Assessment of Anguillid Eels’ Bio-Ecology and Ecological Status in Athi-Galana-Sabaki and Ramisi Rivers

Abstract

The conservation of anguillid eels in Kenya’s Athi-Galana-Sabaki (AGS) and Ramisi Rivers is crucial due to their ecological and economic significance and the increasing threats from habitat degradation. This study aimed to: analyse the population structure of anguillid eels, explore their ecological niches, and determine the ecological risks they face in these rivers. Fieldwork was conducted over 12 months, during which 393 eels were sampled using fyke nets. Population health was assessed through length-weight relationships and condition factors, while ecological niche dynamics were determined via stomach content analysis and habitat characterization. Bayesian models were applied to evaluate current and future ecological risks. The results revealed differences in population structure and ecological dynamics between the rivers. The Ramisi River supported a larger eel population (mean density 1.1 individuals/net/day) than the AGS River (0.8 individuals/net/day). Anguilla bengalensis was the most abundant species (65%), and yellow eels in the AGS River exhibited a higher mean length (48.9±13.7 cm) compared to those in the Ramisi River (34.4±12.4 cm). Silver eels also demonstrated greater lengths in the AGS River (88.0±2.3 cm) than in the Ramisi River (66.8±8.4 cm). Ecological niche analysis revealed the utilization of nine dietary items, with the highest dietary overlap observed between A. bengalensis and A. bicolor (Schoener’s index: 0.82). Anthropogenic activities, including water abstraction and riparian encroachment, were more pronounced in the AGS River, leading to greater habitat degradation (habitat integrity score of 56.6% compared to 92.3% in the Ramisi River). Risk analysis identified the yellow eel stage as the most vulnerable under future scenarios, underscoring the need for proactive conservation. This study concludes that anguillid eels in Kenya’s East-flowing rivers recruit across diverse size classes and habitats but face significant threats from human activities. It recommends urgent habitat restoration, regulation of water extraction, and community-driven conservation initiatives to safeguard these ecologically and economically vital species.