow smelt was highest in June, lowest in July, and high in September owing to differences in thermal stratification and habitat shifts by age‐0 fish. Between July and September, age‐1+ rainbow smelt consumed 0.1–11% of the age‐0 population each day. This resulted in a 38–93% mortality of age‐0 fish due to cannibalism. These estimated mortality rates did not differ significantly from observed declines in age‐0 rainbow smelt abundances between sampling dates. Age‐1+ rainbow smelt are probably the primary predators on age‐0 rainbow smelt during the summer and early fall in Lake Champlain.