Simulating Mesh-size and Selection Pattern Impacts on the Lake Turkana Fisheries Sustainability in Kenya

Abstract

The study compared impacts of different gear mesh sizes and landing sizes (Lcap) through gillnet and knife-edge selection patterns on Tilapia and Nile perch of Lake Turkana. The results will help contribute to the debate on whether to capture large and protect small fish or vice versa. R simulation employing the 1934 Thompson and Bell's model for data-deficient stocks was used to simulate fish cohorts using secondary input data for the two species. Gillnet selection pattern with recommended mesh size (127 mm) produced an 80% increase in Tilapia YPR while ESS and SPR ratios reduced by 12.5%. and 14.3% respectively when fishing mortality (F) was tripled to 3.0. Nile perch's YPR increased by 90% while ESS and SPR dropped by 70 and 75% each when F increased to 0.8. Tilapia YPR maximized by reducing mesh size to 101.6 mm at F=2, and maintained ESS and SPR at 20% each, while the Nile perch YPR dropped by 20% and 66.7% respectively at F=1.0. Small mesh-sized (<40 mm) gears had no effect on Tilapia’s ESS and SPR but produced the lowest YPR ratios. The 152.4 mm mesh for Nile perch produced a 25% YPR increase and maintained optimal ESS and SPR at F=0.5. Knife-edge selection pattern utilizing 127 mm, 20 cm Lcap and F=1.5, produced 50% YPR, ESS, and SPR for Tilapia, and maintained at F=3.0. Large-sized Nile perch using 152.4 mm gear resulted in about 40% YPR of the stock. Reduction of Lcap using 127 mm gear, produced a 75% increase in YPR. ESS and SPR depicted Nile perch collapse at F>0.65. Mesh size and selection pattern impacts are dependent on F. Small mesh gillnets are not damaging if F is controlled and caution should be taken with them in a knife-edge selection. This study recommends 101.6 mm and 152.4 mm mesh sizes for Tilapia and Nile perch respectively in Lake Turkana