Indirect behavioral effects of a piscivore on trophic interactions in stream enclosures

Abstract

Abstract:The indirect behavioral effect of piscivorous pike (Esox lucius) on the growth and diet of brown trout (Salmo trutta) as well as the consequences of the pike-trout interaction for benthic macroinvertebrates and periphytic algae were studied in nine 2 × 3 m field stream enclosures. The indirect effect of pike on trout behavior was studied by holding trout density relatively constant, which was done by replacing consumed trout during the course of the experiment. Three treatments were established, each replicated three times: fishless controls and two fish treatments, one with 15 brown trout (2.5 trout m-2) and one with 15 trout and 1 northern pike (0.17 pike m-2). Growth of brown trout was lower in the presence of pike than in the absence of pike. Pike affected the biomass of the leech, Erpobdella, whose biomass was greater in the presence of pike than in its absence. The biomass of grazing heptageniids was lower in the trout only treatment than in the other two treatments. Moreover, the number of grazing Baetis observed over algal-cultured ceramic tiles was lower in the two different treatments with trout than in the fishless controls. Periphytic chlorophyll-a biomass reflected trout's effect on Baetis as chlorophyll-a biomass was similar in all enclosures with trout (with and without pike), and these enclosures had higher biomasses of chlorophyll-a than the fishless controls.