考研英语阅读命题思路透析和真题揭秘(59)

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.

Dr. Myers and Dr. worm argue that their work gives a correct baseline, which future management efforts must take into account. They believe the date support an idea current among marine biologists, that of the "shifting baseline". The notion is that people have failed to detect the massive changes which have happened in the ocean because they have been looking back only a relatively short time into the past. That matters because theory suggests that the maximum sustainable yield that can be cropped form a fishery comes when the biomass of a target species is about 50% of its original levels. Most fisheries are well below that, which is a bad way to de business.

当史前的人类到达世界的一个新的地方时，那里的大型动物就会一个发生奇怪的现象。它们突然灭绝了，而小型动物却存活了下来。体形庞大 、生长缓慢的动物往往成为狩猎的目标，并且很快被狩猎到灭绝。现在相类似的事情可能会发生在海洋中。

前些年我们就已经知道现在海洋正在受到过度捕捞。Ransom Myers和Boris Worm等研究者这些年所展示的只不过是事情正在迅速地变化着。他们研究了半个世纪以来世界上所有的渔场，其研究方法并不是为了试图估算特定区域的海洋中实际生物总数（活着生物的总数），而是研究单位面积中海洋生物数量发生的变化。根据他们最近在《自然》杂志上发表的文章，在一个新渔场中大型食肉海洋生物（一种杀死并吃掉其它动物的生物）的总量在15年的时间里平均减少了80%。而在一些大型的捕鱼场，这个数字又进一步减半。

Worm博士承认这些数字是保守的，其中一个原因是当今的捕鱼技术得到巨大的提高，可以通过卫星和声纳这些在50年前不可能有的技术来寻找猎物，这意味着更多的海洋生物正在被捕获，因而现在和过去的真正差距可能会比之前通过捕获区记录变化得到的数据更大。在早年的时候，延绳钓可能对鱼不那么起作用，因此有一些鱼种逃过一劫，因为那时没有带诱饵的鱼钩抓它们，因此过去的鱼储量就被低估了。此外，早期用延绳钓捕鱼的日子里，有许多鱼在上钩后被又鲨鱼吃掉了。现在这不再是个问题，因为海里的鲨鱼已经少多了。

Myers博士和Worm博士声称，他们的工作已经确定了一个正确的基本方针，这个方针是未来管理中必须要使用的。他们相信自己的数据恰恰体现了一个目前在海洋生物学家中非常流行的观点，那就是"改变基线"。这个概念就是说人们没有观察到海洋中发生的巨大变化，因为他们只是回顾从过去以来相对较短的一段时间。这一点很重要，因为从理论上看，当一种目标生物的生物量大约达到原始水平的50%时，就可以从一个渔场收获到最大程度的可持续发展的鱼储量。然而大多数渔场远远低于这个数字，这种方式是非常不好的。