Low reproduction rates (26% pregnancy rates, 3-4 year calving intervals) in two geographically isolated common dolphin Delphinus delphis populations in North East Atlantic (NEA) and New Zealand (NZ) have been found compared to other common dolphin populations where higher pregnancy rates (>40%) exist. The causes of these low rates of reproduction in NEA and NZ are not known but contributory/causal factors may include nutritional stress, immune suppression, disease, pollutants and density dependent population effects. Studies that quantify the complex interacting and potentially confounding effects of multiple stressors on reproduction and development in cetaceans (including synergistic, antagonistic, additive and temporal effects) are urgently required. This project will systematically quantify a range of potential causal factors underpinning the low female fecundity rates in NEA and NZ populations by directly assessing stress and reproductive hormone levels, nutritional condition, reproductive and health status using state of the art (histo)pathological, molecular and other analytical techniques on samples obtained from a large number of postmortem examinations. Interactions between these and other potential causal factors and mating strategies in both sexes derived from behavioural studies of wild common dolphins in Hauraki Gulf, New Zealand will be robustly tested statistically at individual and population levels.