Gillnets of large mesh sizes: a threat to sharks and rays

Written by Kennedy Osuka

IUCN SSC Shark Specialist Group | Africa Regional Group | Member CORDIO East Africa

Gillnets are widely used gears among Kenya’s coastal artisanal fishers. However, their mesh sizes are inadequately monitored or regulated; therefore, their impacts on threatened species are poorly understood. For the first time, a team of researchers from Coastal Oceans Research and Development in the Indian Ocean - CORDIO - East Africa assessed the effects of different gillnet mesh sizes on fish and fishers’ catches in Kenya’s coastal waters. The study, which has been published in the African Journal of Marine Science, found that large mesh gillnets ranging from 20.3 to 30.5 cm in stretched-mesh sizes were the primary gillnet type that captured >60% of threatened shark and ray species. The proportion of species assessed as Near Threatened, Vulnerable, or Endangered on the IUCN Red List of Threatened Species was double that of the medium mesh sizes (7.6 to 12.7 cm stretched-mesh size).

The dominant species caught in medium and large mesh sizes were Mackerel Tuna (Euthynnus affinis) and the Vulnerable Coach Whipray (Himantura uarnak). Other notable Vulnerable species captured in large mesh sizes comprised the Blacktip Reef Shark (Carcharhinus melanopterus), Whitetip Reef Shark (Triaenodon obesus), and Spotted Eagle Ray (Aetobatus ocellatus). Endangered species included the Giant Manta Ray (Mobula birostris). The Whitespotted Guitarfish (Rhynchobatus djiddensis) and Bluespotted Lagoon Ray (Taeniura lymma) that were recently assessed as Critically Endangered and Least Concern, respectively, also formed part of the landings. The observed dominance of threatened species highlights the need for focused gillnet management regulations. In other words, it contradicts the general recommendation of large mesh sizes for artisanal fishing. Thus, phasing them out in Kenya’s coastal waters has a great potential of reducing the capture of threatened sharks and rays. This would also help in lowering the incidental capture of marine mammals and turtles. Instead, the least-damaging medium-mesh sizes should be promoted as an alternative.

Sharks and rays are highly susceptible to fishing mortality owing to their slow growth and late maturation. This, coupled with overexploitation and high demand for their products, has led to alarming declines in shark populations. The loss has potential ecological and socio-economic implications, including inversion of trophic pyramids and loss of livelihood activities such as tourist attractions and food resources. In Kenya, sharks are a target species for some artisanal fishers, such as those on the north coast, where there has been a shark fishery for centuries. To this resource user group, species protection measures, including changes in gillnet mesh sizes, need to be preceded by awareness-creation on the overall benefits of sharks to the ecosystem and the importance of their conservation. This is likely to reduce resistance to mesh size regulations recommended by the study.

Kenya has domesticated resolutions made by the Indian Ocean Tuna Commission (IOTC) on the capture, conservation, management and transhipment of sharks and rays in the Fisheries Management and Development Act 2016. However, no fisheries conservation and management measures are in place for any species of sharks and rays in the coastal fisheries. This calls for a proactive approach towards their management, and a ban of large mesh gillnets would be a step in the right direction.

Overall, shark and ray populations in Kenya appear to experience considerable exploitation pressure, which has led to substantial population declines. However, clear quantitative measures are lacking due to inadequate monitoring and little research. This coupled with a lack of species protection measures, suggests an urgent need to revise and implement monitoring and regulations for these threatened species.

Reference: Osuka, K., Kawaka, J. A., & Samoilys, M. A. (2021). Evaluating Kenya’s coastal gillnet fishery: trade-offs in recommended mesh-size regulations. African Journal of Marine Science, 43(1), 15-29. doi.org/10.2989/1814232X.2020.1857836