Dr David Sims of the Marine Biological Association and colleagues examined diving behaviour of four basking sharks (Cetorhinus maximus) - two in the shallow sea off Plymouth and two in the deep water off the shelf-edge southwest of Ireland and in northern Clyde Sea in Scotland - using pop-up tags that measure swimming depth, water temperature and light levels. The tags were programmed to detach themselves from the sharks at a set time, float to the surface and then drift with the currents like a "electronic messages in bottles", before being washed up on beaches and found by the public.
Sims found that while the sharks in deep waters exhibited normal diving behaviour, tracking the zooplankton Calanus up to the surface at dusk and then downward at dawn, sharks in the western English Channel did the reverse. This is the first time this behaviour has been observed among plankton-eating sharks, the authors say, and shows that shark diving behaviour differs predictably between deep waters and in shallow seas close to plankton-rich boundaries in water temperature.
Although the mechanisms underlying this behaviour are unclear, the results indicate that the sharks are responding to changes in vertical migration by the zooplankton. Zooplankton have evolved a range of behaviours to try and avoid being eaten, sometimes staying at greater depths during the day and then feeding near the surface at night but at other times reversing this behaviour in an attempt to
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