Catch composition and conservation management of a human–dolphin cooperative cast-net fishery in the Ayeyarwady River, Myanmar
Introduction
Mutualism between humans and wildlife is a rare ecological phenomenon that has been attributed to a few specific relationships between fishermen and delphinid cetaceans (see Mead, 1961, Thomson, 1967, Busnel, 1973, Cousteau and Diole’, 1975, Hall, 1984, Hall, 1986, Pryor et al., 1990). One example is a cooperative fishery practiced between Irrawaddy dolphins (Orcaella brevirostris) and cast-net fishermen in the Ayeyarwady River, Myanmar (Fig. 1; Anderson, 1879, Thein, 1977, Smith et al., 1997). During this fishery, cast-net fishermen, operating from small, two-person canoes, attract the dolphins to help them by (1) repeatedly striking the side of their canoe with a conical wooden pin in short-pulsed decrescendo drum rolls, (2) slapping the water surface sharply with the flat end of their paddle (Fig. 2), (3) letting the lead weights of their cast-net dangle on the deck of their boat making a tinkling sound, (4) vocalizing a guttural call from the throat which sounds similar to a turkey, and (5) making practice net casts. If the dolphins ‘agree’ to engage in cooperative fishing, they herd fish into a concentrated mass by swimming in ever-tighter semi-circles towards the fishermen. Then one dolphin pushes its half-submerged flukes in the direction of the fisherman, which serves to further concentrate the fish school and provides a ‘signal’ to the fisherman for casting his net (Fig. 3). The fishermen benefit from the dolphins’ behavior because they can more easily capture fish concentrated by the herding and fluke push. The dolphins presumably benefit by preying on fish whose movements are confused by the sinking net and those that are momentarily stuck on the mud bottom and fall out of the net as it is pulled to the surface (Fig. 4; Smith et al., 1997).
Irrawaddy dolphins in the Ayeyarwady River are among only three riverine populations of the species. The other two are in the Mekong and Mahakam Rivers (Smith and Jefferson, 2002). All are classified as “critically endangered” in the IUCN Red List (Kreb and Smith, 2000, Smith, 2004, Smith and Beasley, 2004). The Ayeyarwady population is estimated to number only 59–72 individuals concentrated in three braided channel segments that make up 69% of their total range of about 400 linear km with the downstream extent located about 970 km from the sea (Smith and Tun, 2007). There is believed to be only very limited demographic interaction among the dolphin groups occupying these segments as they are separated by defiles (alluvial channels that become abruptly narrow, deep and fast-flowing as they cut through a mountain range) where no dolphins have been sighted during numerous visual surveys (Smith and Tun, 2007).
The human–dolphin cooperative fishery is limited to the farthest downstream river segment located between Mingun and Kyaukmyaung (∼74 linear km), which supports about 24% of the total population (Smith and Tun, 2007). In December 2005, the Department of Fisheries, Myanmar, announced the establishment of a protected area for Irrawaddy dolphins in this river segment (Fig. 5). Key provisions of the protected area include requiring fishermen to immediately release dolphins if found entangled and alive in their nets, prohibiting the catching or killing of dolphins and trade in whole or parts of them, prohibiting the use of electricity to catch fish, and prohibiting gill nets that obstruct the water-course, are more than 300 ft (91.4 m) long or spaced less than 600 ft (183.9 m) apart (Smith and Tun, 2007).
The present paper reports on a comparative study of the catch composition of cast-net fishermen while fishing cooperatively with dolphins and while fishing without them. It then discusses measures being taken in the Ayeyarwady Dolphin Protected Area to conserve the dolphins and cooperative cast-net fishery.
Section snippets
Materials and methods
A study of the human–dolphin cooperative fishery in the Ayeyarwady River was conducted during 18 November – 3 December 2006 and 24 November – 15 December 2007. Prior to the study extensive casual observations and interviews with cast-net fishermen were carried out during surveys to assess the status of the dolphin population and plan for the establishment of the protected area mentioned above (see Smith and Tun, 2007). Four main areas were identified where the cooperative fishery is practiced:
Overall catch
Altogether a total of 4139 fish from 42 species and five crustaceans from a single species (Machrobrachium sp.) were recorded in 1099 net casts made during 33 fishing sessions (defined as a continuous sampling period – generally one in the morning and/or one in the afternoon) that took place within dolphin habitat (i.e., not including hilly areas with shallow water and emergent vegetation). The weight of the overall catch was 51.6 kg and the total value to the fishermen was 68.6 $US.
The economic
Catch size and composition
The number of fish caught and the total weight and value per cast were substantially greater during cooperative versus non-cooperative fishing. When blank casts were included, the number of fish, total weight and economic value to the fishermen from cooperative casts versus non-cooperative casts were about double during 2006, when the overall catch was relatively low, and about triple during 2007 when the overall catch was substantially larger. Interestingly, despite the larger catches in 2007,
Acknowledgements
We gratefully acknowledge the enthusiastic participation of cast-net fishermen from the villages of Hsithe, Indown, Myay Zun and The‘ Kyun. Special appreciation is given to U Khin Maung Aye, U Than Tun (ret.) and U Khin Ko Lay from the Department of Fisheries, Myanmar for their strong support. The assistance of U Than Myint from the WCS Myanmar Program was invaluable. This study was funded by grants received from the Blue Moon Fund, National Geographic Conservation Trust, Ocean Park
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