Sea turtle migration

Sea turtle migration refers to the long-distance movements of sea turtles (superfamily Chelonioidea) notably as adults but may also refer to the offshore migration of hatchings. Sea turtle hatchings emerge from underground nests and crawl across the beach towards the sea. They then maintain an offshore heading until they reach the open sea. The feeding and nesting sites of adult sea turtles are often distantly separated meaning some must migrate hundreds or even thousands of kilometres. Several main patterns of adult migration have been identified. Some such as the green sea turtle shuttle between nesting sites and coastal foraging areas. The loggerhead sea turtle uses a series of foraging sites. Others such as the leatherback sea turtle and olive ridley sea turtle do not show fidelity to any specific coastal foraging site. Instead, they forage in the open sea in complex movements apparently not towards any goal. Although the foraging movements of leatherbacks seem to be determined to a large part by passive drift with the currents, they are still able to return to specific sites to breed. The ability of adult sea turtles to travel to precise targets has led many to wonder about the navigational mechanisms used. Some have suggested that juvenile and adult turtles might use the Earth's magnetic field to determine their position. There is evidence for this ability in juvenile green sea turtles.

Efficient movement of hatchlings away from the beach and shallow coastal waters is important in reducing the length of time that they are vulnerable to predators, which target the hatchlings on the beach or in shallow waters. Therefore, sea turtle hatchlings move offshore as an innate behaviour. The first part of the hatchling migration is called the 'frenzy period' which involves almost continuous swimming for the first 24–36 hours.

Studies of loggerhead and leatherback hatchlings have shown that moonlight reflected from the sea is an important visual cuee in guiding movement from the beach to the sea. This navigational mechanism becomes a handicap if nesting sites are affected by artificial lighting since this can mean that hatchlings head towards the artificial lights rather than offshore towards the moonlit sea. Hence, the use of moonlight by turtle hatchings as a navigational cue can be considered an 'evolutionary trap'. Loggerhead and green turtles can detect the orbital movement of waves and use this information to swim perpendicular to the waves crests. This means they swim offshore, since close to the shore, wave crests run parallel to the beach. Further offshore the Earth's magnetic field is used to maintain an offshore direction and therefore head towards the open sea The ability to head in a given direction without reference to landmarks, is called a compass mechanism and where magnetic cues are used to achieve this it is called a 'magnetic compass'. Hatchling loggerheads mature within the North Atlantic Gyre and it is important that they stay within this current system since here water temperatures are benign. It has been shown that loggerheads use the magnetic field to stay within the gyre. For example, when exposed to fields characteristic of a region at the edge of the gyre they responded by orienting in a direction which would keep them within the gyre. These responses are inherited rather than learned since the hatchlings tested were captured before reaching the ocean. Adult turtles may learn aspects of the magnetic field and use this to navigate in a learned rather than innate way.

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