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

Abstract

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

“…A Nile perch sampled in the Gulf may have been feeding on haplochromines there, resulting in a higher proportion of fish in stomach analyses, but may have been feeding previously on Caridina shrimps outside the Gulf where Caridina densities are higher and where it is still a major prey in Nile perch diets (Ngupula & Mlaponi, 2010;Cornelissen et al, 2015). Nevertheless, Nile perch isotope values correspond more to an invertebrate-based diet and haplochromines are a less important food source for Nile perch than is suggested by stomach content analyses alone.…”

“…However, macrophytes probably contribute little to the food chain of haplochromines and Nile perch in Mwanza Gulf because they do not feed on macrophytes (Katunzi et al, 2003;Kishe-Machumu et al, 2008;Cornelissen et al, 2015), nor do their prey that feed on detritus, phytoplankton, zooplankton and invertebrates. This is consistent with studies in a satellite lake of Lake Victoria, where macrophytes contribute little to consumers in the food web (Hecky & Hesslein, 1995;Mbabazi et al, 2010).…”

“…Traditional diet analyses showed that during the Nile perch boom, Nile perch preyed heavily on the endemic haplochromines and, when haplochromines declined, they switched their diet to the increasingly abundant Caridina shrimp, their own juveniles and Dagaa (Hughes, 1986;Mkumbo & Ligtvoet, 1992;Katunzi et al, 2006). Since the 1990s, a recovery of some pelagic haplochromine species was observed in the Mwanza Gulf in the south-east of Lake Victoria (Witte et al, , 2007Kishe-Machumu et al, 2012) and Nile perch is observed to switch to haplochromines again (Kishe-Machumu et al, 2012;Cornelissen et al, 2015). Other abundant species of Lake Victoria also changed their diets.…”

“…Nile perch distributions were primarily driven by water temperature, depth and oxygen and not by the availability of particular prey species (Goudswaard et al, 2011;Cornelissen et al, 2015). As a result the diet of Nile perch also varied spatially and temporally, since it feeds opportunistically on prey which is available in its abiotic-driven distribution range (Cornelissen et al, 2015).…”

“…Mwanza Gulf is spatially and seasonally heterogeneous in water quality and the distribution and diets of fish (Cornelissen et al, 2014(Cornelissen et al, , 2015. Therefore, we hypothesize that fish trophic interactions will also vary spatially and seasonally in relation to environmental variables (O'Neill & Thorp, 2014;Schriever, 2015).…”