When prehistoric man arrived in new parts of the world, something strong happened to the large animals; they suddenly became extinct. Smaller species survived, the large, slow-growing animals were easy game, and were quickly hunted to extinction. Now something similar could be happening in the oceans.

　　That the seas are being over-fished has been known for years what researchers such as Ransom Myers and Boris Worm have shown is just how fast things are changing. They have looked at half a century of data from fisheries around the world. Their methods de not attempt to estimate the actual biomass (the amount of living biological matter) of fish species in particular parts of the ocean, but rather changes in that biomass over time. According to their latest paper published in Nature, the biomass of large predators (animals that kill and eat other animals) inanes fishery is reduced on average by 80% within 15 years of the start of exploitation. In some long-fished areas, it has halved again since then.

　　Dr. Worm acknowledges that these figures are conservative, one reason for this is that fishing technology has improved Today’s vessels can find their prey using satellites and sonar, which were not available 50 years ago that means a higher proportion of what is in the sea is being caught, so the real difference between present and past is likely to be worse than the one recorded by changes in catch sizes. In the early days, too, longlines would have been more saturated with fish. Some individuals would therefore not have been caught, since to baited hooks would have been available to trap them, leading to an underestimate of fish stocks in the past. Furthermore, in the early days of longline fishing, a lot of fish were lost to sharks after they had been hooked. That is no longer a problem, because there are fewer sharks around noise.

    更多信息请访问：新浪考研频道 考研论坛 考研博客圈

　　特别说明：由于各方面情况的不断调整与变化，新浪网所提供的所有考试信息仅供参考，敬请考生以权威部门公布的正式信息为准。