Assessment of the optimal gillnet mesh size for harvesting Brycinus nurse (Rüppell, 1832) in Lake Albert, Uganda

Abstract

Brycinus nurse (locally known as Ragooge) within the Albertine region, is one of the commercially important fish species in Lake Albert. The species is harvested using undersize mesh gillnets that are prohibited under the Ugandan fishing regulations, raising concerns among fisheries stakeholders. These concerns are further exacerbated by the absence of scientific information on key life history traits such as sizes at maturity and optimal mesh size for harvesting this species in Lake Albert, hindering formulation of sustainable management strategies for the species. This study evaluated gillnet mesh size selectivity for B. nurse in the Ugandan waters of Lake Albert and identified the optimal mesh size for sustainable harvesting of the species to help develop species-specific management regulations. The study was conducted at four sites (Wanseko, Butiaba, Kaiso, and Ntoroko), covering both inshore and deep open water habitats. Fish sampling involved a range of gillnet mesh sizes from 25.4 mm (one inch) to 101.6 mm (four inches, which is the minimum legal mesh size for Lake Albert in Uganda). Life histories traits, including size structure, length-weight relationships, relative condition, and size at first maturity, were evaluated to assess status of the species. The length (fork length – FL) at 50% maturity of B. nurse in Lake Albert was estimated at 92.1 mm for females and 84.8 mm for males. The smaller gillnets (25.4 mm – 50.8 mm) caught a high proportion of both the target species and immature stages of various large-bodied species, raising concerns about stock sustainability. Gear selectivity analysis showed that 38.1 mm mesh gillnets were effective in catching mature B. nurse above the size at 50% maturity but also caught a wide variety of by-catch species. Based on these results, the 38.1 mm gillnet mesh size is recommended as the most effective and sustainable gear for harvesting B. nurse.