Abstract
M.Sc.
Hydrocynus vittatus, commonly known as the tigerfish, plays an important role in riverine ecology. It is a top predator which roams the open waters of most larger river systems in southern Africa. Their presence in a freshwater ecosystem has a dramatic impact on the fish community structure. It is known that dams and weirs have a negative effect on the migration of the tigerfish. It is also evident that tigerfish do not occur in certain areas in some of the rivers where they have been present historically. The Olifants and Letaba Rivers in the Kruger National Park (KNP) are two of a few rivers within South Africa where tigerfish do occur. The KNP represents the edge of the most southern distribution of tigerfish in southern Africa. It was therefore expected that the tigerfish do not function optimal in the Olifants and Letaba Rivers as they are subjected to waters
with high concentrations of silt and low flow which influences migration and successful breeding. Breeding migrations does however take place during the summer months after which the tigerfish returns to the Massingire Dam in Mozambique to avoid the colder winter temperatures in the rivers. Gonad development coincide with the yearly summer rainfall patterns. A deviation of the expected 1:1 male:female sex ratio to favour the males was experienced in both rivers, which may be the result of over population. Females were found to grow to a larger size than the males and were extremely fecund. Although H. vittatus is believed to be mainly piscivorous, other food items such as invertebrates, played an important role in the diet of small and large tigerfish in both the Olifants and Letaba Rivers. Invertebrates were mostly preyed upon which implies that optimal feeding conditions for the tigerfish does not prevail in these systems and that they have to adapt to satisfy their feeding requirements. Tigerfish is more abundant in the Olifants than in the Letaba River. The overall growth performance or phi prime (4)) values for H. vittatus in the Olifants River was determined and
compares well to the overall growth performance of tigerfish in the Okavango River and Lake Kariba. However the maximum length calculated for tigerfish in the Olifants River (Lco = 52.40 cm ) is smaller than the Lco values (56.06 cm) for the Okavango River. The mortality rate of tigerfish in the Olifants River exceeds those in the Letaba River which means that the life expectancy is longer in the Letaba as opposed to the Olifants River.
Successful artificial spawning revealed some of the secrets of the reproduction strategy of this species. Tigerfish has semi pelagic eggs, are very small (0.65 mm), negatively buoyant and slightly adhesive for bentic and epibiotic incubation, and it is expected that tigerfish would spawn in open water, on a sandy substrate in the vicinity of aquatic vegetation. First hatching took place at 22h 30 min after fertilization. Vertical movement of the larvae lasts for two days, which allows for downstream movement and dispersement of the larvae. It was found that tigerfish replace their teeth on a regular basis as they grow larger. Transition from conical to functional dentition takes place 45 days after hatching. Replacement of sets of teeth occurs during all phases of its lifespan. It is a quick proses of three to six days during which all teeth are replaced in both the upper and
lower jaws.