You may have heard the popular saying, "there are always more fish in the sea". But as many new studies show, fish populations are changing, and not necessarily for the better. Consider the case of big, predatory   fish. These giants, like sharks and cod  , eat other smaller fish. Big fish are an important part of the marine ecosystem   because they keep down the numbers of smaller fish. Without fish that eat other fish, populations of smaller swimmers would increase, eating more plants, and leaving less vegetation for other creatures, including future fish. As fierce as predators can be, they're no match for fishing technology. Many people love to catch predatory fish like sharks, cod and tuna, and new studies suggest that over-fishing is threatening these creatures near the top of the marine food chain. In one study, Villy Christensen and his colleagues looked at 200 past studies of marine life to learn how fish populations have changed over time. Christensen reported that between 1910 and 1970, the numbers of big predators decreased slowly, and that in 1970 their populations really started to drop. Around that time, fishing ships began using new tools that led to more fish being caught. The numbers have been falling quickly ever since. Reg Watson looked at the problem from the perspective of the fishermen. In the middle of the 20thcentury, Watson reported, fishing boats didn't go far from home, and most fish were caught near the shore. That was no longer true by the 1980s. By then, fishing had moved farther from shore, into the open oceans, and was helped by the development of new technologies. But since the 1990s, something has changed. Despite new technologies and more efforts, fishing operations have not continued to develop. Scientists study historical data to understand the present, and this research is needed to forecast the future of fish and fishing. And the forecast doesn't look good: large predator fish are becoming harder to find, and soon may disappear. Why did the number of big predators begin to fall quickly in 1970?
Fishing ships began using new tools with which they could catch more fish. There were fewer smaller fish than before. There were more people catching big predators. Their environment was being polluted by fishing ships.