Tunas are amazing nit just in taste. I am intrigued by blue fin tuna and for a very good reason. They are an intelligent species and a species that although universal by name, vary from ocean to ocean. The species that lives in the pacific (Northern Blue Fin) migrates as far as Australia and returns to Japan to spawn. What is fascinating is it is a warm-blooded species and it can regulate its own blood temperature. Hence it is not subject to water depths and it travels long distances as fast at 50kph: http://en.wikipedia.org/wiki/Bluefin_tuna
The magnetcis of blue fin is up for discussion: http://link.springer.com/article/10.1007%2Fs00265-009-0818-2
The authors suggest that the shallowest portions of the spike dive correspond with the times of day when the tuna would get the clearest reading of “celestial polarization patterns,” which are apparently best viewed just below the water’s surface and are known to aid other animals compasses. The authors suggest that the tuna’s diving behavior allows them to both obtain cues from polarized light near the surface, and perhaps obtain a vertical column of other environmental factors (such as light-independent factors at depth, or map information) that combine to help them create and calibrate their internal navigational compasses.
They suggest as one possibility that blue fin tuna may be gathering information at depths related to creating or maintaining a geomagnetic map, which their light cues then help them to navigate. But suggest alternate possibilities that the blue fin tuna navigate via olfactory or acoustic maps, electric fields, or “temperature stratification and associated gradients at depth.” They also acknowledge that if the spike dives are not related to navigation, they may be related to hunting and keeping track of prey which also migrate up and down the water column in patterns similar to the spike dives of tuna.
But it is clear from the emphasis of the paper that the author’s believe there is a strong connection between the diving behavior (mapping), the light-receptive pineal gland (compass), and the blue fin tuna’s navigation. The adult southern blue fin tunas migrate thousands of miles but return to within a few kilometers of their spawning sites to breed. So it follows that they would be strongly selected for any behavior or physical navigation mechanisms that assist them in this journey to breed, and that the best navigators pass on their genetic disposition to the next generation of southern blue fin tuna.