Gear selectivity of fishery target resources in Lake Koka, Ethiopia: evaluation and management implications

We evaluated the statistical models that best describe the selectivity of common fishing gears used in Lake Koka for four freshwater species widely distributed in Ethiopia and elsewhere: tilapia Oreochromis niloticus (L.), catfish Clarias gariepinus (B.), common carp Cyprinus carpio (L.), and barb Labeobarbus intermedius (R.). Sampling was conducted from October 2012 to March 2013 using gillnets (60, 80, 100, and 120 mm stretched mesh), longlines (4/0, 6/0, and 8/0 hooks size), and a beach seine with covered codend. Size at maturity (Lm) was determined for each species. The SELECT method was used to explore unimodal selectivity models for gillnet and longline gears, while the logistic function was used for beach seine. Results show that a lognormal model best described gillnet selectivity for all species, while a normal scale model best described longline selectivity for C. gariepinus. Gillnets with C100 mm mesh and longlines with[4/0 hooks could be safely used, as they allow target resources to attain Lm before becoming vulnerable to fishing. The mesh size of the beach seine needs to be enlarged, as the estimated length of capture was much smaller than all Lm values. The results have important management implications for protecting juveniles and mega-spawners of the studied species in Lake Koka and beyond.