High and dry: the stranding of whales and dolphins It’s been another beautiful summer’s day in Golden Bay. Families are relaxing, enjoying all sorts of aquatic activities. Suddenly a report sweeps through the local communities that a mass stranding of pilot whales has occurred. Department of Conservation (DOC) staff plus community volunteers rush to the beach and cover the pilot whales with wet sheets to protect them from sunlight and overheating. The DOC staff make decisions on which whales are likely to survive. Those with good chances of survival are fitted with whale rescue pontoons and gently re-floated in the sea.
Long-finned Pilot whales (Globicephala melas) are a species of mammals. They belong to the cetacean order of whales, dolphins and porpoises. It is thought there may be 200,000 of them living in the deep waters of the Southern Ocean.
Looking for reasons
DOC runs a New Zealand Whale Stranding Database and has been collecting information on where the stranding happens and the time of year it occurred. From 1976 to 2000 we find that 68% of the pilot whale strandings take place in spring and summer. This is not surprising as pilot whales migrate from place to place in search of giant squid, their favourite food. Most calves are born in summer and they are social animals that stick together in groups of 20 – 90.
While pilot whale strandings occur all over New Zealand’s long coastline, 48% of them occur in the Nelson, Northland and Chatham Island regions. These three areas have long, gently sloping beaches. We know that echo-sounders receive a weaker echo from these beaches than steeper beaches or rocky coasts. Whales and dolphins use echolocation to navigate and if the signal is weak then navigation will be more difficult.
While no one is sure scientists have come up with other possible causes. It is possible harmful worms in the brain affect co-ordination and balance or they become trapped as they follow prey inshore. Predators such as killer whales or sharks may force the whales and dolphins inshore. Finally, if an individual is sick, or old and dying, the rest of the herd will come to its aid, even if they put themselves in danger by swimming too close to the shore.
Being stranded can also crush a whale’s internal organs. Why would this happen?
Why do we get whale strandings in Nelson and not usually in Kaikoura?
Do we know why strandings occur? What are some of the ideas about why whales strand?
Some cetaceans are capable of echolocation. These cetaceans use sound in the same way as bats—they emit a sound (called a click), which then bounces off an object and returns to them. From this, cetaceans can tell the size, shape and movement of the object, as well as how far away it is. With this ability cetaceans can search for, chase and catch fast-swimming prey in total darkness. Echolocation is so advanced in most toothed whales that they can tell the difference between prey and non-prey (such as humans or boats).
The sounds produced may be divided into: (1) pure tone whistles in the frequency range 500 Hz to 20 kHz, used mainly for communication; and (2) clicks varying from 500 Hz to 150 kHz, used mainly for echolocation. The loudness for both types of sound is estimated to be between 150 and 200 dB.
Pitch of sound (deep or high), measured in Hertz (Hz)
20Hz is lowest pitch sound humans can hear.
256Hz is the pitch a middle C tuning fork makes.
512Hz is the pitch a C sharp tuning fork makes.
20kHz or 20,000Hz is the upper limit that humans can hear.
150kHz or 150,000Hz that dolphins use is far higher than what humans can hear.
Loudness of sound, measured in decibels (dB)
Quiet whisper in Library is 30dB
Normal conversation is 60 – 70dB
Motorcycle is 100dB
Loud rock concert is 115dB
Sound becomes painful at 125dB
If we could hear dolphin’s whistles and clicks we would find the sound extremely painful!
How are speed camera traps like cetacean echo-location?
What information does a cetacean know about its prey, even in the dark?
What advantage is it to humans not to hear above 20,000Hz?
A navy connection?
In 2000 the U.S Navy were having training exercises in the Bahamas. The ships used sonar, use underwater radar called sonar to locate underwater threats, such as a submarine. Within 36 hours fourteen whales were found beached on three nearby islands. In June 2008, twenty six dolphins beached and died in shallow waters off the Cornwall coast, England. Again the Royal Navy had been carrying out exercises off the Cornwall coast. Some people are suggesting the Navy’s sonar causing the deaths; others blame killer whales chasing the dolphins.
Scientists from St Andrews University (Scotland) and Woods Hole Oceanographic Institution (Massachusetts, USA) have been trying to think through the puzzle. Many non-Navy ships use sonar without cetaceans being stranded and some species will strand themselves without the Navy being near.
The scientists used underwater microphones to listen to the different sounds the dolphins make. So far their research has shown that dolphins are really loud and that dolphins can hear each other over distances up to 25km.To overcome the problem of a lot of background noise dolphins have developed signature whistles that tell the caller’s identity and location. Dolphins also copy each other’s whistles if they are separated by long distances, so they can stay in touch.
A call that is common in bottlenose dolphins around Scotland is the bray call. Dolphins produce these when they find fish. When other dolphins in the area hear the call they swim to the caller’s position as fast as they can.
Scientists have also found that the frequency given off by some Navy sonar’s is about the same as that of killer whales. Dolphins and smaller whales are favourite food for killer whales. In an experiment scientists played the sound of killer whales through underwater loudspeakers. When dolphins heard the sound they switched to what is called “stealth mode”. The dolphins stopped sending out signals that detect prey and glide silently and slowly to the surface for air, then leave the area.
Civilian ships and fishing boats tend to use sonar with higher frequencies than Navy ships and this may explain why they do not seem to affect dolphins. Like all good scientists however, their research must be repeated before a conclusion can be made.
7) How do dolphins react if they pick up killer whale signals?
8) What evidence is there that Navy ships may cause dolphins to strand?
9) What evidence is there that it may be something else that causes dolphins to strand?
A New Zealander’s story
During World War One much of Europe was at war. The British Royal Navy ruled the seas with large battleships, but the German Navy had found a way to threaten this. Their U-boat submarines were under the instructions to sink merchant ships. On 7 May 1915, U-20 sank the liner RMS Lusitania with a single torpedo hit. The sinking claimed 1,198 lives.
The U-boats sunk 1.4 million tons of shipping between October 1916 and January 1917. Ernest Rutherford left his research to work with the Royal Navy and find a way of detecting the submarines, so that the merchant ships bringing vital supplies to Britain could be protected. He developed a hydrophone with 2 ears that could rotate to detect submarines. Rutherford also suggested using sound waves reflected from submarines as a future method of detecting submarines. In 1917 he was sent over to the United States to transfer all the knowledge that he and other British scientists had developed on submarine detection.
Cetaceans and the future
Scientists have noticed that Navy sonar is not the only threat. The biggest threats are being hit by ships or caught in fishing gear.
In New Zealand, DOC is responsible for the protection and management of marine mammals. They have developed a plan to manage the whale- and dolphin-watching industry. A marine mammal sanctuary was established in 1988 around Banks Peninsula to protect South Island Hector’s dolphin. Set net controls were introduced in Canterbury and the North Island’s West Coast to protect both the South Island Hector’s and Maui’s dolphins. Some rules we can all follow are
Avoid using set nets, especially when you cannot remain with your net.
Don’t swim with the dolphins except with authorised tourist operators.
Avoid wearing suntan lotion or insect repellent if swimming near dolphins.
Do not try to touch dolphins.
If in a boat use a ‘no wake’ speed within 300m of dolphins.
Do not feed dolphins. Human food is harmful.
Keep their environment clean. Take your rubbish home, and if you find any floating at sea or on the coast, please pick it up.
10) Why was Rutherford’s submarine work important?
11) What is a bigger threat to dolphins than Navy sonar?
12) Why can rubbish be harmful to dolphins?
High and dry: the stranding of whales and dolphins Student tasks
The article has a number of words that may be new to some readers. To increase wordpower complete the following exercises 1) to 6).
High and dry: the stranding of whales and dolphins
Missing Word Exercises In New Zealand the Department of _____________ look after cetaceans (whales and dolphins). Most strandings occur in spring and summer and often in shallow beaches in ________, Northland and Chatham Islands.
There are a number of reasons why whales and dolphins may become stranded. One reason is that of __________ beaches where their sonar does not work as well. The animals may be chasing food or fleeing from predators such as ________ _______. They are also very __________ animals and will stay with members of the group that are sick, even if they swim into danger. Another reason could be due to _____ ships using sonar which is similar to the sounds killer whales.
Dolphins and whales use ______________ to find food and communicate with each other by ___________.
Ernest _____________ helped develop hydrophones that could detect ____________. These underwater vessels were sinking lots of ships in World War One.
Whales and dolphins are ____________ in New Zealand. This means we can’t have set nets where they live and have to be careful a boat does not hit them.
List: Conservation echolocation killer navy Nelson protected Rutherford shallow social submarines whales whistles