Conservation management in the face of uncertainty: effectiveness of four options for managing Hectors dolphin bycatch

New Zealand's endemic Hector's dolphins Cephalorhynchus hectori overlap with gillnet and trawl fisheries throughout their geographic range. The catch rate in commercial gillnets has been estimated using independent observers. No quantitative estimates are available for bycatch in recre- ational gillnets or trawl fisheries. The catch rate (per dolphin, per km of gillnet, per year) in commer- cial gillnets was used in a population viability analysis to estimate past and future population sizes. Total population size today (7873, coefficient of variation (CV) 0.16) was estimated at 27% of popula- tion size in 1970 (29316, CV 0.16), before a major expansion of commercial gillnetting. The species matches IUCN criteria for 'endangered'. The North Island subspecies matches criteria for 'critically endangered'. Current management, which includes 2 protected areas, is not sufficient to halt popu- lation declines. Hector's dolphin populations are predicted to continue declining to 5475 (CV 0.20) by 2050. Creating 4 strategically placed protected areas would allow population recovery towards 1970 levels, with an estimated 47% probability of reaching 50% of 1970 population size by 2050 (propor- tion of 5000 model runs in which final population size was at or above this reference level). Reducing fisheries mortality to levels approaching zero shows the strongest promise of meeting national and international guidelines for managing dolphin bycatch, with a 59% probability of reaching 50% of 1970 population size by 2050.