Differential decline and recovery of haplochromine trophic groups in the Mwanza Gulf of Lake Victoria

Abstract

Lake Victoria had a fish fauna dominated by 500+ species of haplochromine cichlids that made up more than 80% of the fish mass. The five main trophic groups caught with bottom trawlers in the sub-littoral areas of the Mwanza Gulf were: detritivores, zooplanktivores, insectivores, molluscivores and piscivores. The detritivores (13+ species) formed the most important guild, making up 60-80% of the number of individuals, followed by the zooplanktivores (12+ species), which comprised 10-30%. In the 1980s the haplochromines from the sub-littoral and offshore areas (estimated at some 200 species) vanished almost completely. Commercial trawl fishery, the upsurge of the introduced Nile perch, and an increase of eutrophication were potential causes of this decline. In the 1990s, when Nile perch was heavily fished, a recovery of some haplochromine species was observed. We studied the decline and partial recovery of the different haplochromine trophic groups in the northern part of the Mwanza Gulf. The rate at which the trophic groups declined differed; the relatively large piscivores, insectivores and molluscivores were the first to disappear from the catches. The small detritivores and zooplanktivores declined at lower rates, especially the latter group. From the beginning of the 1990s a resurgence of both groups was observed. By 2001, the zooplanktivores had reached their previous level of abundance, but their diversity declined from more than 12 species to only three. Though four detritivorous species began being regularly caught again, they constituted only about 15% of the number of individuals, while the zooplanktivores made up more than 80%. The patterns of decline and recovery indicate that, though fishery played a role locally, predation by Nile perch and eutrophication were the main factors determining the fate of the haplochromines. However, it has so far been impossible to establish the causal relationship between the two, and the relative impact of each of these phenomena separately. The potential effects of the changed trophic dominance, and the importance of the haplochromines for the ecosystem and a sustainable fishery, are discussed.

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

“…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.…”

Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services

“…Surprisingly, haplochromine biomass started recovering in the early 1990s (Seehausen et al 1997b) and quickly achieved precollapse pelagic stock biomass (Witte et al 2007b). Haplochromines now represent up to 80% of the pelagic fish biomass, and together with dagaa, they constitute more than 50% of the lake's total fish biomass (Tumwebaze 1997, Wanink 1999, Kayanda et al 2009).…”

Cichlid species diversity in naturally and anthropogenically turbid habitats of Lake Victoria, East Africa

“…The division in these three periods is based on the following observations in the Mwanza Gulf: (1) dominance by weight, and great diversity, of haplochromine cichlids in bottom trawl catches in sub-littoral waters of the Mwanza Gulf until 1984 (Witte et al 1992); (2) the almost complete collapse of the haplochromine populations in sublittoral waters of the Mwanza Gulf (except for those in rocky areas with clear water) in the period between 1986 and 1990 (Witte et al 1992;Seehausen et al 1997a), together with algal blooms, and low DO concentrations and Secchi values since 1986 (Wanink et al 2001;Witte et al 2005); (3) recovery of some haplochromine species in sub-littoral waters since the 1990s (Witte et al 2007b).…”

“…It appears that over-fishing may have had a negative impact on haplochromines locally, but this is unlikely to be a major driver of the lake-wide loss of cichlid diversity (Witte et al 2007b). Discriminating between the effects of Nile perch predation and eutrophication is more difficult, as they occurred in the same period (Seehausen et al 1997a;Verschuren et al 2002;Witte et al 2007a, b;Kolding et al 2008;Hecky et al 2010).…”

“…The same gear was used as at stations A to D, but trawl shots lasted 10 min (for further details see Witte et al 1992Witte et al , 2007b. The methods for the three sampling periods were the same, except that in the pre-collapse period all fish [5 cm SL were considered as adults, and in the last two periods all fish [4 cm SL because size at first maturity had decreased strongly in the course of the 1980s .…”