Gillnet selectivity and its’ effects on population structure of two key species in Lake Victoria (Kenya)

Successful fisheries management requires estimation of gear selectivity for optimum exploitation of the resource. The aim was to document the effects of gillnet selectivity on population structure of two key species in Lake Victoria: Lates niloticus (Nile perch) and Oreochromis niloticus (Nile tilapia) and come up with the most appropriate gillnet mesh sizes to be used in Lake Victoria for sustainable management of the fisheries. The study was conducted in the lake where predefined sampling areas were identified and sampled. Standard gillnetting methods were used in the sampling sites. A fleet of monofilament gillnets whose mesh size ranged from 1” to 8” at ½” intervals were set during the day for a period of not less than 3 hours, while a fleet of multifilament gillnets with mesh sizes ranging from 1” to 8” stretched mesh size at 1” intervals were set overnight in triplicates.The catch rates increased with an increase in mesh size and was significantly different between mesh sizes for Lates niloticus and Oreochromis niloticus (p ˂ 0.001). The mesh sizes 1” to 2” were very efficient in catching Nile perch, contributing about 86.8% of the catch. The 4” mesh size gillnet was most efficient in catching Nile tilapia, contributing about 83.7% of the catch. The models for gillnet selectivity showed that gillnets were optimally selective from mesh sizes 7" and 5" for Nile perch and Nile tilapia respectively. Nile perch and tilapia gillnet fishery is currently being overexploited due to rampant use of illegal and undersize gillnets. The future of Nile perch fishery is threatened by fishing pressure, thus, this study recommends shifting the minimum stretched mesh size for Nile perch to 7” and above and retention of the 5" mesh size for tilapia.