Cetacean sightings around the Republic of the Maldives, April 1998

In April 1998, as part of a project to collect biopsy samples of putative pygmy blue whales (Balaenoptera musculus brevicauda) in the waters around the Republic of the Maldives, Indian Ocean, incidental sightings of cetaceans encountered were recorded. Using modified line-transect methods and handheld binoculars, a total of 267 sightings of 16 species of whales and dolphins were recorded during 20 at-sea days in the northeastern part of the atoll. Significant results include the following: (1) cetaceans were abundant and species diversity was high, including nearly every pantropical species of pelagic cetacean; (2) the spinner dolphin (Stenella longirostris) was by far the most common species encountered (56 sightings) and also had the largest mean school size ( = 50.3 individuals); (3) blue whales were rare; only four individuals were sighted; (4) a large concentration of Bryde’s whales (28 sightings in two days) was apparently feeding in nearshore waters; (5) this paper reports the first records for the Maldives of Cuvier’s beaked whale (Ziphius cavirostris), Blainville’s beaked whale (Mesoplodon densirostris) and the dwarf sperm whale (Kogia sima): the latter was particularly common (17 sightings); (6) the spotted dolphin (Stenella attenuata) was rare and almost always associated with yellowfin tuna (Thunnus albacares), spinner dolphin, or seabirds, as has been reported in the eastern Pacific and western Indian oceans.

Modeling Age-Specific Mortality For Marine Mammal Populations

A method is presented for estimating age-specific mortality based on minimal information: a model life table and an estimate of longevity. This approach uses expected patterns of mammalian survivorship to define a general model of age-specific mortality rates. One such model life table is based on data for northern fur seals (Callorhinus ursinus) using Siler’s (1979) 5-parameter competing risk model. Alternative model life tables are based on historical data for human females and on a published model for Old World monkeys. Survival rates for a marine mammal species are then calculated by scaling these models by the longevity of that species. By using a realistic model (instead of assuming constant mortality), one can see more easily the real biological limits to population growth. The mortality estimation procedure is illustrated with examples of spotted dolphins (Stenella attenuata) and harbor porpoise (Phocoena phocoena).