Evidence of a Possible Decline since 1989 in False Killer Whales (Pseudorca crassidens) around the Main Hawaiian Islands.

Abstract

Recent evidence indicates that there is a small, demographically isolated, island-associated population of false killer whales (Pseudorca crassidens) around the main Hawaiian Islands. Although it is known that false killer whales in Hawai‘i are sometimes killed or seriously injured in the Hawai‘i-based longline fishery, it is not known whether such interactions have resulted in a reduction in population size or whether other factors have been negatively influencing population size. We report the results of an aerial survey in June and July 1989, the purpose of which was to obtain a minimum count of the number of false killer whales around the main Hawaiian Islands. The false killer whale was the third most commonly seen species of odontocete off the island of Hawai‘i during the survey, representing 17% of sightings. Groups of more than 300 individuals were seen on three different days, with minimum counts of 380, 460, and 470 individuals in these groups. The encounter rate, relative species ranking, and average group size from the 1989 survey were all substantially greater than those from more recent aerial and ship-based surveys. The largest group observed in 1989 (470) contained almost four times as many whales as estimated for the entire main Hawaiian Islands from recent aerial surveys (121 individuals, CV ¼ 0.47) or mark-recapture analyses (123 individuals, CV ¼ 0.72). Therefore, the population of false killer whales around the main Hawaiian Islands may have declined substantially since 1989. The cause or causes of such a decline are uncertain.