Why do marine mammals strand?

Strandings are natural and have been occurring for millennia.  There are many theories why whales and dolphins strand, but in many cases the cause is unknown and unlikely to be due to any one factor alone.  For many strandings, it can be very difficult to determine the reasons or to link them to other events.

Sick, distressed or dying whales, or mothers having difficulty giving birth may come into shallow water and strand. This is especially the case for single whales or mother/calf pairs. For mass strandings, strong social bonds may lead to a herd following a sick or disorientated pod member into trouble.  

Species which echolocate can run into problems off gently shelving shores.  A gently sloping seabed of sand and mud may interfere with the sonar signals they use for navigation, giving a false indication of deeper water.

Bad weather and rough seas, with dirty water and reduced visibility and sonar effectiveness, could confuse whales and dolphins. 

Mistakes while chasing prey in shallow waters are thought to be a major cause of strandings for orca and dolphins. Conversely, large predators such as orca might cause other marine mammals to panic and strand.

Other theories include magnetic anomalies, earthquakes, and underwater volcanoes. 

The largest recorded mass stranding in New Zealand was in 1918 when about 1,000 pilot whales stranded on the Chatham Islands.

Many Māori view whales and dolphins as taonga (treasured) species and will often be involved in stranding events ensuring correct tikanga (Māori custom and protocols) is followed.

Pilot whales and Farewell Spit

Farewell Spit is a naturally occurring “whale trap” that occurs along a migratory route for long-finned pilot whales in New Zealand. The spit hooks around the northern entrance into Golden Bay forming extensive intertidal sand flats flanked by gently shelving waters offshore. Whales may be easily deceived and caught out by the gently sloping tidal flats and a rapidly falling tide. 

Can pilot whale strandings be prevented in Golden Bay?

A number of proposals have been suggested over the years to prevent whales from stranding or to deter them from an area such as Golden Bay. The use of beacons, sonar reflectors, nets, noise generating devices (including ‘pingers’, distress calls and orca sounds) and other creative solutions to help prevent whale strandings have been suggested and considered for many years. Their use is not being considered at this time for the reasons below.

Golden Bay is habitat for various other cetacean species we don’t want to keep out or harm, including resident Hector's dolphins as well as dusky dolphins, bottlenose dolphins and visiting orca.

Pilot whales are thought to follow traditional migratory routes, and will probably continue to do so, despite any efforts to dissuade them.  They come into Golden Bay to feed and we would be hard pressed to stop them if they are hungry and food is plentiful.

Any system that might keep whales away from the beaches could equally hem them in.  Navigational errors are also only part of the overall picture, and whales may strand for many other reasons which navigational “aids” would not necessarily assist with.

Beacons - buoys, sonar reflectors or noise generators

Pilot whales belong to the dolphin family so are inquisitive.  A series of beacons would not necessarily dissuade them from passing through and such devices could well attract rather than deter them.

An individual beacon would be seen by a pilot whale as an individual obstacle.  The main Golden Bay stranding zone extends over 20 km and even with 400 beacons they would still be 50 metres apart, leaving ample space for pilot whales to swim under or around them.  Nor is there evidence to suggest pilot whales would follow a line of beacons rather than simply swim through them.  For example, dolphins and orca travel through, and under, marine farm structures.  

Cetaceans also learn from experience, and will ignore “illusionary walls” once they discover the illusion.  To illustrate this point, DOC has trialled the use of a bubble curtain to herd pilot whales back out to sea.  This system comprised a long, perforated hose, connected to a compressor to create a wall of bubbles which reflect sonar signals.  This apparatus worked, but only for 20 minutes or so. Once one or two whales discovered it was effectively an illusion, they all began to ignore it. 

Auditory deterrents

Pilot whales are highly social, and using their distress calls to warn them of danger could trigger or contribute to a stranding.

Likewise, orca sounds could well cause panic in a pilot whale pod, and lead to greater problems.

Shark nets

Nets pose a significant risk to whales and dolphins which can become entangled and drown.

Logistical feasibility

To be effective, any system of beacons (or nets) would need to extend over many kilometres and involve a very large number of structures.  Such a system would be expensive to manufacture, install and maintain.  Buoys are used to mark the boundaries of some marine reserves and well-illustrate the very high costs involved.  Bio-fouling is a huge problem and creates costly and on-going maintenance issues.

Devices close to shore would be particularly difficult and costly to maintain given they would have to cope with large waves from strong onshore winds.

Large parts of Golden Bay are trawled and dredged by the commercial fishing industry, and it would be difficult to obtain approvals to install any devices which would interfere with these activities. Such devices could also be easily lost or damaged because of fishing.