Towards sustainable exploitation of Nile perch consequential to regulated fisheries in Lake Victoria

Abstract

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.

The Environment of Lake Victoria (East Africa): Current Status and Historical Changes

“…By 2007 the total catch had risen to around 1,000,000 t with the Nile perch catch being around 240,000 t (25% of the total) with Rastrineobola, Oreochromis and haplochromines making up the balance. The decreased proportion of Nile perch in the catch led to concerns that the stock was becoming overfished (MATSUISHE et al, 2006;NJIRU et al, 2007;MKUMBO et al, 2007) and the biomass of Nile perch in the lake has indeed declined significantly over the last decade (LVFO, 2008). SIMONIT and PERRINGS (2005) asserted that landings of all [our italics] fish had declined since 1994 and, while recognising the contribution of overfishing, suggested that eutrophication was an important factor.…”

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

“…In contrast, during the Nile perch boom, from 1987 to 1990, Nile perch densities were homogeneous across the regions of Mwanza and Speke Gulf (Goudswaard et al, 2008). More recent stock assessments through bottom trawling and catch assessments of Nile perch in the Tanzanian part of the lake from 1997 to 2002 showed that Nile perch aggregated on a very local and temporal scale in shallow and deep waters and at different depths of the water column (Mkumbo, 2002;Mkumbo et al, 2007). However, on a large spatial scale, there was no consistent pattern in Nile perch densities between the east, west and south side of the Tanzanian part of the lake or between different depths up to 50 m (Mkumbo, 2002;Mkumbo et al, 2007).…”

“…Since the start of the fisheries concerns have been voiced that Nile perch stocks are declining and may collapse if fishing pressure will continue to increase (Mkumbo, 2002;Balirwa et al, 2003;Getabu et al, 2003;Ogutu-Ohwayo, 2004;Matsuishi et al, 2006;Mkumbo et al, 2007). However, stock assessments based on Nile perch catches and population size structure showed no signs of overfishing or long-term decline of the Nile perch stock, although, since the 1990s the stock shows short-term decadal fluctuations (Kolding et al, 2008).…”

“…More recent stock assessments through bottom trawling and catch assessments of Nile perch in the Tanzanian part of the lake from 1997 to 2002 showed that Nile perch aggregated on a very local and temporal scale in shallow and deep waters and at different depths of the water column (Mkumbo, 2002;Mkumbo et al, 2007). However, on a large spatial scale, there was no consistent pattern in Nile perch densities between the east, west and south side of the Tanzanian part of the lake or between different depths up to 50 m (Mkumbo, 2002;Mkumbo et al, 2007). In contrast, lake-wide acoustic surveys from 2007 to 2010 showed a seasonal spatial variation in Nile perch distributions, with higher densities in shallow waters and in the upper layers of the water column as a result of vertical and horizontal migrations related to oxygen (Taabu-Munyaho et al, 2013).…”

“…Most studies on the Nile perch stock in Lake Victoria focussed on whole lake dynamics over long periods and on the impacts of the Nile perch fisheries (Mkumbo, 2002;Getabu et al, 2003;Mkumbo et al, 2007). In addition, Nile perch distribution was generally studied in open waters of more than 15 m depth where most of the commercial Nile perch fishery is concentrated (Mkumbo, 2002;Getabu et al, 2003).…”