An analysis of New Zealand whale strandings (1990)
AuthorsBrabyn, Mark Williamshow all
The New Zealand Whale Stranding Data Base (NZWSDB) was set up in association with the Department of Conservation. The NZWSDB contains 1140 records of whale strandings involving a total of 8287 individuals, 35 species, 163 herd strandings, and 304 known live strandings. The geographical distribution and seasonality of strandings are given for 25 species of whales. Major hotspots for strandings occur at Whangarei, Hawke Bay, Farewell Spit, and Chatham Islands (Waitangi and Okawa). Herd stranding is most pronounced for offshore delphinids, in particular the pilot whale. Herd strandings are highly clumped in distribution. In contrast, single-dead strandings (indicative of the population distribution) are evenly scattered. There is a summer high and winter low in the seasonality of strandings. Physical aspects of herd stranding sites are compared with random sites. Slope and bay indentation of stranding sites are significantly different from random sites. Coastal configurations of world multiple herd stranding sites are compared and show similar protruding coastlines with long gently sloping beaches. These configurations and associated currents may trap migrating whales. Klinowska's theory relating stranding sites to geomagnetic topography is tested for 126 herd strandings and 147 single-live strandings. New Zealand herd strandings show no relationship to perpendicular geomagnetic contours or magnetic minima, and whales do not appear to avoid magnetic gradients. Weather conditions at the time of and 24 hours prior to, whale stranding dates are studied for a set of 24 pilot whale herd strandings and 4 sperm whale herd strandings. A significant relationship is found between strandings and increasing barometric pressure. No obvious relationship between whale stranding dates and the lunar cycle is found. Whale strandings are divided into nine categories based on distributional and site patterns. Theories on the causes of whale strandings are discussed. Those theories that do not explain the highly clumped nature of strandings are rejected.