Pacific salmon vary in abundance, size and behavior, but all members of this species share the same general life cycle. Young salmon emerge from the gravel in streams or lakes in spring and then, over various periods of time, migrate to the ocean. After living at sea between one and four years, they return to their natal streams to spawn and die. The young salmon are quite small when they leave freshwater, weighing from less than 1 gram to about 20 grams, and they are quite big when they return, ranging from 2 to 10 kilograms or more. Consequently, even though most juveniles die at sea, the return migration and death of adult salmon produce a large net flow of nutrients and energy from the ocean to stream and lake ecosystems.
This influx of energy from the ocean can have an extraordinary effect on freshwater systems because the nutrient composition of the fish and their densities are so great. For example, an adult male chum salmon on the spawning grounds contains an average of 130 grams of nitrogen, 20 grams of phosphorus(磷)and more than 20,000 kilojoules of energy in the form of protein and fat. Multiplying the average nutrient composition of salmon by the total number of returning fish, we found that a 250-meter reach of stream in southeastern Alaska received more than 80 kilograms of nitrogen and 11 kilograms of phosphorus in the form of chum salmon tissue in just over a month.
The behavior of the bears that feast on the salmon-brown bears-is the other part of the equation. Salmon are a crucial resource for the bears because the survival and reproductive success of these large mammals depend on the amount of fat they can deposit in the late summer and fall. Once bears enter their dens in the early winter, they neither eat nor drink for up to seven months. Yet bears are not true hibernators-their body temperature stays above surrounding levels-so they must produce body heat to maintain metabolic(新陈代谢的)functions throughout the cold months. In addition, females give birth and lactate during this time.
【英语六级考试快读阅读练习及答案(7)】相关文章:
最新
2016-10-18
2016-10-11
2016-10-11
2016-10-08
2016-09-30
2016-09-30