Cascading Effects of the Introduced Nile Perch on the Detritivorous/Phytoplanktivorous Species in the Sublittoral Areas of Lake Victoria

Abstract

In the 1980s an explosive increase of the introduced Nile perch (Lates sp.; Harrison 1991) in Lake Victoria caused the destruction of approximately 65% of the endemic haplochromine cichlids. The eradication of approximately 200 vertebrate species in less than a decade may well represent the largest extinction event among vertebrates during this century. The introduction of the Nile perch can be considered as a large‐scale, albeit unintended, experiment. Farreaching changes in the food web are taking place. We present data on the importance of haplochromines in the ecosystem prior to the Nile perch boom. An analysis of the pelagic community in the sublittoral area of the Mwanza Gulf revealed that the phytoplanktivores formed 18% of the biomass of the total haplochromine community. In the benthic community, the detritivores that frequently included phytoplankton in their diet comprised 31% of the biomass. We examine the hypothesis that algal grazing was reduced by the disappearance of haplochromine phytoplanktivores and detritivores. The disappearance of these groups may have contributed to the recent algal blooms. To investigate this hypothesis we consider a second major change in the system. Coinciding with the decrease of the haplochromines, the number of atyid prawns, Caridina nilotica, has strongly increased. We present preliminary evidence that the stock of the detritivorous haplochromines that formerly comprised most of the demersal ichthyomass has (partially?) been replaced by the prawn. We discuss the possible mechanisms underlying this major change in the food web. With respect to the conservation of the Great African Lakes, tbe collapse of Lake Victoria’s ecosystem should serve as an example of how easily a complex ecosystem can be irreversibly destroyed.

The invasion of an introduced predator, Nile perch (Lates niloticus, L.) in Lake Victoria (East Africa): chronology and causes

“…Apart from zooplankton and insect larvae, juvenile shrimps also appeared to be an important food item for small (<10 cm) Nile perch (Goudswaard et al 2006;Katunzi et al 2006). Both shrimps and insect larvae increased after the disappearance of the haplochromines (Kaufman 1992;Mbahinzireki 1992;Goldschmidt et al 1993;Goudswaard et al 2006), which were their predators. In this situation adult Nile perch facilitated the food availability for juvenile Nile perch by eradicating the former competitors of their juveniles.…”

“…The effects of the Nile perch boom on the indigenous fauna of Lake Victoria, has been discussed in several publications (Barel et al 1985;Hughes 1986;Acere 1988;Harrisson et al 1989;Ogutu-Ohwayo 1990a;Kudhongania et al 1992;Witte et al 1992a, b;Goldschmidt et al 1993;Bundy and Pitcher 1995;Kudhongania and Chitamwebwa 1995;Goudswaard and Witte 1997;Goudswaard et al 2002a, b). Remarkably, the process by which Nile perch established itself successfully in Lake Victoria is an underexposed aspect in the whole discussion.…”

The shift to smaller zooplankton in Lake Victoria cannot be attributed to the 'sardine' Rastrineobola argentea (Cyprinidae)

“…Average catch rates (kg·h -1 ) per year of all haplochromines were taken from Witte et al (1995). We assumed that 27% (in weight) of the total haplochromine catch consisted of zooplanktivores (Goldschmidt et al, 1993). Fresh weight (FW) was converted into dry weight (DW) using the relationship DW = 0.246 FW (Douthwaite, 1976), in order to achieve an estimate of biomass comparable to the dry biomass estimated for haplochromines and Nile perch in later years.…”

“…However, in 1982 the abundance of dagaa, in terms of fresh weight, has been estimated to be one-third of that of the haplochromine zooplanktivores (Goldschmidt et al, 1993). After converting the fresh weight of dagaa to dry weight with the relationship DW = 0.224 FW (Douthwaite, 1976), we have used this as a basis.…”

Responses of Aquatic Non-Native Species to Novel Predator Cues and Increased Mortality

“…Impacts of aquatic invaders can result in huge economic costs through damage to infrastructure (Galil & Zenetos 2002, Leppäkoski et al 2002, Davidson & de Rivera 2012 and predation on fishery species (Glude 1955, Travis 1993, Jamieson et al 1998. Biological invasions have also triggered shifts in community structure, size structure of native species, and in extreme cases local extinctions (Zaret & Paine 1973, Goldschmidt et al 1993, Grosholz et al 2000, Grosholz 2002, Blackburn et al 2004. Numerous laws, educational efforts, techniques and technologies have been developed to prevent the further distribution of propagules (Hewitt & Campbell 2007, Tsolaki & Diamadopoulos 2010, Cooper et al 2012.…”