Question

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 planets 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, or central, idea of the passage?
because cod used to be abundant in the atlantic, it was once the backbone of the u.s. fishing industry.
because the gulf of maine is warming unusually quickly, atlantic cod are disappearing.
female cod are producing fewer offspring in the gulf of maine.

Explanation:

The passage starts by stating the Atlantic cod's importance to the US fishing industry, then focuses on how ocean - warming along New England shores, especially in the Gulf of Maine, is causing the cod stock to disappear despite fishing restrictions. The main idea is about the cod's disappearance due to Gulf of Maine warming.

Answer:

Because the Gulf of Maine is warming unusually quickly, Atlantic cod are disappearing.