tailieunhanh - BIOMES OF THE EARTH - OCEANS Phần 7

Tham khảo tài liệu 'biomes of the earth - oceans phần 7', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 148 OCEANS instinctively or they learn by following other individuals. Environmental changes such as increasing daylight hours in spring can act as a trigger for migration. Some fish species make marathon migrations between freshwater and seawater. The larvae of European and American freshwater eels hatch from eggs in the Sargasso Sea. The larvae travel on ocean currents for two years or more to reach rivers in Europe and North and Central America. The young eels called elvers swim upriver. For 10 years or so until fully grown they live in freshwater. Then they migrate downstream enter the sea and swim to the deep waters of the Sargasso Sea where they spawn and die. Salmon of Atlantic and Pacific Oceans spawn in rivers but grow to maturity in the sea the reverse situation of that of eels. As adults salmon usually return to spawn in the very same river where they hatched. Scientists suspect that salmon follow familiar ocean currents navigate by local magnetic fields and then recognize the scent of their home river when they approach its estuary. In the South Atlantic some adult green turtles that feed on sea grass near the Brazilian coast migrate to breed at Ascension Island in the middle of the Atlantic some 1 300 miles 2 100 km away. Ascension Island is only five miles 8 km wide a tiny destination in a big expanse of ocean. Like fishes turtles probably use a variety of environmental clues smell ocean currents and local magnetic fields to find their way. The longest-distance migrator of all is a seabird the arctic tern. Breeding birds travel up to 20 000 miles 32 000 km in a year. By crossing the equator and experiencing the summer in both Northern and Southern Hemispheres they probably encounter more hours of daylight each year than any other creature. Adult arctic terns begin their journey at breeding and feeding grounds along northern coasts of Europe and North America. In the northern autumn they fly south crossing the equator to arrive at feeding grounds in .