Apart from the convenience of researchers, film producers and beach wardens, why do sharks spend so much time displaying just their fins?
鈥� Very few species of shark make a habit of poking a dorsal fin out of the water. The main exception is the basking shark, which has a large dorsal fin and often swims close to the surface to feed on plankton. Other sharks show their fins rarely, and when they do, just like the basking shark, it is incidental to activity that brings them to the surface.
To describe exposing the fin as a 鈥渄isplay鈥� is misleading, because the shark is not doing it deliberately, any more than you 鈥渄isplay鈥� your foot to worms if you step into mud. The fin is simply appearing when the shark approaches the surface.
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The dorsal fins of many sharks that swim constantly act as keels, helping to keep them upright and to provide control in high-speed turns. Most bottom-dwelling sharks don鈥檛 need that kind of control, so many of them don鈥檛 have conspicuous dorsal fins.
Jon Richfield, Somerset West, South Africa
鈥� Fin display is not a deliberate action, but a consequence of a shark putting its mouth and eyes where the food is 鈥� in this case near the surface. One other consideration is that surface water has a slightly higher oxygen level and is also slightly warmer, hence less dense, so this may also tempt sharks to the surface.
Andrew Doherty, Tongala, Victoria, Australia
鈥� Over the past 50 years, I have done more than 2000 ocean swims, totalling approximately 8000 kilometres. I occasionally see sharks, but I have never seen a shark with its fin out of the water.
It is worth noting one consequence of sharks spending a lot of time near the surface: they produce melanin and can develop a suntan. As for why people think sharks show their fins above the water a lot, there are two reasons. First, movies love to portray them this way because it is easy to make a fake shark fin, poke it out of the water and film it. It is much harder to make a realistic fake shark. Second, most people are at the surface when they see sharks, so inevitably only see them at this point. This is a form of availability bias, where we tend to think that what we see represents the general case when actually it probably isn鈥檛.
Stephen Johnson, Eugene, Oregon, US
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