The experimental cage culture was conducted at Shirati bay, Lake Victoria from February to August 2013, to investigate the impacts of the small scale cage culture on the environment. Three locations along the cages, at the intermediate and one in the offshore (control) were sampled for water quality parameters, phytoplankton and macro invertebrates. A notable increase in nutrient concentration was observed after the set of cages among the stations. However DO, pH, and water transparency showed no major changes and was within the recommended ranges. Cyanophytes an indicator of inorganic pollution dominated before and after the set of cages, an increase in phytoplankton numerical abundance was observed after stocking of fish in cages. In addition there was an increase in the invertebrate community especially bivalves and gastropods. In conclusion we found no consistent environmental change caused by cage culture, and therefore it can be allowed in Lake Victoria, Tanzania part, with close monitoring of its impacts.

Understanding fluctuations in the abundance and distribution of fishery resources over space and time is fundamental in order to address concerns about their sustainability and the basis for their management. Stock assessment studies were conducted on Lake Victoria from 1997 to 2001, with the aim to assess the status of the stocks, in particular of Nile perch. This paper summarises the findings for the Tanzanian part of the lake and makes reference to the Kenyan and Ugandan national waters for comparison. Both fisheries independent data from bottom trawl surveys and fisheries dependent data from catch assessment and frame surveys were used. Current exploitation levels and practices are analysed and linked to growth, mortality and reproductive characteristics of the stock. Abundance estimates and distribution patterns are discussed in relation to the exploitation levels, and key threats to the sustainability of the resources highlighted. Options are provided for a sustainable management of the Nile perch fishery. The 2000 frame survey revealed an intensive fishing effort. In addition, the Nile perch fishery was found to depend largely on juvenile fish. The size at first maturity was at 54.3 cm TL (1.6 yr) and 76.7 cm TL (2.5 yr), for males and females, respectively. Over 80% of the commercial catch was below the size at first maturity for males and 99% below that for females. Bottom trawl data (88% juveniles) suggest good recruitment to the stock, but yield per recruit modelling indicates unsustainable exploitation tendencies. The current annual yield (estimated at 138 324 ± 6 229 t) is well above the estimated sustainable level (109 000 t). It is recommended to reduce the exploitation rate by 50% and to increase the size at first capture. Co-management is considered to be the most effective option to implement monitoring, control and surveillance strategies.

Nile Perch fishing in Lake Victoria is done using gillnets and long lines. These gears have been deployed in the lake indiscriminately to the extent that in early 2000s the species showed signs of over-fishing. To address this situation, a slot size of 50–85 cm total length (TL) was instituted by the partner states to protect immature fish, and large adults to replenish the stocks while at the same time harvesting mature individuals. Catch Assessment Surveys, have been conducted regularly by the partner states between July 2005 and December 2008 to generate data for setting policies, development planning, and management intervention. A study involving two-stage stratified sampling on the Tanzanian side of the lake showed that Nile Perch caught in the minimum legal 5” gillnets were immature by more than 80%. In contrast Nile Perch of ≥85 cm total length, targeted by long-lines, were seldom encountered to validate the upper limit of the slot size. We advise to abolish the upper slot size limit and introduce a lower legal limit of 7” for gillnets.

Two mesh sizes (5 and 10 mm) usually used to harvest Rastrineobola argentea in Lake Victoria were assessed from April to September 2001 to determine their suitability. It was observed that 13.2% of fish caught in 5 mm and 11.2% caught in 10 mm for April, May, June and July were of a size below maturity (44 mm standard length, [SL]). Mean size of fish caught with the 5 mm mesh was 46.8 ± 1.2 mm and that of fish caught with the 10 mm mesh was 46.8 ± 0.5 mm (SL). Catches per unit effort (CPUE) from the two meshes were not significantly different, with Haplochromis laparogramma (50.1%) dominating the catch followed by R. argentea (48.9%). Juveniles of Nile Perch, Brycinus jacksonii, Brycinus sadleri, and Barbus spp. (0.9%) were caught in small numbers. About half of the females matured at 43.5 mm and the males at 46.6 mm SL. Fish caught with the 5 mm mesh had on average 1050 ± 400 eggs and those caught with the 10 mm mesh had 918 ± 281 eggs, but the difference was not significant. However, a significant difference was noted between non-infected fish (1050 ± 400 eggs) and fish infected by the cestode Ligula intestinalis (663 ± 316 eggs). Given the absence of differences in key parameters under investigation between the fish caught with the 5 and 10 mm mesh sizes, the two meshes can be used interchangeably.