It’s commonly said that sharks, especially great white sharks, Carcharodon carcharias, mistake surfers for seals when looking at them from below and thus bite them by mistake.
Researchers from Florida and Vienna, though, have found that this is not the case.
They discovered that the majority of damage to surfers and their boards is at best moderate in nature and does not reflect the level of damage needed to immobilise or stun a seal. Not only that, but the sharks biting surf boards tend to be smaller than those that bite seals.
The working basis of the mistaken identity theory is the silhouette similarity between a seal and surfer when looked at from directly below. However, the average depth among the examined cases was was only around 4m, so the shark is not coming directly from below but at a shallow angle.
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The original research is at
Erich Ritter and Alexandra Quester, “Do White Shark Bites on Surfers Reflect Their Attack Strategies on Pinnipeds?,” Journal of Marine Biology, vol. 2016, Article ID 9539010, 7 pages, 2016. doi:10.1155/2016/9539010