New Zealand has one of the highest stranding rates in the world. On average, about 300 dolphins and whales strand each year. Most stranding are of individual animals, but mass strandings are common and can involve hundreds of animals at a time.
Strandings are complex events and there are many reasons why dolphins and whales may strand. In most cases the exact cause is unknown but any one of the following factors, or a combination of them, can be the cause.
Old or sick
- Old whales may find it difficult to keep up with their pod or resist heavy swells or inshore currents. Because of failing strength these animals may strand. Generally they are found to be in poor health
- Whales can suffer from a number of diseases. It may be a temporary affliction or something more severe. The effects of such disease leave them weak and disorientated, or with impaired echolocation and they strand
Environmental factors
- Natural toxins can poison whales. Because they are at the top of their respective food chains, pollutants tend to bioaccumulate in their blubber.
- A shortage of food caused by overfishing can result in malnourished whales. Separated dependent calves and old adults may also be malnourished. Sadly many stranded whales have been found with large volumes of litter or plastic in their gut. Ingestion of these materials can lead to disease and malnutrition
Birthing
- Calving whales will often seek out sheltered bays to give birth to their young. If they come too close to shore they may strand. In particular, whales which are having problems birthing are more likely to strand
Injured
- Whales that escape net entanglements or are hit by vessels may sustain serious injuries, such as broken teeth and jaws, deep lacerations, flipper dislocations or fractures, spinal or muscle damage or severed fins or flukes. This may result in them stranding
- Underwater explosions caused by sonar, seismic testing or underwater sea quakes can have a devastating impact on whales. These loud explosions can damage their hearing and affect their ability to communicate, hunt and navigate
Navigational errors
- When chasing prey, whales may accidentally beach themselves. They can be picked up by a wave and thrown onto a beach or be left high and dry by the receding tide
- They may also come too close to shore to avoid predators, such as orca
- Gently shelving, sandy beaches may not reflect echolocation signals back to the whale, leading them to believe they are in deeper water. Combined with a fast dropping tide whales can be left high and dry
- Some whales may use geomagnetic contours to navigate and where these cross the beach or an outcrop of land, this can result in a whale following the line of the contour on to the shore
- Unfamiliar coastal configuration or unusual weather patterns, particularly electrical storms, may also cause whales to strand through navigational error
Social bonding
The strong social bonding of some species of whales can cause mass strandings. Whales that strand in groups are usually deep water species with highly evolved social structures. In New Zealand, the most common species to mass strand are long-finned pilot whales. Whatever the reason for the initial stranding, the strong social bonds of these animals can draw the rest of the pod in.
One of the most common patterns with mass strandings is that one or two whales will initially strand. These animals will send out distress signals and members from their pod may attempt to help or mill slightly off-shore. A receding tide will then catch these animals out and soon the whole pod will become stranded.