Strong Effects of Foraging Minnows on a Stream Benthic Invertebrate Community

Abstract

Behavioral responses to piscivorous fish are known to alter the spatial distribution of small invertebrate—feeding fish in streams, producing concentrations of small fish in safer areas. We asked whether such variation in the local density of small juvenile creek chubs (Semotilus atromaculatus) would, in turn, produce local alterations in the stream benthic invertebrate community. Replicated treatments of 0, 2, and 6 juvenile chubs per 0.5—m2 enclosure were established in a warm—water, soft—sediment stream. Over a 3—mo period, the presence of fish resulted in reductions of 79—90% in total invertebrate volume relative to the zero—fish treatments. Numbers were reduced 55—61%. The two major taxa, Oligochaeta and Isopoda, showed strong fish effects, and size distributions of these two taxa showed shifts to smaller sizes. Densities of less abundant taxa (e.g., Chironomidae, Sphaeriidae) showed no statistically significant fish effects. Evenness of the invertebrate community increased in the presence of fish. No differences in invertebrate densities were detected between the two— and six—fish treatments; however, the above—sediment activity of Oligochaeta was reduced more in the six—fish treatment than in two—fish treatment, and growth of the fish was slower in the six—fish treatment. Unlike previous experimental studies in stony streams, this experiment shows that foraging fish can significantly alter the behavior, population structure, and community structure of stream benthic invertebrates.