Population trends and vulnerability of humpback dolphins Sousa chinensis off the west coast of Taiwan

Predictive modeling of population trends can indicate the rate of population decline and risk of extinction, providing quantitative means of assessing conservation status and threats. Our study tests the rate of population change and risk of extinction of the Indo-Pacific humpback dolphin Sousa chinensis off the west coast of Taiwan, the only humpback dolphin population classified as Critically Endangered (CR) by the IUCN Red List of Threatened Species. Under the most optimistic assumptions, almost 60% of simulations (out of 250 replications x 5000 iterations) predicted that population decline will exceed 80% within 3 generations, while the mean estimate of population decline within 1 generation was >50% of the current population numbers. Status classification performed using IUCN Red List Categories and Criteria Version 3.1 supported previous CR classification, while risk assessment models that factored in anthropogenic impacts further increased the estimated extinction risk. At an adult survival rate of 0.95, a modeled increment of annual bycatch rate by 1% of population size increased the probability of extinction within 100 yr by 7.5%; this increase was lower at a higher adult survival rate. The estimated extinction risk was greatest under the impact of habitat loss, reaching a hazardous level when habitat carrying capacity dropped to less than 50%, indicating that habitat fragmentation and alteration of coastal environments pose the greatest threats to this population, even if the cumulative sum of fragmented patches of habitat may superficially appear to be large.