Non-recovery of two spotted and spinner dolphin populations in the eastern tropical Pacific Ocean

Populations of northeastern offshore spotted dolphins Stenella attenuata attenuata and eastern spinner dolphins S. longirostris orientalis have been reduced because the dolphins are bycatch in the purse-seine fishery for yellowfin tuna in the eastern tropical Pacific Ocean (the 'tuna-dolphin issue'). Abundance and trends of these dolphin stocks were assessed from 12 large-scale pelagic surveys carried out between 1979 and 2000. Estimates of abundance were based on a multivariate linetransect analysis, using covariates to model the detection process and group size. Current estimates of abundance are about 640 000 northeastern offshore spotted dolphins (CV = 0.17) and 450 000 eastern spinner dolphins (CV = 0.23). For the whole period from 1979 to 2000, annual estimates of abundance ranged from 494 000 to 954 000 for northeastern offshore spotted dolphins and from 271000 to 734 000 for eastern spinner dolphins. Management actions by USA and international fishing agencies over 3 decades have successfully reduced dolphin bycatch by 2 orders of magnitude, yet neither stock is showing clear signs of recovery. Possible reasons include underreporting of dolphin bycatch, effects of chase and encirclement on dolphin survival and reproduction, longterm changes in the ecosystem, and effects of other species on spotted and spinner dolphin population dynamics.