Empirical Assessment of the Effectiveness of Minimum Legal Size Limits in Managing Nile Tilapia Fisheries in Kainji Lake, Niger State

Minimum legal size limits have been established for Nile tilapia in the Kainji Lake fishery under Nigerian federal fisheries regulations, but empirical evaluation of the biological effectiveness and compliance levels associated with this management tool has not been conducted. This study empirically assesses the effectiveness of minimum legal size limits for Oreochromis niloticus in the Kainji Lake artisanal fishery through an analysis of six years of catch size frequency data from monthly market sampling, combined with compliance monitoring observations at six landing sites. Size-at-first-maturity, growth parameters, and exploitation rate were estimated from length-frequency analysis using FiSAT II. The proportion of fish harvested below the minimum legal size of 20 cm total length was computed for each year and landing site. Results showed that 48% of all Oreochromis niloticus landed were below the minimum legal size, with minimal change in this proportion over the six-year period despite the regulations being in force throughout. Exploitation rate was estimated at F/Fmsy = 1.34, confirming overfishing. Compliance monitoring revealed that only 12% of fishing trips using gill nets deployed legal mesh sizes. Fishers cited economic pressure and lack of alternative income sources as reasons for harvesting undersized fish. The study demonstrates that minimum legal size limits have been largely ineffective as a management tool in Kainji Lake due to systemic non-compliance and recommends complementary approaches including gear modification incentives, alternative livelihood programmes, and community-based rule enforcement.

Keywords: minimum legal size, Oreochromis niloticus, compliance, Kainji Lake, fisheries management