TUNA dive fast and deep twice a day because they use an internal compass to navigate, a new study suggests.
It has long been known that tuna dive around dawn and dusk but no one has been quite sure why. To find out, Jay Willis at the University of Tasmania in Hobart, Australia, and colleagues attached tags to 21 southern blue fin tuna (Thunnus maccoyii) and used them to monitor water temperature, time, depth and light levels for 135 days (Behavioral Ecology and Sociobiology, ).
The team found that the tuna initiated these 鈥渟pike dives鈥� when the sun was precisely 6 degrees below the horizon, 30 minutes before dawn and 30 minutes after sunset.
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At this time of day magnetic interference created by the solar wind is at its lowest. Since some fish can detect and navigate using magnetic fields, Willis thinks that diving at this time may help tuna to get a clearer magnetic signal. As surface wind and waves also cause interference, Willis suggests that they dive deep to 鈥渇ine-tune their personal compass鈥�.
Others are not so sure. 鈥淭here may be other reasons besides geolocation at work here, namely keeping track of food,鈥� says Molly Lutcavage of the University of New Hampshire in Durham. She points out that tuna鈥檚 prey migrate to depth at around the same time.
