Feeding and grazing in Lake George, Uganda

Abstract

The fauna of Lake George is dominated by herbivores, in particular the fish Tilapia nilotica and Haplochromis nigripinnis , and the cyclopoid copepod Thermocyclops hyalinus . Digestion, and the assimilation of carbon from algae, by these herbivores were studied in detail. It was found that, contrary to previous observations reported in the literature, both fish and zooplankton were able to digest and assimilate blue-green algae. The diurnal feeding patterns of the herbivores were examined, and methods devised for assessing, in terms of carbon, the daily ingestion of algae. Hence, using biomass data, the total amount grazed by the herbivores was calculated. Estimates of carbon requirements at other trophic levels were made, as was an assessment of the required level of net production by the algae for comparison with figures for standing crop and net algal production measured by other means. Food selection by secondary and tertiary producers is discussed, and in several species age correlated changes in selectivity were examined. Both the major herbivorous species of fish adopt phytoplankton feeding after a period of carnivorous or omnivorous feeding as fry. Thermocyclops hyalinus is herbivorous all its life, but the size of particle taken changes with age. There are more species of carnivorous than herbivorous fish and these exploit a wider variety of food sources; the few species studied also show changes in food preference with age.

Seasonal variations in body length and fecundity of 2 copepod species: Thermocyclops crassus (Fischer, 1853) and Eudiaptomus drieschi (Poppe & Mrázek, 1895)

“…However, it appears that chlorophyll-a concentration is not always sufficient to explain the observed egg production (Saiz et al, 1999). Furthermore, adults of T. crassus consume not only algae (Moriarty et al, 1973;Infante, 1978;González, 1998) but also nauplii, rotifers, and cladocerans (Gras et al, 1971). In this study, however, chlorophyll-a concentration affected egg production of both species.…”

Did the loss of phytoplanktivorous fish contribute to algal blooms in the Mwanza Gulf of Lake Victoria?

“…To estimate phytoplankton consumption we used the formulae of Moriarty et al (1973), who calculated ingestion rates based on diurnal changes in dry weight of stomach and intestine contents, for the two main phytoplanktivorous fish species of Lake George: H. (Enterochromis) nigripinnis Regan and Oreochromis niloticus (L.).…”

“…In the diet of the phytoplanktivores, cyanobacteria dominated. They were shown to be digestible by tilapiine and haplochromine cichlids at stomach pH values below 2.0 (Moriarty et al, 1973).…”

“…In this study we determined the biomass and trophic composition of the major fish taxa in the entire column of the sublittoral (6-20 m deep) waters of the Mwanza Gulf in 1979-1982, just before most of the phytoplanktivorous fish started to disappear. Next, we estimated the daily intake of phytoplankton by fish in the area, using these biomass data and data on ingestion rates of related cichlid species from Lake George (Moriarty et al, 1973).…”

Zooplankton capturing by Nile Tilapia, Oreochromis niloticus (Teleostei: Cichlidae) throughout post-larval development

“…This is associated with loss of visual acuity in large fish, which causes them to interrupt their selective visual predation (HJELM et al 2000). This change in eating habits may occur abruptly (MORIARTY et al 1973, BEVERIDGE & BAIRD 2000, or more gradually (WHITFIELD & BLADER 1978).…”