Can differential resurgence of haplochromine trophic groups in Lake Victoria be explained by selective Nile perch, Lates niloticus (L.) predation?

Abstract

The introduced Nile perch, Lates niloticus Linnaeus 1758 in Lake Victoria is considered to be a major contributor to the decline of haplochromine species. In the 1990s, the abundance of Nile perch declined and the recovery of some haplochromine species, mainly detritivores and zooplanktivores was observed. However, the resurgence of detritivores group was much slower than that of zooplanktivores. Differential Nile perch predation is thought to be the main cause for the differential recovery of these two groups. To test this, the frequency of occurrence (FOO) of the detritivores and zooplanktivores in the stomachs of the Nile perch and their FOO in the lake were compared. On their vulnerability to predation, body shape of the two groups was measured and compared with Nile perch mouth gape. The FOO for haplochromines in Nile perch stomach contents were 13.7 % detritivores and 86.3 % zooplanktivores, whereas in trawl catches they were 24.3 and 75.7 % respectively. The FOO of detritivores in the Nile perch diet was significantly lower than in the environment. The same holds for the FOO of juvenile haplochromines (13 % in the diet and 45 % in the environment). Body depth of detritivores (0.8–1.9 cm) in the diet of Nile perches was significantly larger than that of the zooplanktivores (0.6–1.7 cm). However, based on Nile perch mouth gape (2.0–5.3 cm), only Nile perches <13 cm TL (<1.9 m mouth gape) would not be able to swallow the largest detritivores. Thus, selective predation by Nile perch cannot explain the relatively slow resurgence of detritivores.