The increase in the number of recorded strandings, from 2.5 per year between January 1990 and April 2001 to 5.5 per year between May 2001 and December 2003 (see above), probably reflects an increase in reporting (due to the greater awareness of local people about the dolphins and efforts to conserve them; see below), the developing capacity of local agencies to respond to stranding reports, and possibly a rise in mortality within the population itself. It is unlikely that any real increase in mortality could be explained by an increasing population size, given the lack of recent sightings despite extensive survey effort (see above).
The proportion of stranded calves has also increased, which could mean that some adults are habituating to the presence of nets and fish traps, or that the calves are dying at an unusually high rate for altogether different reasons; the causes of death for these carcasses could not be determined (Somserm Choorak, pers. comm.). Although we have no supporting evidence, a possible explanation for the high proportion of stranded calves could be that they were stillborn or died shortly after birth due to high toxic loads from agrochemicals used intensively along the shores of the lake.
Although incidental mortality in gillnets and fish traps is the principal known threat to the population, prey depletion is another potential threat. This could be the result of overfishing and/or the sedimentation and eutrophication caused by shoreline development and deforestation. A closure dam across a narrow channel that previously connected Thale Luang to the sea has undoubtedly contributed to the cumulative effects of human activities (Beasley et al. 2002, Smith, unpublished). Year-round availability of fresh water after the dam?s closure has allowed an increase in shoreline agriculture, which uses large quantities of pesticides and herbicides. The dam?s construction also may have eliminated a movement corridor for dolphins between Songkhla Lake and the Gulf of Thailand.
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