New Zealand is one of the main locations for whale and dolphin (cetacean) strandings. Records of strandings have been kept since 1840. Even though early records are sketchy and were usually of strandings near centres of population, they do show trends. About 13,000 individual cetacean strandings have been recorded, and by 2006 over 2,000 creatures had been successfully rescued.
Certain areas are stranding hotspots: Ōpoutama (Māhia Peninsula), Golden Bay and the Chatham Islands. Some species are attracted to particular areas: sperm whales to Kaipara Harbour and pilot whales (which belong to the dolphin family) to Doubtless Bay, Ninety Mile Beach and Stewart Island. In 1998 Doughboy Bay, western Stewart Island, was the scene of 328 pilot whales stranding.
Just six species account for 88% of strandings: the long-finned pilot, sperm, false killer, pygmy sperm, Gray’s beaked whales and the common dolphin. Three of these species, the long-finned pilot, false killer and the sperm whale, are mass stranders. In 207 incidents, more than 9,000 long-finned pilot whales have beached themselves, with just over 2,000 being successfully refloated. In 1918, New Zealand’s (and the world’s) worst recorded stranding occurred when around 1,000 pilot whales came ashore at Long Beach, Chatham Islands.
Pilot whales: common stranders
Pilot whales are members of the same family as dolphins. The long-finned whale (Globicephala melaena) is larger than the short-finned (Globicephala macrorhynchus), and more common around New Zealand. Both frequent deep water, which accounts for the difficulties they experience when they encounter unfamiliar shallow waters.
Why do whales become stranded?
Scientists believe there are three main reasons why New Zealand’s coastline is such a hazard for cetaceans:
- Certain beaches such as Golden Bay are ‘whale traps’, with protruding coastlines and long, gently sloping beaches. These are usually the locations of mass strandings. Offshore cetaceans, such as long-finned pilot whales and beaked whales unfamiliar with the coast, are likely to get into difficulties here.
- Toothed cetaceans use echolocation to navigate, but this might not work on gently sloping beaches. Scientists using navy echo-sounders receive a weak echo from these beaches compared to steep, shingle boulder beaches or rocky coasts.
- Local oceanic conditions may account for strandings. Golden Bay and Hawke Bay have currents that appear to explain strandings there.
There are other possible causes:
- Cetaceans may become stranded for biological reasons. Theories abound, from parasitic worms in the brain affecting co-ordination and balance, to whales being trapped as they follow prey inshore.
- Predators such as orcas or sharks may force the marine mammals inshore. In a 1993 pilot whale stranding at Golden Bay, orcas were seen patrolling outside the spit. The whales might have sought sanctuary in shallow waters because of the threat.
- Deep-water toothed whales with strong social bonds become stranded in a group more frequently than other species. If an individual is sick, or old and dying, the rest of the herd will come to its aid, even if they endanger themselves by venturing too close inshore.