Using examples and details from the lecture, describe the causes and effects of problems that threaten the world’s oceans.

Now listen to part of a lecture in a marine biology class. The professor is discussing problems that threaten the world’s oceans.
A major study shows that the world’s oceans face several serious problems related to over fishing, pollution, climate change, and loss of habitat.
At least one-third of the fish stocks are "over fished." This includes several groundfish species that will take decades to recover. The estimated number of large ocean predators--tuna, marlin, sharks, and halibut--has fallen 90 percent globally in half a century. One reason for overfishing is bottom trawling, the practice of dragging nets along the sea bottom to catch huge quantities of fish. Bottom fishing with nets can destroy habitat and contribute to fish declines.
Pollution is a serious and growing danger. Land-based pollution in the form of erosion puts silt into the ocean, killing marine life close to shores. Cargo ships, container ships, and cruise ships dump at least 65 million gallons of petroleum products a year into the seas. Even small amounts of oil can damage sensitive marine environments. Scientists estimate that nearly 30 million gallons of petroleum seep into North American waters every year, most of it from runoff, seaplanes, and small boats.
Climate change is causing new disease outbreaks in ocean environments, affecting everything from coral reefs to oysters. Diseases threaten many corals in the Pacific. Since coral reefs provide rich habitat for fish, the loss of coral contributes to fish declines. Climate change also hurts large mammals that depend on fish. In the Arctic, melting ice may be preventing polar bears from reaching their fishing areas, and this affects their breeding patterns and the survival of their young cubs.