PLANET EARTH Shallow Seas
Our planet's continents are fringed（围绕着） by shallow seas. Rarely more than two hundred meters deep they lie on the continental shelves which may stretch sometimes for hundreds of miles before the sea floor drops into deeper darker waters. All together they constitute a mere eight percent of the world's oceans but they contain the vast majority of its marine life（海洋生活）.
A male humpback whale sings to attract a mate. The whales have just returned to their breeding grounds in the shallow seas of the tropics. The calf is no more than a few weeks old. Despite being three meters long and weighing nearly a ton he is nonetheless vulnerable. but his mother watches over him and, as he begins to tire, she supports him close to the surface so that he can breathe more easily. These shallows around the equator are excellent nurseries. They're warm, calm, and contain very few predators.
The playful calf is now drinking five hundred liters of milk a day but his mother must starve. There is nothing for her to eat here. Like many tropical shallow seas these crystal clear waters are virtually lifeless. They receive year round sunlight but they lack the nutrients essential for the growth of plankton（浮游生物）. The mother will be trapped here for the next five months until her calf is strong enough to make the journey to the feeding grounds, near the poles.
