Population-specific Data and Policy Support Can Effectively Mitigate Entanglement: a South African example

Bryde’s whale lunge feeding in Plettenberg Bay, South Africa. ©Gwen Penry

By Dr. Gwen Penry*

The inshore population of Bryde’s whales in South Africa was assessed regionally as Vulnerable (using IUCN criteria to apply to the population that is found in the waters off South Africa, Lesotho and Swaziland) owing to its small population size and genetic isolation (Penry et al. 2016). At the time of assessment, a new threat was recognized; fatal entanglement in bottom-mounted fishing gear. Between 2014 and 2020, the South African Whale Disentanglement Network recorded 10 fatal Bryde’s whale entanglements, 6 of which were in gear used for an exploratory octopus fishery. Examination of the carcasses revealed that most had rope through the mouth, indicating that the whales had been feeding at the time of entanglement. Identifying the behaviour leading to such incidents became a research priority. In April 2018 and 2019 scientists deployed suction-cup data loggers on 12 Bryde’s whales in Plettenberg Bay in the hope that they would provide data to explain the entanglement process.

They did! Data from the loggers revealed a novel behaviour: Bryde’s whales chase prey near the seafloor at high speeds (up to 7.4 m/s), followed by upward lunges and mouth opening (Segre et al., 2022). This explained how the rope connecting the octopus traps became lodged in the whale’s mouths, anchoring them to the seafloor.

The original gear configuration for the octopus fishery included a set of traps (~30) spaced 10–20 m apart connected by floating groundlines that arc up into the water column. This design was intended to minimize damage to reefs, facilitate gear retrieval, and allow sufficient slack in the rope for an entangled whale to surface and breathe until disentanglement teams arrived. The same configuration had been used elsewhere (e.g. Australia) without major incident and was therefore not expected to be a fatal entanglement concern in South Africa.

In 2019, while the tagging fieldwork was still underway, two Bryde’s whales died from entanglement in this gear and the Minister of Forestry, Fisheries and the Environment implemented a temporary moratorium on the fishery and called an urgent meeting with her advisors, fishery representatives and scientists. Several options to mitigate whale-gear interactions were discussed, including seasonal closures. However, because Bryde’s are resident in South African coastal waters, seasonal closures would not help, as they would for migratory species such as humpback and southern right whales. The only viable option whereby the fishery could continue without entanglements was to remove all rope from the water column by using sinking groundlines and acoustic or time-release surface-marker buoy lines. Revised regulations were issued specifying required gear modifications. Replacement of floating line with sinking line is extremely expensive and, because the fishery was still in an exploratory phase, only one of the permit holders was prepared and able to invest in the necessary gear modifications. Since then (late 2019), no entanglements have been reported. The mitigation measures, though provisionally deemed successful, remain to be fully tested if the fishery becomes active at a larger and commercial scale.

Lessons learned

This situation highlights the importance of conducting science-based risk assessments before, and indeed while, new fisheries are explored. Although the original octopus gear configuration apparently had not led to fatal whale entanglements elsewhere, the unique behavior of Bryde’s whales in South Africa led to unfortunate deaths in this Vulnerable population.

Fortunately, there were only a few active permit holders (6) at the time, which allowed for manageable discussions and interactions with scientists and decision-makers. New fisheries being explored should begin with a limited number of allocated permits, to allow any issues to be resolved amongst a small group of people, and before large investments are made.

This example shows that location- and species-relevant science can support solutions to at least some bycatch problems. The South African government acted decisively to make data-driven policy changes and are commended for their swift action. It remains to be seen whether the mitigation measures will be effective and allow for large-scale expansion of the octopus fishery. The novel finding that Bryde’s whales use high-speed chases near the seafloor to catch their prey highlights the value of using species-specific, behaviour-based information to support conservation and management recommendations.

Bryde’s whale in Plettenberg Bay, South Africa. ©Gwen Penry

Research team investigating a Bryde’s whale that died from entanglement in lines of experimental octopus fisheries in South Africa. ©Gwen Penry

1. Penry G, Findlay K, Best P. 2016. A conservation assessment of Balaenoptera edeni. In Child MF, Roxburgh L, Do Linh San E, Raimondo D, Davies-Mostert HT, editors. The Red List of Mammals of South Africa, Swaziland and Lesotho. South African National Biodiversity Institute and Endangered Wildlife Trust, South Africa.
2. Segre, PS; Di Clemente, J; Kahane-Rapport, SR; Gough, WT; Lombard, A.T; Goldbogen JA; Penry, GS(2022). High-speed chases along the seafloor put Bryde’s whales at risk of entanglement. Conservation Science and Practise, 4(5). https://doi.org/10.1111/csp2.12646

*Dr Gwen Penry
Research Associate – Nelson Mandela University, Institute for Coastal and Marine Research
Research Fellow – University of Pretoria, Mammal Research Institute Whale Unit
IUCN SSC CSG member

https://orcid.org/0000-0003-0545-7723