Line-transect surveys conducted in April ? November 2001 covered 884 km of trackline in the entire Sound (total area 230.7 km²), and resulted in a total population estimate of 77 individuals (CV=27.4%), confined to the inner portion (133.7 km²; Smith et al., in press). Seasonally stratified estimates were calculated as 67, CV=38.6%; 78, CV=78.1% and 81, CV=31.7% for pre-monsoon, monsoon and post-monsoon seasons, respectively. The absence of Irrawaddy dolphin sightings in the outer Sound (see Dolar et al. 2002, Smith et al. in press, and unpublished reports of WWF-Philippines) and the close agreement among abundance estimates from surveys conducted during pre-monsoon, monsoon and post-monsoon seasons strongly suggest that the subpopulation is resident within the inner Sound.
There are no direct data on population trends. However, for small cetaceans generally, it is recommended that yearly removals should not exceed 1?2% of the overall population size (Wade 1998). Using a minimum estimate of two dolphins killed per year (see above), this represents 2.6% of the population, according to the best estimate of abundance made during line-transect surveys (77 dolphins). Using the minimum abundance estimate (45 dolphins), the estimated minimum yearly mortality rate would be 4.4% of the population size. Even this latter figure may be an underestimate because it considers only the two confirmed kills recorded during a 7-month study (Smith et al. in press). Considering that the small size of the Malampaya population already makes it vulnerable to extirpation from demographic stochasticity, inbreeding depression and catastrophic environmental and epizootic events, the current rate of incidental mortality in gillnets will almost certainly lead to extirpation.
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