Habitat-based spatial and temporal variability in life history characteristics of female common dolphins Delphinus delphis in the eastern tropical Pacific

Spatial and temporal variability in mean age, breeding season and mean body length were examined for female short-beaked common dolphins Delphinus delphis in the eastern tropical Pacific (ETP). Specimens were obtained from central stock D. delphis incidentally killed in the tuna purse-seine fishery between 1973 and 1993. To examine spatial variability in life history parameters, oceanographic variables were used to characterize 2 primary habitats within the range of central stock animals: the Costa Rica Dome (CRD) and the surrounding areas of the CRD. Mean age of mature females did not vary through time or between habitats, and calving occurred throughout the year in both habitats. However, mean body length of D. delphis sampled 'on' the CRD was 10 cm shorter than those sampled 'off' the CRD during the years 1973-1974 but was not significantly different between the 2 habitats in the years 1988-1989. The difference between spatial groups in 1973-1974 is likely attributed to animals moving within the region in response to changes in their environment. That is, animal size and location is consistent with southern stock D. delphis moving into the higher quality habitat of the CRD area during the strong El Nino of 1972-1973 when preferred habitat was likely reduced in the area that the southern stock inhabits. Our evidence of animal movement suggests animals cross stock boundaries during periods of significant environmental change. This result has implications for development and implementation of management plans and estimation of life history parameters for D. delphis inhabiting the ETP.