Kenya Aquatica

KCSAP Project

The population sizes of key commercial fish species were estimated using the catch curves and gear selectivity data obtained from experimental fishing by monofilament and multifilament gill nets.  The objective of the study was to assess the selectivity of gill nets on fish catches in Lake Turkana to inform fisheries management, and share findings with stakeholders.  Fish samples were collected using both monofilament and multifilament gill nets, and comparisons made with the fishers catches using similar multifilament gill nets.  The gill nets were set at Kerio River and Turkwel River Mouths Delta, Napasinyang River Mouth and Omo River Mouth Delta on different dates from September 2019 to December 2019 in Lake Turkana.  The gill nets were set from a fibre glass boat measuring 30 m long mounted with a 75 HP outboard engine.  A set of monofilament and multifilament gill nets consisted of ten different meshes with geometric increment in their mesh sizes (25, 38, …., 203 mm). In this study gillnet used ranged from mesh sizes of 25 mm (1 inch), 38 mm (1.5 inches), 51 mm (2 inches), 63 mm (2.5 inches), 76 mm (3 inches), 88 mm (3.5 inches), 102 mm (4 inches), 114 mm (4.5 inches), 127 mm (5 inches), 140 mm (5.5 inches), 152 mm (6 inches), 178 mm (7 inches) to 203 mm (8 inches).  Gillnet selectivity index influences the population structure of the key commercial species including Alestes baremose, Bagrus bayad, Bagrus docmak, Barbus bynii, Citharinus citharus, Clarias gariepinus, Distichodus niloticus, Hydrocinus forskalii, Labeo horie, Lates niloticus, Oreochromis niloticus, Schilbe uranoscopus, Gymnarchus niloticus and Synodontis schall in Lake Turkana, Kenya.  The mean size of fish for each mesh size were 22 cm TL in the 25 mm mesh, 25 cm TL in the 38 mm mesh, 31 cm TL in the 51 mm, 38 cm TL in 63 mm mesh, and 42 cm TL in the 76 mm mesh, and 200 cm in 203 mm mesh sizes.  The mean length for Lates niloticus caught in 88 mm mesh, 101 mm mesh, 114 mm mesh, 127 mm mesh and 203 mesh size were 29, 30, 32 and 33 cm and 200 cm TL respectively.  The higher catches of fish noted in 38 mm for Alestes baremose might be related to the selective properties of gill nets giving the resulting catches in 25 mm lower than the higher meshes.  This study recommends the optimal mesh sizes of more than 127 mm for both Oreochromis niloticus and Labeo horie.  Thus, for Lates niloticus sustainability and fish stocks replenishment, there is need for a minimum of 127 mm mesh size, targeting exploitation of older mature fish.  The significance and use of gillnet selectivity knowledge of the selectivity of any gear, especially gillnets, and allows better management of a commercial fish stock through the choice of mesh size to suit the available population of fish.  Gillnet selectivity also allows an independent estimate of the population structure of fish from the commercial catch data, something not feasible with uncorrected gillnet catch data. READ MORE