Wherever gillnets and cetaceans occur together there will be entanglements and mortality (see International Whaling Commission 1994). The fact that gillnets were present in higher frequencies in areas where dolphins were reported to occur historically but were not observed during the 2002 survey, implies that these fishing gears are at least partially responsible for the range decline of the species. For small cetaceans it is generally recommended that yearly removals not exceed 1–2% of the population size (Wade 1998) – the lower bound being more applicable to very small populations that are already vulnerable to extirpation due to demographic, genetic and other factors. If there are only about 60 animals in this subpopulation (‘best’ estimate of abundance from December 2003 survey), any more than a single death every one or two years may be unsustainable.
During the December 2003 survey electric fishing was occasionally observed during daylight hours and reported by local villagers to be practiced widely and surreptitiously at night. A local veterinarian reported that he had examined a stranded dolphin that was killed by electrocution (Smith, unpublished). Electric fishing has been cited as being responsible for the largest number of recent known deaths of the baiji (Lipotes vexillifer), a Critically Endangered dolphin in the Yangtze River of China, and has come to be regarded as the main anthropogenic threat to the survival of that species (Zhang et al. 2003).
Potential additional threats
Smith (2003) also recorded a total of 890 gold mining operations during 15 days of the 2002 survey. These were concentrated primarily in areas of reduced current, above and below defiles and near channel convergences – the same areas that constitute the preferred habitat of Irrawaddy dolphins (see above). Large boat dredges (15.8% of the total operations) and hydraulic land blasters (13.4% of the total operations) introduce, break up, and redistribute large quantities of gravel and fine sediments. This causes major changes in the geomorphological and hydraulic features of river channels that make them suitable for dolphins. These operations are also very noisy, which may interfere with the ability of dolphins to navigate, detect and catch their prey and communicate.
Gold mining operations use mercury to amalgamate the gold. Relatively high levels of THg and MeHg have been found in the muscle of 104 fish belonging to 22 different species sampled from fishermen's catches and fish markets along the entire length of the river. In the most widely distributed genus (Ompok) mean muscle Hg from multiple similar individuals was found to increase significantly in the downstream reaches, as compared to upstream reaches (Slotton et al. 2004). The Irrawaddy dolphin is a predatory species and the biomagnification potential of mercury makes pollution by this element a source of concern to the dolphin population. The risk may be increased by the animals’ affinity for counter-currents, where entrained metals may settle in higher concentrations than elsewhere, and tributary mouths fed by lower-order streams, where gold mining may be even more intense and dilution limited by lower flows relative to the mainstem. Although we have no information on the effects of mercury on Irrawaddy dolphins, a casual link has been suggested between liver disease and high levels of the metal in bottlenose dolphins (Tursiops truncatus) and long-finned pilot whales (Globicephala melas) (Bowles 1999).
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