Catch distribution and size structure of Nile tilapia (Oreochromis niloticus) in Lake Tana, Ethiopia: Implications for fisheries management

Abstract

Nile tilapia (Oreochromis niloticus) is the most important species for consumption and economic purposes in Lake Tana, yet, tilapia stocks have declined drastically over the past decades because of anthropogenic impacts. Thus, effective conservation management is required urgently. The aim of this study was to investigate the spatiotemporal distribution and size structure of O. niloticus populations in Lake Tana in order to evaluation conditions to promote sustainable exploitation. Data were collected monthly at three sites on the lake from April 2016 to March 2017. The size at first maturity was computed using logistic regression. The catch per unit effort (CPUE) of O. niloticus in Lake Tana has sharply declined from 75 kg trip⁻¹ in 2001 to 24 kg trip⁻¹ in 2016/2017. The mean CPUE values between sites and seasons were significantly different. The size at first maturity for O. niloticus was 21 cm in 2016/2017. Of the catches taken, more than 24% of those were motorised and reed boat fisheries and 46% of the chase-and-trap fishery catches were below the size at first maturity in 2016/2017. Additionally, large size specimens were rarely recorded. Widening the mesh size of the gillnets would give a chance for immature fish to spawn at least once. Considering the fish size distribution and social context of the fishers, limiting mesh size could be a more feasible management option than a closed season.

Keywords: catch per unit effort, CPUE, chase-and-trap fishery, immature fish, length at first maturity