What is dolphin-assisted cast net fishing in Kerala, which researchers want to study?

The dolphin assisting cast net fishing in Ashtamudi Lake was first studied and documented in 2012 by Prof A Biju Kumar, R Smrithy and K Sathasivam, of the Department of Aquatic Biology and Fisheries, University of Kerala. The study, published in the Indian Journal of Fisheries, found that traditional cast net fishermen were taking advantage of the behaviour pattern of humpback dolphins.

The feeding-related behaviours noted were a sudden rush towards the prey (mullet) with shuttling or speedy zigzagging backwards and forwards, and fish stunning by tossing them into the air. Dolphins chase fish (mullet) toward the shore while fishermen cast their net to coincide with dolphin movements — resulting in improved catches for fishermen and successful foraging for dolphins. The fishermen cast their net in front of the advancing dolphins.

The catch recorded in the dolphin-assisted cast net fishing was significantly higher than fishing without the support of dolphins. Mullet, which is the primary prey of dolphins, was found abundantly in the lake. This cooperation often helps fishermen get a very good catch of the mullets when they cast nets concomitant with the chase of the dolphin for its preferred prey, the mullet.

Dolphin-fishermen interactions elsewhere in the world

Similar interactions have been observed in Brazil and Myanmar. Wild dolphins in the coastal town of Laguna, Brazil, have formed a strong symbiotic bond with fishermen working in the bay. The dolphins herd schools of fish towards the shoreline and then give distinct signals telling the fishermen when to cast nets. This mutualism between dolphins and fishermen has been reported from Brazil in 1991 and Myanmar in 1997.

Stakeholders of the study

Experts from the Oregon State University (USA), the Universidade Federal de Santa Catarina (Brazil), the Australian National University (Australia), the University of Kerala and the Dakshin Foundation, Bangalore, are working on the project, which is supported by the National Geographic Society. Oregon State University is globally coordinating the project. They are studying a few remaining cases of human–wildlife cooperation, where both species actively coordinate their behaviour to achieve mutual benefits.

Non-invasive and collaborative research

The research in Ashtamudi Lake will last till 2028 and will compare the dolphin-fishermen cooperation reported in Brazil and Myanmar. The study uses non-invasive research methods, including photo and video documentation, behavioural observations, and semi-structured interviews with local fishers. No animals will be captured, handled, or disturbed.

Dolphins on the Kerala coast

Kerala has reported the presence of eight species of dolphins, which are found in herds in the sea. While most species are found in the deep sea, the humpback dolphins are found in shallow areas near the coast. They move in small herds in coastal waters and estuaries from East Africa to Southeast Asia.

The Indian Ocean humpback dolphin (Sousa plumbea) is particularly seen along the Kerala coast. Considered as endangered, this species has been bracketed within Schedule I of the Wild Life (Protection) Act, 1972. Their main fodder is mullet, which is found in abundance in the Ashtamudi Lake, which is directly open to the Arabian Sea through an estuary.