Biotic Interactions between Catadromous Anguillids with Sympatric Riverine Ichthyofauna along Athi-Galana-Sabaki and Ramisi Rivers, Kenya

Abstract

Biotic interactions are among the most powerful ecological drivers influencing fish species occurrence. Biotic interaction especially competition and predation are of particular concern, precipitating population declines especially migratory fish such as eels. Along the Western Indian Ocean (WIO) region, declines of three of the four catadromous Anguillid; Anguilla bicolor, A. bengalensis, and A. mossambica, have been reported and primarily blamed on anthropogenic activity but the role of biotic interactions among eels and sympatric fish remains poorly studied. This study elucidate the biotic interactions between sympatric fish and eels along two east flowing rivers in Kenya, Athi-Galana-Sabaki and Ramisi. Eels and sympatric fish from Athi and Ramisi were captured monthly using fyke nets from April 2021 to March 2022. Species occurrence data was used to evaluate species composition and habitat utilization. Data on gut-content, evaluated diet breadth and overlap between eels and sympatric fish guilds. One way Analysis of Similarity (ANOSIM) was used to detect differences between sympatric fish assemblage and eels. Data on habitat utilization was analyzed using Permutational Multivariate Analysis of Variance. ANOSIM was employed to demonstrate diet preferences between sympatric fish guild and eels. Results indicated significant differences in both sympatric fish and freshwater eel assemblage along the two rivers (R=0.49, p˂0.05). At both rivers, sympatric fish (57%) and eel (84%) were prevalent in pools but also demonstrated an active habitat selectivity on vegetation (0.03±0.002 and 0.26±0.02) respectively which were not significant different (F=0.69, p=0.81). The influence of environmental variables on occurrence of fish species was comparable (p=0.17). Eel primarily consumed assorted fish (43%), shrimps (14%) and prawns (13%). Sympatric fish on the other hand, consumed detritus (27%), shrimps (20%) and fish (14). The most significant dietary overlaps were observed between predatory sympatric carnivorous fish and A. bicolor (0.81±0.01) at Athi. At Ramisi, diets of A. bicolor (0.74±0.11) overlapped with those of omnivorous guilds. Although significant differences in diet preference occurred among fish types (R=0.09, p=0.0005), eel and carnivorous guilds were comparable (p=0.08), especially for predatory carnivorous (p=0.1). It is apparent that eel co-occur with sympatric fish, share habitats, but mostly importantly compete with exotic carnivorous fish. Therefore, with these observations, the findings imply that these species are at greater risk of previous undescribed competition attesting to their higher IUCN ranking. Further integration of stable isotopes analyses should be given credence to elucidate feeding habits and trophic interactions.