Title: Establishing a hydroacoustic survey protocol for major commercial fish species in Lake Albert, Uganda.

Lake Albert has a total surface area of 5,300 km², shared between Uganda and the Democratic Republic of Congo at 54% and 46% respectively, with an average depth of 25 m and a maximum depth of 58 m. Lake Albert is a multi-species fishery that has supported the livelihoods of riparian communities for many decades by providing food, nutrition, and employment opportunities. The commercial species contributed to the 2020 total annual catch of 348,000 tonnes, valued at US$175.2 million in Uganda. Lake Albert’s fisheries have experienced changes in fish stocks, with a decline in the number and sizes of fish species caught (e.g., Nile perch) and the emergence of small pelagics (Ragoogi and Muziri). Information and data to guide management have remained scanty, limited to fish catches and fishing effort; however, appropriate long-term management would require hydroacoustic studies for ecosystem-based stock management. This study aimed to assess water parameters and their effects on sound speed and acoustic backscatter, characterise fish backscatters, and map the Nautical Area Scattering Coefficient (NASC) alongside fishing areas and landings of the major species. Acoustic sampling was conducted using an EK80 120 kHz transducer, and water parameters were sampled at several hydrostations. The data were processed using the Large-Scale Survey System (LSSS), and the results of the acoustic sampling protocol are discussed.

Fish distribution and acoustic backscatter varied in the three regions of Lake Albert, which could be attributed to the variability in environmental water parameters. There were significant differences in sound speed among the three regions and zones. We distinguished four fish acoustic categories: Nile perch, small pelagics, other demersal species, and other species, with the highest NASC averages of Nile perch and small pelagics in the central region. The main landing sites of E. bredoi were concentrated in the central region, with 90% of the catch proportion. B. nurse landings dominated the northern region, with 80%, whereas 75% of tilapia were recorded in the southern region. Lates spp. are the most important commercial species, and their landings are distributed across all three regions.

Trawl and hydroacoustic surveys with additional echosounder frequencies are recommended to estimate fish biomass, density, and population structure more accurately. Target strength experiments, periodic fish stock assessment studies, and water parameter sampling during stratified and mixed seasons are also recommended for the future.