To: Jacques Chitte who wrote (18361 ) 5/8/2000 10:23:00 AM From: Neocon Respond to
Other natural marvels:britannica.com Experiments in Canada and the United States, in which young salmon migrating to the Pacific have been tagged, have shown that a high proportion of the fish return to the river where they hatched. Tagging of Atlantic salmon has shown that a few survivors have migrated two or even three times to a particular river in successive years. Adults reared from experimentally transplanted eggs return to the stream where they were hatched or grew, not to the stream where the eggs were laid. Aside from other means of orientation, such as reference to celestial features, topographical features are believed to play an important part in recognition of the original habitat. The sense of smell, or olfaction, however, has the most important role. Experiments have shown that migrating salmon are attracted to the waters of the stream in which they are going to spawn. Experiential imprinting at an early stage of development enables a grown fish to respond to waters that contain substances with a particular odour or that have a characteristic temperature. European eels and North American eels spawn in warm saline waters of the Atlantic, at depths of 400 to 700 metres (about 1,300 to 2,300 feet), in an area centred near latitude 26§ N longitude 55§ W called the Sargasso Sea. The pelagic eggs develop into leptocephali--transparent, leaflike forms with relatively small heads--that are carried by the Gulf Stream to the shallow waters of the continental shelves. When they are about two and one-half years old and about eight centimetres long (a little more than three inches), a metamorphosis occurs. The leptocephali are transformed into so-called elvers, which are bottom-dwelling, pigmented, and cylindrical in form. They arrive in coastal waters as glass eels and begin to swim upstream in freshwater streams in spring. Their migration upstream is spectacular, as the young fish gather by millions, forming a dense mass several miles long. In freshwater the eels grow to full size, becoming yellow eels. They live as such for 10 to 15 years before changing into silver eels, with enlarged eyes; they swim downstream to the sea, return to the spawning grounds (Sargasso Sea), and die.The migration of these eels is not entirely understood, particularly their return to the Sargasso Sea. It may be that European eels and North American eels belong to the same species.
