Observation and sampling of a large aggregation of sperm whales (Physeter macrocephalus) on the Offshore Grounds, Equatorial Pacific

Sarah D. Davidson
Affiliations: The Ocean Alliance, 171 Weston Road Lincoln, MA 01773
Graduate School of Oceanography, University of Rhode Island,
South Ferry Road Narragansett, RI 02882

During a Pacific leg between the Galapagos and Marquesas Islands in July/August of 2000, the Ocean Alliance's research vessel Odyssey encountered a large aggregation of sperm whales in the eastern equatorial region of the Pacific. This area was historically referred to as the Offshore Grounds by whalers. The group of whales was tracked for 8 days using visual detection of surfacing individuals and acoustic monitoring of clicks during daylight hours, and just acoustic surveillance at night. The aggregation consisted of an estimated 60 to 70 individuals including adult females, subadults, calves, and 9-10 mature males. During the 8 days, acoustic recordings were made of click trains and codas, and 54 skin/blubber biopsies, representing an estimated 80-93% of the aggregation, were collected for forthcoming genetic and toxicological analyses. The whale aggregation was stable and cohesive, and showed no signs of dispersing through the 8th day, when contact was lost. Although it has been hypothesized that smaller social units of P. macrocephalus may converge into large aggregations for migration, this aggregation did not maintain a fixed direction of travel. Instead, individuals were swimming, socializing (including breaching and lobtailing), and apparently feeding within a relatively constrained area. We hypothesize that this aggregation may have formed and persisted because of localized favorable food conditions . The unique bathymetry in this area of the Offshore Grounds may contribute to increased biological productivity, and numerous squid catches by the Odyssey crew while with the whales also lend support to this hypothesis.