[00:08.14]When prehistoric man arrived in new parts of the world,
[00:12.17]something strange happened to the large animals:
[00:15.51]they suddenly became extinct.
[00:18.03]Smaller species survived.
[00:20.54]The large, slow-growing animals were easy game,
[00:24.07]and were quickly hunted to extinction.
[00:27.09]Now something similar could be happening in the oceans.
[00:31.42]That the seas are being overfished
[00:33.96]has been known for years.
[00:36.18]What researchers such as Ransom Myers
[00:38.95]and Boris Worm have shown
[00:40.86]is just how fast things are changing.
[00:43.79]They have looked at half a century of data
[00:46.92]from fisheries around the world.
[00:49.46]Their methods do not attempt to estimate the actual biomass
[00:53.68](the amount of living biological matter)
[00:56.48]of fish species in particular parts of the ocean,
[00:59.86]but rather changes in that biomass over time.
[01:03.98]According to their latest paper published in Nature,
[01:07.50]the biomass of large predators
[01:10.04](animals that kill and eat other animals)
[01:12.96]in a new fishery is reduced on average by 80%
[01:17.37]within 15 years of the start of exploitation.
[01:21.42]In some long-fished areas,
[01:23.52]it has halved again since then.
[01:26.55]Dr. Worm acknowledges that these figures are conservative.
[01:31.20]One reason for this is that fishing technology has improved.
[01:35.83]Today's vessels can find their prey using satellites and sonar,
[01:41.07]which were not available 50 years ago.
[01:44.91]That means a higher proportion of
[01:47.23]what is in the sea is being caught,
[01:49.86]so the real difference between present and past
[01:52.88]is likely to be worse than the one recorded
[01:55.14]by changes in catch sizes.
[01:58.68]In the early days, too, longlines would have been
[02:01.68]more saturated with fish.
[02:03.88]Some individuals would therefore not have been caught,
[02:07.21]since no baited hooks would have been available to trap them,
[02:10.93]leading to an underestimate of fish stocks in the past.
[02:15.28]Furthermore, in the early days of longline fishing,
[02:18.90]a lot of fish were lost to sharks after they had been hooked.
[02:23.05]That is no longer a problem,
[02:25.36]because there are fewer sharks around now.
[02:28.78]Dr. Myers and Dr. Worm argue
[02:31.33]that their work gives a correct baseline,
[02:34.13]which future management efforts must take into account.
[02:38.47]They believe the data support an idea current
[02:41.70]among marine biologists,
[02:43.71]that of the "shifting baseline".
[02:46.13]The notion is that people have failed to detect
[02:48.26]the massive changes which have happened in the ocean
[02:51.69]because they have been looking back
[02:53.14]only a relatively short time into the past.
[02:57.07]That matters because theory suggests
[02:59.49]that the maximum sustainable yield
[03:01.61]that can be cropped from a fishery comes
[03:04.13]when the biomass of a target species
[03:06.77]is about 50% of its original levels.
[03:10.36]Most fisheries are well below that,
[03:13.00]which is a bad way to do business.内容来自 听力课堂网：http://www.tingclass.net/show-8686-251462-1.html