|dc.description.abstract||Following preliminary observations in 1975 the
Lake Kariba Fisheries Research Institute implemented a research
program in March 1976 to investigate various aspects of the
biology and population dynamics of the mussel crop of Lake
Kariba, with a view to greater understanding of their status
in the lake, and establishing a basis for management, if
required. Aspects investigated were densities, distribution,
species composition, predation by fish, reproduction, age and
growth, gross biochemical composition and crude production
rates. The mussel community comprised four species, a small
pill clam, Corbicula africana, and three mussels, Caelatura
mossarnbicensis, Aspatharia wahlbergi and Mutela dubia. Only
the latter three were studied. Mussel beds occupied all the
gently shelving cleared and uncleared areas, and the bulk of
the populations occurred from 3 m - 9 m depth, but extending
to 11 m. Predation by fish was found to be extremely low.
The reproduction study showed two species bred all year
C. mossambicensis and M. dubia, whereas A. wahlbergi bred
seasonally during the rainy season. The two year-round
breeders were repetitive spawners, capable of breeding several
times a year. Indications are that A. wahlbergi is essentially
a fluviatile species, with a life cycle adapted to taking
advantage of riverine conditions. Sex ratios in all three
species were approximately 1 : 1 , with females predominating
slightly. Fecundity in one species, C. mossambicensis, was
investigated. The complete parasitic cycle of M. dubia was
elucidated and found to be similar to that of M. bourguignati,
as described by Fryer (1961). Host species were noted amongst the cichlid and mormyrid families, and the conclusion drawn
that the species is not host specific. various aspects of the
life-cycles of the other two species were noted, but complete
life-cycles were not successfully elucidated. population
composition according to age showed the most common species,
c. mossambicensis, to be comprised mainly of 2 - 5 year-old
individuals, indicating a young and vigorous population.
Juvenile pre-adult mussels were very scarce. In the population
of A. wahlbergi, older mussels formed a greater proportion
of the biomass than in the population of c. mossambicensis, and
young were also scarce. M. dubia were not recorded in sufficiently
large numbers to estimate age composition accurately.
The production rate (whole wet mass) of the most common
species, c. mossambicensis, was calculated to be 2,45 kg
ha¯¹ day¯¹ in the sanyati East cleared area (30,5 km²), while
the overall production rate of all three species was calculated
to be 3,34 kg ha¯¹ day¯¹. Calorific values of all three
species were obtained, while gross body composition in terms
of water, fat, protein, ash and amino acid composition were
determined. The ash component of A. wahlbergi was analysed.
The Lake Kariba results were supplemented by a
follow-up study of the composition of the mussel population
of a eutrophic, highveld dam, Lake McIlwaine, in 1978/1979.
This much smaller lake was populated by only two mussel
species, M. dubia and C. mossambicensis, although A. wahlbergi
and Unio caffer were recorded in the riverine upper reaches.
Observations on seasonal breeding of the two lake species were
made. A detailed study on the draw down zone of a gently
shelving beach showed that at the time the extreme shallows had been dominated by C. mossambicensis, whereas from a depth
of approximately 1,6 m M. dubia dominated very significantly.
Fulling lake level was found to trigger off migratory responses,
thus placing a proportion of individuals in deeper water and
improving chances of survival.
The mussel composition of Lake Kariba and Lake
McIlwaine was considered in conjunction with preliminary
observations of mussel distribution a nd composition in other
water bodies, and some inferences drawn regarding the ecological
factors which appear to influence the composition and
diversity of the mussel fauna in rivers and lakes.||en