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When prehistoric man arrived in new parts of the world, something strange happened to the large animals: they suddenly became extinct.

当史前的人到达世界的一个新的地方时，那里的大部分动物就会发生奇怪的事情，它们突然灭绝了。

Smaller species survived.

小部分动物种族存活了下来。

The large, slow-growing animals were easy game, and were quickly hunted to extinction.

但大部分的，成长缓慢的动物则成了狩猎的目标，并且很快被狩猎到灭绝。

Now something similar could be happening in the oceans.

现在相类似的事情发生在大海中.

That the seas are being overfished has been known for years.

这些年在海中的捕捞活动太过频繁。

What researchers such as Ransom Myers and Boris Worm have shown is just how fast things are changing.

就像 Ransom Myers 和 Boris Worm 这些年所研究的，事物在迅速地变化着。

They have looked at half a century of data from fisheries around the world.

他们研究了半个世纪以来世界上所有的鱼场。

Their methods do not attempt to estimate the actual biomass (the amount of living biological matter) of fish species in particular parts of the ocean,

他们的方法不是为了试图估算特定区域的海洋中实际动物总数（活着生物的总数），

but rather changes in that biomass over time.

而是研究单位面积中海洋生物数量的变化。

According to their latest paper published in Nature,

根据他们最近在《自然》杂志上发表的文章，

the biomass of large predators (animals that kill and eat other animals) in a new fishery is reduced on average by 80% within 15 years of the start of exploitation.

大型食肉的海洋 生物（一种捕杀猎食其它动物的生物）的总量在 15 年的时间里已经减少了平均 80%。

In some long-fished areas, it has halved again since then.

在一些大型的捕鱼场，这个数量已经减半。

Dr. Worm acknowledges that these figures are conservative.

Worm 博士承认这些数字还只是保守数字。

One reason for this is that fishing technology has improved.

一个原因是今天的捕鱼技术已经大大改善。

Today's vessels can find their prey using satellites and sonar, which were not available 50 years ago.

可以通过卫星和声波来发现猎物。这些在50年前是不可能的。

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.

这意味着同50年前相比可以捕获更多的海洋生物，所以现在和过去的真正差距可能会比之前在捕获区记录的数据所显示的更糟糕。

In the early days, too, longlines would have been more saturated with fish.

在早期的时候，多勾的鱼杆线已经对鱼不起作用了。

Some individuals would therefore not have been caught, since no baited hooks would have been available to trap them, leading to an underestimate of fish stocks in the past.

因此有一些鱼种没被抓到，往后没有鱼饵的鱼钩根本捕不到鱼，导致过去的捕鱼业受到轻视。

Furthermore, in the early days of longline fishing, a lot of fish were lost to sharks after they had been hooked.

而且，早期用鱼钩捕鱼的日子， 有许多鱼在它们上钩后被鲨鱼给吃了。

That is no longer a problem, because there are fewer sharks around now.

那不再是个问题，因为现在鲨鱼已经很少了。

Dr. Myers and Dr. Worm argue that their work gives a correct baseline, which future management efforts must take into account.

Dr. Myers 和 Dr. worm 声称说，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。

They believe the data support an idea current among marine biologists, that of the “shifting baseline.”

他们认为数据恰恰体现了一个海洋生物学家都支持的观点。这个观点就是“改变基线”。

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.

这个概念就是说人们已经很难观察得到大范围海洋中发生的变化，因为他们只是回顾从过去以来相对短的时间。

That matters because theory suggests that the maximum sustainable yield that can be cropped from a fishery comes when the biomass of a target species is about 50% of its original levels.

这确有其事，因为理论告诉我们如果一个渔场的总储量如果连起初的最大可承受的捕鱼量的50％都不到，就应该降低捕鱼数量了。

Most fisheries are well below that, which is a bad way to do business.

大多数渔场远远低于这个数，这是对以后的捕鱼业非常不利的。