Effects of bycatch on the population viability of the narrow-ridged finless porpoises in Ariake Sound and Tachibana Bay, Japan

The narrow-ridged finless porpoise Neophocaena asiaeorientalis is a coastal ceta - cean that is threatened by various human activities. Bycatch mortality is a particular danger to the porpoise population in Ariake Sound and Tachibana Bay, Japan. To evaluate the impact of bycatch mortality on the viability of this population, we simulated changes in population size over the next 100 yr, using a Leslie matrix model. The simulation trials were repeated for 3 scenarios of possible bycatch mortality rates. If bycatch mortality remains at the reported level, the estimated annual rate of decrease is 0.671 to 3.87%, and the estimated population size reduction over 3 gen- erations is 29.6 to 86.3%. A population size reduction of ≥30% was predicted in almost half the simulation trials, even in the most optimistic scenario. The reported bycatch mortalities would pose a serious threat to the viability of this porpoise population; therefore, the population should be classified at least as 'Vulnerable' according to Criterion A4, as defined in the IUCN Red List. Our predictions were sensitive to annual bycatch mortality rate estimates. Hence, the collection of current abundance and bycatch data should be promoted in order to minimize uncertainty in risk assessments.