Abstract
Increase in anthropogenic pressures on freshwater ecosystems, globally and locally in South Africa, has made it difficult to meet environmental flow requirements and maintain these systems. The Letaba-Olifants catchment is one such example, where the upstream water demands place pressure on the river downstream as it flows through the Kruger National Park. We used the activity rates of Hydrocynus vittatus as a line of evidence to assess (1) the effects of telemeter tagging on their activity rates in three potential post-tagging recovery periods and (2) their vulnerability to predation and the causality related to the environmental stressors placed on the river catchment in the Olifants River Gorge, Kruger National Park, South Africa. We determined H. vittatus activity rates as locomotive movement using radio telemetry methods linked remotely to an online data management system. We telemeter tagged nine individuals from 08 May to 28 June 2018. However, only seven fish were successfully tracked for the duration of our study, and two individuals moved out of range of the remote network shortly after release and could not be located. The tagged H. vittatus individuals were found to have reduced activity at least within the first 7 days after tagging compared with the time after that. The results showed that three individuals were preyed on by predators after the tagging procedure recovery period. This coincided with abnormal low flow conditions, where the Letaba River ran dry. African fish eagles Haliaeestus vocifer were the only confirmed predator, whilst predation by other species was also evident.Conservation implications: Monitoring of H. vittatus using telemetry is a viable tool to use when assessing environmental stressors in remote locations. The abnormally low water levels in the Olifants and Letaba Rivers (cessation of flow in the Letaba River) during our study may have compromised the predator avoidance strategies of tagged H. vittatus and may further be affecting the viability of the H. vittatus populations in Kruger National Park. The synergistic effects of natural and anthropogenic stressors impact negatively H. vittatus populations and potentially other aquatic biotas.
Highlights
Fish are well established around the world as indicators of ecological integrity (Harrison & Whitfield 2006; Mims & Olden 2013; Sosa-López et al 2005)
Telemeter tagged H. vittatus were monitored for a minimum of 22 days, with the longest tracked for 156 days
Telemetry methods should, where possible, have larger sample sizes and make use of smaller and lighter tags (< 20 g air mass) with proportionally less body to tag mass ratios for adult fish. These recommendations will improve the confidence in results and the recovery process as well as increase the representable size classes in telemetry studies of this nature. This will further ensure the inclusion of smaller fish species as well as earlier life stages in telemetry studies
Summary
Fish are well established around the world as indicators of ecological integrity (Harrison & Whitfield 2006; Mims & Olden 2013; Sosa-López et al 2005). Fish kills and population declines are often reported worldwide as a result of deteriorating aquatic ecosystems (Ferreira & Pienaar 2011; Thronson & Quigg 2008) This deterioration has been attributed to several factors, including river fragmentation, flow modification, pollution, the introduction of alien species and global climate change (De Moor 1996; Ellender & Weyl 2014; O’Brien et al 2019). Fish make use of the available physical habitat as refugia, for feeding and spawning, and as nurseries for their offspring (Godin 1997; O’Brien et al 2013) Maintaining these environments is essential, but they are often inaccessible and difficult to sample, making observations hard to http://www.koedoe.co.za
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