Mass stranding of Baird's beaked whales at San Jose Island, Gulf of California, Mexico

Abstract

The Baird's or giant beaked whale ( Berardius bairdii ) is endemic to the North Pacific Ocean. In the eastern North Pacific, seasonal movements are poorly understood. Historic California catches suggest two peaks of abundance, in July and October. Mass strandings of Baird's beaked whales are rare. Prior to this report only two records were known, in the Bay of La Paz, Mexico in 1986 and Sagami Bay, Japan, in 1987; both mass strandings included male and females. Here we report a mass stranding of 10 Baird's beaked whales at San Jose lsland in the Gulf of California on 31 July 2006. The group was composed only by males ranging from 9.97m to 11.05m. The estimated age, based on the dental group layers, ranged from 9 to 42 years. The cause of this stranding is unknown, although with the large number of squid also found dead in the area, it is possible that the whales could have been following the squid, one of their main prey. The mass stranding of 10 mature males reported here supports the idea of a seasonal presence, during summer, of this species in the waters of the southwest Gulf of California. The group composition supports a reproductive strategy where the old males have a major role in the guidance and protection of inexperienced weaned and young males, unlike sperm whale bachelor groups, where all males are typically the same age.