Exploitation of Brycinus nurse (Rüppell, 1832) and Engraulicypris bredoi (Poll, 1945) in lake albert, Uganda: implications for management

Abstract

Lake Albert supports a diverse artisanal fishery characterized by multi-species exploitation using a wide range of fishing gears. The fishery is increasingly dominated by two small pelagic fish species (SPS), Brycinus nurse and Engraulicypris bredoi, which are primarily harvested using vertically stacked small seine nets operated in conjunction with artificial light attraction. Despite their growing importance, the ecosystem-level impacts of these fishing techniques on both target and non-target fish stocks remain poorly understood. The use of these light assisted gears has also generated conflicts with fishers targeting large-bodied species, particularly Nile perch (Lates niloticus), due to perceptions that these gears negatively affect the recruitment and availability of larger species stocks. This study was conducted on the Uganda portion of Lake Albert between September 2019 and July 2022, with the aim of evaluating the socio-ecological effects of light-attraction fishing techniques as a basis for informing sustainable fisheries management. A mixed-method approach was adopted, integrating literature reviews, experimental fishing, catch assessment surveys, hydroacoustic assessments, and stakeholder consultations. These methods were used to quantify catch rates, species abundance and distribution, life-history traits, exploitation patterns, and biological reference points relevant to the management of the lake’s artisanal fisheries. Results indicated a substatial light-attraction fishing effort on Lake Albert, accounting for approximately 40% of the lake’s active fishing vessels. Fishing activity peaked during moonless nights and was significantly (P<0.05) associated with higher SPS catch rates. High bycatch, unregulated expansion of light-fishing effort, excessive net stacking, and the increasing use of solar-powered fishing lights emerged as the principal drivers of user conflict within the fishery. Marked spatial variation in SPS abundance was observed across the lake. Engraulicypris bredoi dominated catches in the central region (F (2, 15240) = 172.45, p<0.001), whereas B. nurse was more abundant in the southern region (F (2, 4447) = 41.166, p<0.001). Hydroacoustic survey and artisanal catch statistics consistently demonstrated the dominance of SPS, which constituted approximately 82% and 70% of total biological scatter and catch volume, respectively, indicating a high standing biomass of small pelagic species stocks in Lake Albert. Analysis of life history traits revealed pronounced sexual dimorphism in length at 50% maturity (Lm50) for both species. Male E. bredoi matured at a larger size (31.6 mm) than females (23.9 mm), whereas female B. nurse matured earlier and at a larger size (60.9 mm) than males (56.8 mm). Both species exhibited positive allometric growth and high condition factor: B. nurse (b = 3.21, K = 2.08) and E. bredoi (b = 3.06, K = 1.27), suggesting favourable physiological condition and relatively healthy stock status. Experimental fishing trials demonstrated that light attraction significantly enhanced SPS catch rates (p<0.05), with near-zero catches recorded under control treatments without light. This finding corroborates observed fishing patterns characterized by intensified effort during dark nights and confrims the functional necessity of artificial light in SPS exploitation. Bycatch, particularly of Lates niloticus, Oreochromis niloticus, Hydrocynus forskahlii, and Distichodus niloticus, was predominantly recorded in shallow inshore waters (< 20 m depth). These habitats serve as critical zones for biodiversity conservation, recruitment, and stock replenishment, underscoring the need for their protection from intensive fishing pressure. Net stacking was found to significantly increase both SPS catch rates and bycatch. Optimal SPS yields with comparatively minimal bycatch were achived at a net depth of approximately 16 meters (eight panels), suggesting a a potential management threshold for regulating fishing effort. Estimates of maximum sustainable yield (MSY) and fishing effort at MSY indicated that current exploitation levels for both E. bredoi (~90,000 t; 2500 boats) and B. nurse (~37,000 t; 2000 boats) remained below 2022 observed thresholds, implying that SPS were not yet fully exploited. These findings have important implications for sustainable management of Lake Albert’s SPS fisheries. The study recommends revising existing fishing regulations to incorporate SPS-specific management measures, including species-specific licensing, protection of inshore breeding and nursery habitats, regulation of net stacking, and promotion of standardized solar fishing lights. In addition, strengthening regional collaboration for comprehensive stock assessments and harmonized fisheries management frameworks is essential. These measures, when implemented could contribute to reduction in bycatch, mitigate fishing conflicts, and promote the long-term coexistence of multi-species fisheries within the Lake Albert ecosystem.