Trends and interventions in large whale entanglement along the South African coast

Abstract

The major causes of large whale entanglement in South Africa are static fishing gear, especially the type associated with the West Coast rock lobster Jasus lalandii industry, and large-mesh gillnets that are set off the coast of KwaZulu-Natal (KZN) to reduce shark attacks (shark nets). The prevalence of entanglements is seasonal with the peaks in activity coinciding with the breeding migrations of humpback whales Megaptera novaeangliae and southern right whales Eubalaena australis, the two large whale species that are the most prone to entanglement. Generalised linear models with a Poisson or quasi-Poisson distribution were used to describe the relationship between the number of incidents and time. Taking into account the combined length of shark-net installations per year as an offset variable, entanglement of humpback whales in shark nets increased at 15.1% per year (95% CI = 9.5–21.6%) from 1990 to 2009. This is comparable to the rate of increase in the numbers of this species migrating past the KZN coast, between 1988 and 2002 (9–11%). The number of reported incidents of southern right whales entangled in gear other than shark nets also increased between 1990 and 2009. This was accounted for by the increase in numbers of this species in South Africa (7% per year), so in neither case are the two species at increasing risk of individual entanglement, and anthropogenic factors including entanglement do not seem to be affecting the recovery of these whale populations. Nevertheless, there is concern regarding the vulnerability to entanglement of a small assemblage of humpback whales that habitually visits the West Coast in spring and summer. The continued recovery of whale populations is likely to lead to greater levels of anthropogenic interaction and heighten the need for adequate mitigation measures. The KZN Sharks Board and the South African Whale Disentanglement Network (since 2006) have respectively released (disentangled) 81% and 23% of confirmed entangled individuals, and recorded relevant information on entanglement incidents. Such information is critical for developing mitigation measures and monitoring the prevalence of entanglement.