Abstract
Historical data suggested that the tigerfish (Hydrocynus vittatus) of the Incomati River migrates upstream and downstream as part of their life history. It has been suggested that this movement was a prerequisite for successful spawning in inundated floodplains in Mozambique. Recent advances in aquatic radio telemetry provided a reliable mechanism to monitor fish movement and increase knowledge of the ecology of tigerfish. From 04 January 2003 to 22 December 2003, 41 tigerfish in the Incomati River system were fitted with radio transmitters to record movement patterns and estimate home range size. On average, each fish was tracked 72 times, and the total number of fixes was 2971 over the study period, including 1322 summer fixes and 1649 winter fixes. The mean longest distance travelled by tigerfish was 730 m (range = 75 m to 3200 m). The home range size varied between individual fish, but on average fish stayed within a defined home range of 48 846 m2. Tigerfish showed high site fidelity to specific habitats within specific activity zones and movement occurred primarily within these defined zones. Differences in movement pattern, longest distance travelled and home range size could not be attributed to the sex or size of the fish. No large-scale movement patterns associated with specific life history activity were observed; thus, previous reports of large-scale downstream migrations and spawning migrations appear to be invalid. The presence of weirs in the study area impedes free fish movement as these weirs create migration obstructions.Conservation implications: River regulation such as damming, water abstraction, obstructive barriers and channel modification may have a detrimental impact on the survival strategy of this species. Implementation of these results in a management policy will provide a reliable basis for species specific requirements such as upstream reservoir release management; minimum flow volumes required for downstream ecosystem maintenance and management and planning of structures obstructing natural flow.
Highlights
The freshwater fish genus Hydrocynus is represented by six species, all endemic to Africa
The southern African tigerfish (H. vittatus) has a limited distribution in South Africa, where it is restricted to the lowveld reaches of the Limpopo River system, mainly within the Kruger National Park (KNP), and further south in the lower reaches of the Usutho and Phongolo Rivers (Gaigher 1967)
The Crocodile River flows along the boundary of the KNP, and at the confluence, the border extends across the river to include the lower reach of the Komati River (Figures 1 and 2)
Summary
The freshwater fish genus Hydrocynus is represented by six species, all endemic to Africa They are pikelike predators, commonly termed ‘tigerfishes’ for their prominent dentition and dark lateral stripes (Gery 1977). In southern Africa, one of these species, Hydrocynus vittatus (commonly known as tigerfish), occurs in the Zambezi and Okavango Rivers and in the lowveld reaches of coastal systems (Skelton 2001). Being essentially a lowveld species in South Africa, it is intolerant to cold water and migrates downstream to lower lying reaches of these rivers during winter where water temperatures are higher and more stable (Pienaar 1978; Steyn et al 1996; Van Loggerenberg 1983; Skelton 2001). Mortalities caused by a sudden drop in temperature (< 16.0 °C) related to cold water in the Incomati River were reported on several occasions (Deacon 1991; Gagiano 1997; personal observation by authors; Van Loggerenberg 1983). Gagiano (1997) reported mortalities in the Piet Grobler Dam in the KNP at a temperature of 14.5 °C during the winter period
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