134. Read the following passage, then answer the question below.

In 1992, the Canadian government closed the Grand Banks cod fishery off Newfoundland, one of the richest fishing grounds in the world, and some thirty thousand people lost their jobs. For nearly five centuries, cod had been so plentiful that early explorers reported nets that could not be hauled aboard. The idea that such abundance could vanish seemed absurd, which is partly why it did.

The causes were a combination of technology and misjudgment. After the Second World War, large factory trawlers equipped with sonar and enormous nets began to scoop up fish far faster than smaller boats had done. Catch figures rose until the 1960s, then fell. Yet scientists advising the government relied heavily on catch records and on estimates of how many fish remained, which proved too optimistic. Because cod gather in dense schools even when their total numbers decline, trawlers could keep filling their nets until the stock was nearly gone.

After the closure, many expected the cod to rebound within a few years. They did not. Three decades later, the population remains a fraction of its former size. Some biologists point to the loss of the largest, oldest females, which produce far more eggs than young ones, and to changes in the ecosystem, since other fish and shrimp filled the space cod once occupied and now feed on cod's young. Warming waters may also play a role.

The episode changed fishery management worldwide. Many countries now set catch limits based on cautious estimates, protect nursery areas, and require fishing gear that lets young fish escape. Some stocks, such as Atlantic herring in several regions, have recovered. Still, about a third of the world's fish stocks are fished beyond sustainable levels, a reminder that the lesson of the Grand Banks is easier to endorse than to apply.

The word 'They' in paragraph 3 refers to