AGE and SEX COMPOSITION OF THE STRIPED DOLPHIN DIE-OFF IN THE WESTERN MEDITERRANEAN

During the morbillivirus epizootic that affected the Mediterranean striped dolphins ( Stenella coeruleoalba )in 1990–1992, several thous and individuals are believed to have died. In order to determine the segments of the population that were affected by the event, the sex- and age-frequency distributions of the animals that were washed ashore on the Spanish coasts were studied. The age of the animals was determined by counting dentine growth layer groups in the teeth. The results obtained indicate that both sexes were affected similarly by the disease and that the population component that suffered the largest mortality was that of sexually mature individuals, although substantial mortality of calves, probably still dependent on their mothers, was also observed. Juveniles, in contrast, appeared to be only slightly affected by the epizootic. This pattern of mortality is not consistent with the epidemiology of morbillivirus infections previously observed in other mammals. Possible explanations of this particular age distribution include behavioral factors, increased susceptibility to the disease in adults due to the effects of pollution, high temperatures and decreased food availability, or allopatric geographical distribution of population components and, therefore, of their associated mortality.