歷年考研英語閱讀理解2006年03

[00:05.82]2006 Text3 [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.

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