FORAGING RATES OF A VERTEBRATE AND AN INVERTEBRATE PREDATOR IN STREAMENCLOSURES

We compared prey consumption by brown trout (Salmo trutta) and leeches (Erpobdella octoculata) when they occurred together and alone in stre am enclosures. One density of trout was compared with three densities of leeches during 24-h-long experiments. Separate experiments were con ducted with two prey taxa, Gammarus pulex and Baetis rhodani, using tw o types of substrate, fine gravel and fine gravel plus cobbles. We fou nd that trout had a higher foraging rate over fine gravel bottoms than over cobble bottoms, whereas leeches were more effective over cobble bottoms. Prey type also affected foraging rates, with trout consuming more Baetis than Gammarus, and leeches consuming more Gammarus than Ba etis. A direct comparison of foraging rates of trout and leeches when alone showed that trout consumed more prey than the highest density of leeches, except when Gammarus occurred over cobble bottoms; then trou t consumed prey at the same rate as the lowest leech density. When lee ches and trout were together, trout foraging was unaffected by leeches , but leech foraging was affected by trout when feeding on Baetis but not Gammarus. These results show that one leech consumes much less tha n one trout, but at representative densities, leeches can have similar impacts on prey per area of stream, suggesting that the role of inver tebrates in structuring benthic communities can be similar to that of fish.