I.J.M. Cornelissen

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

We studied the effects of environmental driving factors (maximum depth, visibility, oxygen, temperature, and prey densities) on the distribution and diet composition of Nile perch (Lates niloticus) in south-east Lake Victoria from 2009 to 2011. We tested the hypotheses that (i) Nile perch distribution is regulated by the same environmental factors on a local scale (Mwanza Gulf) and on a regional scale (Mwanza Gulf, Speke Gulf and the open lake in Sengerema district), and (ii) driving factors act differently on different Nile perch size classes. Fish were sampled with gillnets. Nile perch densities were highest in the shallow part of the Mwanza Gulf and during the wet seasons, mainly caused by high densities of juveniles. The environmental driving factors explained Nile perch distributions on both regional and local scales in a similar way, often showing non-linear relationships. Maximum depth and temperature were the best predictors of Nile perch densities. Prey densities of shrimp and haplochromines did not strongly affect Nile perch distributions, but did explain Nile perch diet on a local and regional scale. We conclude that abiotic variables drive Nile perch distributions more strongly than prey densities and that feeding takes place opportunistically.

Heterogeneity in food-web interactions of fish in the Mwanza Gulf, Lake Victoria: a quantitative stable isotope study

Stable isotope analyses and derived population-level metrics were used to quantitatively analyse spatial and seasonal heterogeneity in the fish trophic dynamics in relation to environmental variables in Mwanza Gulf, Lake Victoria (Tanzania). The fish community in Lake Victoria, including the top predator Nile perch, is generally omnivorous with a heavy reliance on invertebrates. This is in contrast to findings based on stomach content analyses of Nile perch, which showed a stronger reliance on fish. We tested two hypotheses: (1) during the rainy seasons multiple carbon sources influence the food-web structure inside the Gulf, leading to increased carbon ranges and trophic diversity. (2) During dry periods, the food-web structure mainly relies on pelagic primary production, reducing carbon ranges and trophic diversity. Carbon sources indeed varied seasonally and spatially, affecting the fish community at the highest trophic levels. With the onset of rains, carbon sources became spatially highly differentiated with enriched δ13C values of fish in shallow water inside the Gulf and depleted δ13C values in open waters. Metrics associated with niche size correlated significantly with seasonally varying environmental variables, while δ13C ranges correlated with spatially varying environmental variables.

Assembling the pieces of Lake Victoria's many food webs: Reply to Kolding

Stockholm University, Department of Systems Ecology, SE-10691, Stockholm, Sweden Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University, P.O. Box 47, NL-6700 AA Wageningen, Netherlands Netherlands Environmental Assessment Agency, P.O. Box 303, 3720 AH Bilthoven, Netherlands Institute of Biology, Leiden University, Sylviusweg 72, 2300 RA Leiden, Netherlands Aquaculture and Fisheries Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH Wageningen, Netherlands Department of Aquatic Ecology, Netherlands Institute of Ecology, P.O. Box 50, NL-6700 AB, Wageningen, Netherlands