This is the VOA Special English Agriculture Report.

A study has warned that seafood supplies from the world's oceans could be almost gone by the middle of the century.

The researchers say there has already been a collapse in wild populations of almost one-third of currently fished seafoods. The study says that means their catch has fallen by 90 percent from their highest levels.

Boris Worm of Dalhousie University in Nova Scotia, Canada, led the international team that did the study. Professor Worm says species have recently been disappearing from oceans at increasing speed. At this rate, he says, all seafood species could collapse by 2048.

Other studies have also warned about the dangers ofoverfishingand the effects on ocean environments. But not everyone thinks the oceans are likely to be empty in fifty years. Some scientists said parts of the world do have problems, but others are doing a good job of protecting fish populations. Government officials in several countries with large fishing industries also questioned the research.

The study appeared earlier this month in Science magazine.

The researchers say damage to oceans affects not only fish populations but also the productivity of ecosystems. These complex systems help control water quality. The scientists say the loss of different kinds of sea life appeared to increase the risk of fish kills and beach closures from harmfulalgaegrowth.

The scientists examined the results of 32 experiments and observed 48 protected areas. They also looked at records of catches worldwide. They studied records from the United Nations Food and Agriculture Organization for 1950 to 2003.

And they examined archeological information and other historical records for twelve coastal areas. That research reached back over a thousand years.

Boris Worm says the findings were, in his words, "beyond anything we suspected." But he also said the situation is not too late to correct. He said that with good fisheries management, some species could completely recover in three to ten years.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. You can learn more about agriculture, and download transcripts and MP3 files of our reports, at voaspecialenglish. I'm Steve Ember.

overfish: 捕捞过度

algae: 藻类