The Role of Resource Partitioning and Size Structure in The Coexistence of Nile Perch (Lates Niloticus Linnaeus 1758) Al'.1) Haplochromine Spp., In Lake Sare, Western Kenya

Abstract

The disappearance of haplochromines in Lake Victoria has been attributed to. among other factors, predation by the introduced Nile perch, Lates niloticus (Linnaeus, 1758). Lake Sare a satellite lake with an open direct dispersal corridor to Lake Victoria exhibits a unique phenomenon of coexistence of Nile perch and haplochromines. Other satellite lakes: Kanyaboli and Namboyo of Lake Victoria are home to several endemic haplochromines but are free from Nile perch invasion. This study has investigated the current size structure of Lake Sare Nile perch to establish their impact on the haplochromine population and the role of resource partitioning between the two fishes in order to determine the factors that control their co-existence. Surveys were conducted and a total of 1317 fish specimens obtained including 375 haplochromines, 315 Nile perch and 627 specimens comprising mainly of Brycinus spp., Mormyrids, Synodontis spp., Gambusia sp. and tilapiines. Species diversity Shannon-Wiener Index (H'= 1.8429), species richness (d =1.2529) and evenness (J =0.8004) were relatively low. The Nile perch population was predominantly immature (85.7%); in contrast, the haplochromine population comprised about 60% mature stages. There was minimal dietary overlap between Nile perch and haplochromines as shown by similarity index of shared food categories; plant materials (0.6090) where intake in Nile perch was of insignificant quantity and probably resulted from incidental intake rather than deliberate feeding, insects (0.2102) and Molluscs (0.1238). Although haplochromines of Lake Sare do not have an absolute size refuge from Nile perch, most of the haplochromines' body depth measurements fall within the same range as most mouth gape measurements of Nile perch (range 12 mm to 45 mm) thus according partial size refuge to the former, leading to their coexistence. Generally, the physico-chemical parameters were uniform throughout the lake. However, secchi disc transparency was highest in the open water portion of the lake, whereas turbidity was highest where River Yala enters the lake. Lake Sare haplochromines could potentially be a source of seeds of resurgence in Lake Victoria as there is an open direct dispersal corridor between the two lakes.