Mass Strandings

What are Mass Strandings?

Mass strandings are events where 2 or more injured or dead cetaceans are found beached in the same area or in the same tidal cycle. Common cetaceans to become stranded are toothed whales and dolphins - and due to them being social groups, if one becomes stranded, its likely the rest will follow suit. Some organisations also use the term "mass stranding" to refer to pinnipeds found dead or injured on land or shallow waters.

Why do strandings happen?

It has been well documented that mass strandings can occur due to the following reasons:

• Injuries from ships,
• Fleeing from predators,
• Entrapment or entanglement from fishing gear,
• Viral, bacterial or parasitic infections,
• Starvation,
• Unusual oceanographic events.

In recent years however, there have been many studies into the effects of noise pollution on marine animals - particularly cetaceans, and many strandings have now been marked as the consequence of this with anthropogenic sources such as military sonar and oil exploration.

Some examples:

• 12 Cuvier's beaked whales in Kyparissiakos Gulf, Greece (12th-13th May 1996)
• 16 beaked whales (multiple species) in the Bahamas (15th-16th March 2000)
  • This mass stranding was pivotal in the awareness of noise pollution, being the first mass stranding directly linked to naval sonar operations being carried out. The Natural Resources Defence Council consequently filed a lawsuit against the US Navy requiring them to adopt certain security measures to protect marine mammals. This included no use within 12 nautical miles of the coast, and shutdown when marine mammals were within 2200 yards. 
• 3 Cuvier's beaked whales in Madeira (May 2000)
• 19 beaked whales in the Canary Islands (24th September 2002)
  • This incident took place just 4 hours after mid-frequency sonar exercises were being commenced. From the autopsies, it was discovered the whales had severe congestion and haemorrhaging, as well as gas-bubble associated lesions. This lines up with previous theories that whales may swim up rapidly during these exercises, and ultimately end up with nitrogen supersaturation - otherwise known as decompression sickness or "the bends" in humans.  
• 6 bottlenose dolphins in Fujian (10th June, 2021)
• 12 melon-headed whales in Zhejiang (6th July, 2021)
• 12 melon-headed whales in Zhejiang (30th September, 2021)
• 12 melon-headed whales in Shanghai (3rd October, 2021)
  • All 3 of these mass strandings in China took place along the continental shelf of Zhejiang Province and Shanghai Municipality, where the Zhoushan Fishing Ground is also located. This fishing ground is the fifth largest in the world, which results in a large influx of ships travelling through the area. Because of this, all 3 of these mass strandings have been attributed to the noise pollution caused by this shipping traffic.