Declines in reproductive output in two dolphin populations depleted by the yellowfin tuna purse-seine fishery

Reproductive data for 2 pelagic dolphin subspecies in the eastern tropical Pacific, the eastern spinner (ES) dolphin Stenella longirostris orientalis and northeastern pantropical spotted (NEPS) dolphin S. attenuate attenuate, were obtained from aerial photographs taken between 1987 and 2003. Two measures of reproductive output were estimated: proportion of adult dolphins with calves ('proportion with calves') and length at which calves disassociated from their mothers ('length at disassociation'). Trends in length at disassociation were investigated to determine if the proportion with calves was affected by possible changes in calving interval, but no changes were found. Proportion with calves for ES dolphins was stable from 1987 to 1993, then declined from 1993 to 2003; proportion with calves for NEPS dolphins decreased steadily from 1987 to 2003. For both species, proportion with calves was related to number of dolphins in the school of the focal species and/or proportion of the school made up of the focal species. For NEPS dolphins, annual number of purse-seine sets on dolphins was a predictor of both proportion with calves and length at disassociation. Because NEPS dolphins are the main species targeted by the fishery, the link between fishing activity and both measures of reproductive output indicates that the fishery has population-level effects beyond reported direct kill. Decline in reproductive output is the proximate cause or one of the proximate causes of the failure of dolphin populations to recover at rates expected after reduction of high bycatch levels.