The effect of fish predation on benthic macroinvertebrates in a seasonal stream in north-western Zimbabwe

Abstract

The cumulative impact of the entire fish assemblage on benthic macroinvertebrate assemblages was investigated over four months in a removal experiment in isolated pools that persist through the dry season, in an intermittent stream in north-western Zimbabwe. Macroinvertebrate taxonomic richness did not differ significantly between sampling dates, indicating that fish removal had no effect on the zoobenthos taxa richness but led instead to large increases in the densities of certain macroinvertebrates. There was a progressive increase in the body size of Odonata in fishless pools 34 and 55 days after treatment and, by 78 days post-treatment, the proportion of large-sized odonates was significantly higher in fishless than in control pools. Peak densities of predaceous invertebrates coincided with a sharp decline in macroinvertebrate densities in the fishless pools about three months after fish removal. The values for Strauss\'s food selection index were low (range –0.220 to 0.180) for all macroinvertebrates, indicating random feeding by fish. Results indicate that, although fish may be important predators, they are not keystone predators because the macroinvertebrate community structure in this temporary habitat was found to be influenced by the assemblages of both vertebrate and invertebrate predators, rather than by a single keystone predator.

Keywords: biomanipulation, invertebrate predators, predation impacts, species assemblages, taxa richness, vertebrate predators

African Journal of Aquatic Science 2007, 32(3): 251–257