Read the passage.
Goodbye, Cod
The Atlantic cod was once the mainstay of the United States fishing industry. However, warming of the ocean along the shores of New England has caused the stock of cod to disappear almost completely. According to scientists at the Gulf of Maine Research Institute, the Gulf of Maine is heating up faster than a majority of the rest of the planet's oceans. As a result of warmer ocean waters, female cod produce fewer offspring, and these fish are less likely to survive to maturity. Even though recreational cod fishing was banned in 2014, and strict limitations were placed on the number of fish commercial fishermen could catch, these measures have not been enough to protect the cod.
What is the main idea of the passage?