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Sunday, July 8, 2012
Myths In The Open Water: Sharks vs. Dolphins
One issue that is commonly reported in the press is that "Dolphins protect swimmers from sharks."
This oft-used quote is a dramatic indication how scary it is to swim with sharks and how comforting it is to swim with dolphins.
But is it true?
Are open water swimmers truly and naturally protected from sharks by dolphins and porpoises as is commonly believed by many channel swimmers? There are a wide variety of opinions on this topic.
Many open water swimmers around the world, especially channel swimmers and marathon swimmers, often tell newcomers that when dolphins or porpoise are near them in the open water, there is no need to fear sharks. It is often said to be a sign of good luck and protection. The belief is that sharks fear dolphins and porpoises that can easily defend themselves against the apex predators - and will, in turn, protect swimmers, their mammalian friends.
Escort pilots of channel swimmers throughout the Pacific, from the Catalina Channel to the Hawaiian Islands, also share this opinion.
According to Peter Klimley and David Ainley who wrote a book Great White Sharks: The Biology of Carcharodon carcharias, Great white sharks attack dolphins and porpoises from above, behind or below to avoid being detected by the friendly mammal's echolocation. The targeted species include dusky dolphins, Risso's dolphins, bottlenose dolphins, Humpback dolphins, harbour porpoises, and Dall's porpoises.
Scientists like Michael Heithaus also raise possibilities that differ from the widely held beliefs of open water swimmers. He presented a study called Predator–prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review, that shares findings similar to television programs like the Discovery Channel's Are Sharks Afraid of Dolphins?. He concluded that sharks remain the apex predator in the ocean and do not fear dolphins or porpoises, except perhaps when out-numbered.
His study in the Journal of Zoology (Cambridge University Press) stated, "White, tiger, bull and sevengill sharks are probably the major predators on nearshore cetaceans, but dusky sharks may also represent a risk. The risk that nearshore cetaceans face from sharks will vary with location. For example, the risk of shark predation is probably higher in tropical waters than in higher latitudes because of the diversity and abundance of large, predatory sharks in warm waters."
Richard Theiss of RTSea Productions believes what the scientific community has presented in published peer-review articles, but he also points out an important truth. "...what Michael had to say in his study is important: different species, different locations, different circumstances – all this makes it difficult to make a definitive statement. [In reference to the] Mythbusters video, there are errors in not making the distinction between a white shark’s role as an ambushing predator and that of a scavenger. Each generates different behavior patterns. As a scavenger, which is what it was doing with the floating tuna head, it can be easily put off by another large fish in the area (did it look like a dolphin or another shark?). It’s just looking for an easy meal and doesn’t want to have to compete for it. I’ve seen this many times with white sharks at Isla Guadalupe (see RTSea Productions video below).
So, sharks will kill dolphins on occasion – seeking a slow, sick, or aged fish, just like the Great white shark does in selecting a pinniped (seal or sea lion). What needs to be examined in a definitive study and not just through anecdotal storytelling is whether dolphins will show any aggressive tendencies towards sharks. It’s not so much as to whether sharks are scared of dolphins, but whether dolphins are fearless when it comes to sharks."
Heithaus, who directs the Shark Bay Ecosystem Research Project in Shark Bay, Western Australia, encourages further study of the natural relationship between sharks and dolphins where more information and understanding about the nature of shark-dolphin interactions is needed.
A MythBusters episode on the Discovery Channel also explored this issue: Are Sharks Afraid of Dolphins?.
Other commonly held beliefs among open water swimmers include:
1. Most body heat escapes through your head in the water (read here).
2. Commercial jellyfish ointments will prevent jellyfish barbs from firing into the skin of open water swimmers.
3. Black wetsuits lead to shark attacks (read here).
4. Shark risks increase at dawn and dusk (read here).
5. More people have been in space than have swum across the English Channel (read here).
Copyright © 2012 by World Open Water Swimming Association
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