Mechanisms of migration
Fishes may move thousands of kilometers through the open and seemingly landmark-free ocean. A great deal of research has focused on the means by which fish undertake longdistance migrations, specifically how they orient toward and locate their ultimate destinations. Research has identified numerous possible cues used in orientation, including sun and polarized light, geomagnetic and geoelectric fields, currents, olfaction, and temperature discontinuities and isolines (Leggett 1977; McCleave et al. 1984; McKeown 1984).
Birds use a sun compass and internal clock to orient. An animal must be able to sense the time of day, the altitude, azimuth (angle with the horizontal), and compass direction of the sun at a given time and date, correcting for the 15°/h movement of the sun across the sky. Experimental evidence suggests that some fishes use such a mechanism. Swordfish (Xiphias gladius) can maintain a constant compass heading in the open sea for several days. Displaced parrotfish return relatively directly to their home locations on sunny days. When the sun is obscured, when fitted with eyecaps, or when held in darkness such that their internal clocks have been shifted 6 h, displaced fish are disoriented or move in a direction appropriate for a 6 h clock shift. Juvenile Sockeye Salmon have a sun compass which they complement with a magnetic compass at night or during overcast conditions. Polarized light can also provide directional cues, and Sockeye Salmon are able to detect and discriminate between vertically and horizontally polarized light, which could aid them particularly during dawn and dusk migrations toward the sea, when light is maximally polarized. Minnows, other salmonids, halfbeaks (Hemiramphidae), damselfishes, and cichlids can also sense polarized light, which often involves detection of ultraviolet radiation undiscernible to the human eye (Quinn & Brannon 1982; McKeown 1984; Hawryshyn 1992; Mussi et al. 2005).
A magnetic compass implies a sensitivity to the earth’s magnetic fields. Such a sensitivity has been demonstrated in elasmobranchs, anguillid eels, salmonids, and tunas. Sharks are theoretically capable of navigating using geomagnetic cues, since they can detect fields 10 to 100 times weaker than the earth’s magnetic field, as well as fields created by ocean currents moving through the earth’s magnetic field, or fields induced by their own movement. An induced field would change as the animal’s compass heading changed, being strongest when moving east or west and weakest when heading north or south, thus giving it directional information. A magnetic compass could be useful in transoceanic migrations undertaken by large pelagic sharks.
Orientation abilities are also needed for homing, as happens when Scalloped Hammerhead Sharks, Sphyrna lewini, return daily to small seamounts in the Sea of Cortez after foraging offshore at night. Scalloped Hammerheads may use a combination of directional cues, including visual landmarks, auditory cues produced by fishes and invertebrates, electrical cues induced by site-specific currents, and geomagnetic fields at seamounts. The use of multiple cues and redundant systems are a general feature of migratory animals. Redundant information increases the accuracy of the information, and backup systems provide information when conditions interfere with or negate the use of other cues (Kalmijn 1982; Klimley et al. 1988; Klimley 1995; Meyer et al. 2005).
Water currents serve to transport fish eggs, larvae, and adults, but may also provide orientational information. Where currents border on other water masses, differences in water density, turbulence, turbidity, temperature, salinity, chemical composition, oxygen content, and color could all act as landmarks to a migrating fish (once inside a current and out of sight of or contact with the bottom or other stationary objects, it is difficult to imagine that a fish could sense the water’s movement, unless the fish could detect induced magnetic fields as discussed above). In shallow waters, many fishes show a positive or negative rheotactic response that causes them to move up- or downstream, respectively. The strength and direction of response may change with season and ontogeny. Selective tidal stream transport (see above) is such a response, whereby a fish moving upriver in an estuary swims actively against an ebbing tide and drifts passively with a flooding tide. Olfactory cues are often carried on currents. Homing of salmon to chemicals in the streams in which they were spawned (see below) probably applies to many stream and intertidal fishes (e.g., minnows, sculpins, blennies), although the age at which a fish learns the chemical fingerprint of a water body will vary. Sensitivities to familiar chemicals are extreme, on the order of 1 : 1 x10−10 or 10−19, depending on species, suggesting that just a few molecules of a substance are necessary for detection (Hara 1993).
Seasonal movement is induced or directed by temperature changes in several migratory species. American Shad, Alosa sapidissima, move north along the Atlantic seaboard in the spring, staying in their preferred water temperatures of 13–18°C. Individuals may winter as far south as Florida and spawn in Nova Scotia, 3000 km away. Some oceanic species follow specific isotherms during seasonal migrations. Albacore Tuna, Thunnus alalunga, move north during the summer along the Pacific coast of North America, staying within a fairly narrow 14.4–16.1°C temperature zone; east–west movements are contained within a temperature range of 14 and 20°C. Onshore arrival of water masses of the preferred temperature serve as predictors of the arrival of the fish. Many other tuna species also migrate to stay within fairly narrow temperature ranges.
Many pelagic fisheries, which rely on oceanic migrations to bring fish into regions on a seasonal basis, are highly dependent on water masses of the correct temperatures moving into specific areas. Cod and Capelin (Mallotus villosus) in the Barents Sea of northern Europe are available to Finnish fisheries in cold years when fish migrate farther west to warmer waters. In warm years, fish restrict their movements to the eastern side of the basin and are then exploited in the Murmansk area. The response to temperature may be a direct, behavioral one involving thermal preferenda, or an indirect response related to food abundance. Often, plankton blooms are associated with changing water temperatures and hence fish may be tracking food availability that responds to temperature. Herring in the Norwegian and Greenland seas migrate in response to the inflow of warm Atlantic water, which in turn stimulates plankton growth and food availability (Leggett 1977; McKeown 1984; Dadswell et al. 1987).
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