Apparent Low Densities of Small Cetaceans in Okinawa May Be Due to Uncontrolled Local Hunting

From October 2011 to May 2012, we conducted the first marine mammal surveys of Okinawan waters, using line-transect vessel surveys and passive acoustic monitoring (PAM). We obtained 913 km of visual survey effort, and 1,039 cumulative hours of PAM data from around the island. Only a single on-effort sighting was made (of two common bottlenose dolphins [Tursiops truncatus]) during visual surveys, and acoustic detection rates were generally low, with the exception of humpback whale (Megaptera novaeangliae) observations during their winter/spring breeding season. The apparent low density of at least some marine mammals observed in Okinawan waters may be due to the depletion of coastal populations. We believe this could result from many decades of heavy and largely-uncontrolled hunting of small cetaceans in waters around the island.