Occurrence of netmark incidences on two species of dolphin commonly caught in bather protection nets along the KwaZulu-Natal coastline, South Africa- can they be used as an indication of entanglement in stranded dolphins?
Stephanie PlÃ¶n, Natasha Roussouw and Sabine Wintner
The Indo-Pacific bottlenose dolphin (Tursiops aduncus) and the Indian Ocean humpback dolphin (Sousa plumbea), are commonly found along the east coast of South Africa. Since 2009, detailed health investigations of the animals incidentally caught in bather protection nets along the KwaZulu-Natal coast have yielded a valuable dataset to determine whether netmarks are a clear indicator of entanglement in stranded dolphins where cause of death is unclear. The aim of this study was to investigate netmark occurrence on two bycaught dolphin species along the east coast. Marine mammal dissection and necropsy reports as well as photographs of 125 animals known to have been caught in bather protection nets between 2010 and 2017 were investigated to determine prevalence of netmarks as well as any correlations with species, sex, age and season. Our results showed that only 23% of the investigated dolphins known to have been caught in nets presented signs of netmarks on the skin. There were clear differences between the two species, with only 14% of Sousa showing signs of netmarks, while Tursiops made up well over half of the bycaught animals with netmarks (86%). Furthermore, females were more likely to exhibit netmarks on the skin when compared to males (59% vs 41%). Adults were least likely to exhibit netmarks (31%), with the majority of dolphins being juveniles (69%; juveniles included calves, neonates and subadults). Seasonal differences were also observed, with more netmarks occurring in winter (38%), while summer appeared to have the least number of dolphins with netmarks (10%). This result was of particular interest as it contrasted with the assumption that increased temperature attributed to decay of dolphin carcasses and thus affected netmark occurrence. Netmarks and other injuries are considered to be a clear indication of entanglement. However, our study found that only a small percentage of bycaught animals actually present these signs of entanglement. Thus, other methods, in addition to netmark presence, are required to reliably identify entanglement cases in strandings.