Assessing the effect of tagging and the vulnerability to predation in tigerfish (Hydrocynus vittatus, Castelnau 1861) in a water-stressed system using telemetry methods

Fish are well established around the world as indicators of ecological integrity (Harrison & Whitfield 2006; Mims & Olden 2013; Sosa-López et al. 2005). Similar to other aquatic organisms, fish species face several challenges from the environment they occur in and are subject to a plethora of stressors, both natural and anthropogenic (Du Plessis 2019; O’Brien et al. 2018). The ever-increasing anthropogenic pressures have generally led to a steady decline in aquatic organisms, including fish species (Dudgeon 2014; Rodell et al. 2018). 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 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.