Acidification of the seas linked to climate change could threaten fisheries production and is already causing the fastest shift in ocean chemistry in 65 million years, a U.N. study showed on Thursday.

Production of shellfish, such as mussels, shrimp or lobsters, could be most at risk since they will find it harder to build protective shells, according to the report issued on the sidelines of U.N. climate talks in Mexico.

It could also damage coral reefs, vital as nurseries for many commercial fish stocks.

“Ocean acidification is yet another red flag being raised, carrying planetary health warnings about the uncontrolled growth in greenhouse gas emissions,” said Achim Steiner, head of the U.N. Environment Programme (UNEP).