Resource use within the crab-eating guild of the upper Kairezi River, Zimbabwe

Abstract

ABSTRACTCape clawless otters Aonyx capensis and African mottled eels Anguilla bengalensis were suspected of reducing numbers of rainbow trout Oncorhynchus mykiss in the upper Kairezi River, Zimbabwe, by preying on them and competing for the river crab Potamon perlatus. The analysis of otter, eel and trout diets showed that this is unlikely. Trout was an unimportant prey item for otters, none were found in the stomachs of 13 large eels, and any predation was probably minimised by the low densities of these predators. Some competition was probably present because crabs occurred in the diets of otters (relative occurrence 41.9%), eels (63.2%) and trout (7.4%), but it was impossible to quantify. Since these predators shared the crab food resource, the guild concept provided the basis for an analysis of their potential competitive relationships. Comparisons between the sizes of crabs selected and those available suggested that resource partitioning was occurring. Trout selected small crabs from the invertebrate drift, and because they were restricted by their mouth gapes. Otters and large eels ate larger, more abundant crab sizes; it is proposed that they instead partition resources spatially by feeding in separate micro-habitats. Intraguild predation by otters on eels may benefit trout by reducing competition for aquatic insects. Competition within the guild probably recedes in the dry season, when eels become inactive.