The behavioural ecology of the Orange–Vaal River yellowfish in lentic and lotic ecosystems, North–West Province, South Africa
Jacobs, Francois Jakob
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Fishes are widely used by biologist as ecological indicators that measure key elements of complex systems, without having to capture the full complexity of a specific system. The Vaal River in South Africa is classified as Africa’s hardest working river and is home to, two yellow fish species that are socially and economically important. Both these yellow fish species are considered to be sensitive to changes in water quantity and quality, habitat destruction and utilisation pressure and are often used as ecological indicators to manage aquatic ecosystems. Very little however, is known about their movement, response to changing environmental variables and interspecies habitat preferences. This study therefore aims to use radio telemetry as a method to characterise and evaluate how yellow fish behaviour is influenced by changing environmental variables. To characterise the behavioural ecology of the Vaal-Orange River yellow fish species in lentic and lotic ecosystems, Labeobarbus aeneus (n=18) and L. kimberleyensis (n=3) were fitted with externally attached radio transmitters in Boskop Dam (L. aeneus, n=4) and the Vaal River (L. aeneus, n=14) (L. kimberleyensis, n=3). Various methods were used to collect yellow fish species including: gill nets, to target mobile individuals, in deep habitats, electro-fishing (electro-narcosis) to collect yellow fish in shallow habitats and angling techniques in a wide variety of habitats. Thereafter yellow fish species were sedated and tagged with externally attached radio transmitters, before being released back into the system. Yellow fish were monitored for eleven months using a remote monitoring system together with manual monitoring surveys. Analyses of data collected showed that L. aeneus follows distinct behavioural patterns, with some individual variations in behaviour. Labeobarbus aeneus exhibited higher movement that are associated with deeper water during daylight hours (04:00-16:00). During nocturnal periods (20:00-04:00) L. aeneus showed a decrease in movement activity and preferred shallower water compared to daytime. However, L. aeneus in the Vaal River seems to be less influenced by bright daylight and this might be due to the turbidity of the river water. Labeobarbus aeneus in Boskop Dam showed higher movement counts during full moon phases whereas L. aeneus in the Vaal River showed higher movement counts during new moon phases. All tagged fishes in Boskop Dam and in the Vaal River preferred deeper water during full moon phases than during new moon phases. Movement were significantly higher (P<0.05) with increased temperatures and shallower water in summer whereas movement significantly decreased (P<0.05) with a decrease in temperature and increased depth in autumn and winter. Seasonal movement data were, however, limited. This study confirms that radio telemetry methods can be used to characterise the behavioural ecology of yellow fish species. In addition, the study has improved the knowledge of how environmental variables may affect the behaviour of yellowfish species. However, due to limited data and our understanding of these species, it is still uncertain how behaviour of yellow fish species can be applied as an ecological indicator of aquatic ecosystems.