Measuring Biological Diversity

The life histories of five populations of moggel, Labeo umbratus, inhabiting small Eastern Cape reservoirs were compared and the differences related to environmental parameters. A significant positive correlation between fish growth and chlorophyll a was noted in four cases. In the eutrophic Dimbaza Reservoir, slow growth rate was attributed to intraspecific competition. Mortality was lower in populations living amongst fewer predatory species, and also where food was abundant. Length- and age-at-maturity were not affected by environmental factors, but were dependent on growth and mortality. There was a weak relationship between the fishery potential of L. umbratus populations and algal biomass, and thus it was concluded that populations in small, shallow, slightly enriched reservoirs would be those more suitable for exploitation.     

Reproductive biology of a riverine cyprinid,Labeo umbratus (Teleostei: Cyprinidae), in small South African reservoirs

The reproductive and recruitment characteristics of moggel, Labeo umbratus, populations were examined in four small South African reservoirs. Reproduction, characterised by an extended spawning season, high fecundity, short incubation time and rapid larval development, appears to be ideally suited to the highly variable environment of small reservoirs. Evidence suggested that L. umbratus spawns in the reservoirs. In two reservoirs where samples were conducted monthly, GSI (gonado-somatic index) was positively correlated with both water temperature and day length, whilst the CPUE (catch per unit effort) of juveniles was not related to any environmental variable. The success of moggel spawning appeared to increase when there was early spring and consistent summer rainfall.