When prehistoric man arrived in new parts of the world , something strong happened to the large animals ; they suddenly became extinct. Smaller species survived , the large , slow-growing animals were easy game , and were quickly hunted to extinction. Now something similar could be happening in the oceans that the seas are being over-fished 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 de 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 ) inanes 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 Dr. Worm acknowledges that these figures are conservative , one reason for this is that fishing technology has improved Today’s vessels can find their prey using satellites and sonar , which were not available 50 years ago 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. In the early days , too , loneline would have been more saturated with fish. Some individuals would therefore not have been caught , since to baited hooks would have been available to trap them , leading to an underestimate of fish stocks in the past. Furthermore , in the early days of longline fishing , a lot of fish were lost to sharks after they had been hooked. That is no longer a problem , because there are fewer sharks around noise. Dr. Myers and Dr. worm argue that their work gives a correct baseline , which future management efforts must take into account. They believe the date support an idea current among marine biologists , that of the “shifting baseline” 。 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. That matters because theory suggests that the maximum sustainable yield that can be cropped form a fishery comes when the biomass of a target species is about 50% of its original levels. Most fisheries are well below that , which is a bad way to be business.