Effects of the louisiana rayfish (procambarus Clarkii) invasion on the food and territorial ecology of the African clawless otter (Aonyx capensis) in the Ewaso Ng'iro ecosystem
Ogada, Mordecai Owidi
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The introduction of the Louisiana red swamp crayfish (Procambarus clarkii Girard) into the Ewaso Ng'iro river ecosystem has been one of the most profound human impacts on this system in recent years. The crayfish has spread throughout the lower Ewaso Ngi'ro, becoming a new food source, competitor and predator to various organisms at different levels in the food chain. This was a two-year study covering two complete long and short rainy seasons as well as the intervening dry seasons. The study also aimed to provide information on the status of the African clawless otter (Aonyx capensis Schinz, 1821), which is unknown in much of its range. The study examined the impacts of the crayfish invasion on the African clawless otter and its primary food source, the indigenous freshwater crab (Potamonautes neumannii), which are predators and competitors to the crayfish, respectively. Trap sampling indicated that crayfish have supplanted indigenous crabs in much of the lower Ewaso Ng'iro River north of the equator. This was confirmed by experimental competition between captive crabs and the crayfish, where crabs in a tank without refuges were killed significantly faster (mean diff--56, 't'value=9.058, df--9, p<0.0001) than those in the tank with rocks for refuge. Crayfish are excluded from the upper Ewaso Ng'iro and its tributaries south of the equator by low water temperatures. Since the crayfish forms such a large proportion of the otter's diet, results of this study showed that the environmental factors affecting the crayfish also influence habitat use by otters. This was investigated mainly by collection of otter faeces within the study area and laboratory analysis of the same. Territorial behaviour of the otters was inferred from the distribution of the faecal deposits. Temperature and depth of water were measured to deduce the effects of these factors on the river ecosystem, within which the otters are functioning as tertiary consumers. The above abiotic factors are influenced by human impacts such as siltation, water extraction and construction of reservoirs. This study has shown that the territorial behaviour (marking frequency) of otters in the Ewaso Ng'iro ecosystem is significantly lower (mean diff -30.158, df--11, `t'value=-5.030, p<0.0005) in areas that have been invaded by the Louisiana crayfish than in the pristine (crab) areas. Crabs were found to contain significantly higher protein content per unit live weight than crayfish (mean diff =7.449,df--19, `t'=17.281, p<0.0001). Consequently, carrying capacity for otters was also reduced by the invasion and density was significantly higher (mean diff.=5.198, `t'value=10.958, df--11, p<0.0001) in the pristine areas. Trap sampling showed a 75% reduction in diversity of the sampled segment of aquatic fauna during the study period. This study recommends how best people can exploit river ecosystems with minimum effect on aquatic fauna. This aspect urgently needs attention because currently, there is no regulation on the introduction of alien species, and sport fishermen continue to stock aquatic ecosystems with exotic species without any due consideration to the ecological impacts of these activities.