That the seas are being overfished 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 do 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 in a new 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.

I am a little confused with the last sentence.

what does "new" exactly mean here?  "Being the later or latest in a sequence" or "Never used or worn before now" ? 

Can i paraphrase the last sentence like this : The biomass of large predators at the beginning of the exploitations  of the fisherys that studied by Ransom Myers and Boris Worm had been reduced on average by 80% 15 years latter.
Hi, copysnake,

No, you may not.

I would say "new" = "virgin," in this context. But I didn't realize there were any left. I think the statistic would be meaningless unless the starting point represented a natural balance. Curiously, we're not told how the 80% compares to the reduction of the combined species which are being fished. (Edit. I suppose the reference explains that, but not how the predators are separated out.)

It was found in fisheries studied by Ransom Myers and Boris Worm that 15 years after exploitative fishing had begun, the biomass of large predators had been reduced on average by 80%.
When you say, "the biomass at the beginning had been reduced 15 years later," it seems to mean they just made a mistake. That is, how can you reduce the biomass at the beginning 15 years later? I guess it's a fine point. It's like saying, "My wife's weight in 1950 had been reduced by 10% in 1960." What does that mean?? They discovered that the old scales were wrong? Not really. It doesn't mean anything. It's self-stultifying. "My wife weighs 10% less than she did before" is correct.

If we could accept the meaning of "a new fishery," the original would be easier to take than your paraphrase - perhaps more so with my modifications.

Best wishes, - A.
copysnake 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.
If you establish a new fishery (as opposed to taking charge of a fishery that has been in existence for a long time), you will find that in 15 years from the date of establishment, 80% of the biomass of large predators in that fishery will probably disappear.

new -- never having existed before

We're going to build a new house.
The boss wants to start a new procedure for using the copy machines.
Scientists found a new way to cure certain kinds of cancer.

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so "a new fishery" refers to a virgin fishery but not  refers particularly to a fishery established recently. And its opposite is not old. Right?
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I'd agree with your statement. Obviously, if a study is to be meaningful, the parameters would need to be stated much more explicitly than in the sentence we have to work with.

I expect finding a new fishery is as rare as finding a new gold mine. Do we define "fishery" as an area where fish are plentiful and commercial fishermen often work, or are we refering specifically to an area where a well-organized "exploitative" fishing enterprise has been set up? (akin to the clear-cutting of timber)

Changing global weather patterns and ocean currents have profound effects on fish populations. Major shifts have been know to occur over periods much shorter than 15 years. Is this taken into account?

If an area has been fished out and abandoned, to what degree can it be restored? By definition (as I understand the sentence) it would never be suitable for this particular study. That is, if it begins to come back, and a "new fishery" is established there, it could not be considered "new" for purposes of the study.

- A.