Dolphins are in danger as temperatures rise with global warming. Since a heat wave struck the waters of Western Australia in 2011, researchers noticed that warmer ocean temperatures caused fewer dolphin births and decreased the animal’s survival rate.

The heat wave caused the water temperature of an area called Shark Bay to rise about 4 degrees above the annual average. After that, the survival rate for some species of dolphins fell by 12%, according to a study published Monday in the journal Current Biology.The dolphins also gave birth to fewer calves.What worries the scientists is that this change in birth rate wasn’t only observed immediately after the year of the heat wave. They sstudied the dolphins that lived in Shark Bay between 2007 and 2017, and the decline in births lasted at least until 2017.

What was a surprise, he said, was that even six or seven years after the heat wave, there was still no clear sign that things were back to normal — survival and reproduction were still lower, so these short term effects have long-term consequences on marine megafauna.”It’s unclear what is causing the change in dolphin survival and birth rate. It may be because fewer newborns survived the higher temperatures. Dolphin parents may have been neglecting their offspring due to the environmental change. Or the heat could have delayed the animal’s sexual maturity. The team hopes to do more research to find out.

Not all dolphin groups were affected. Some of those in Shark Bay use sponges as tools to hunt, and those dolphins weren’t as negatively impacted, at least not in the time period the researchers observed. Long-term, however, they don’t know whether those animals would be similarly affected.Scientists have long known that a warmer ocean is bad news for animals. The warmth stresses the entire ocean food web, according to latest studies. Warmer oceans hold less oxygen, which can cause massive fish kills. Corals, home to many fish and other sea creatures, are also extremely temperature-sensitive. Heat wave between 2016 and 2017 killed half the corals at the GreatBarrier Reef, for instance.

There is a high possibility there will be many more ocean heat waves. Climate change is particularly hard on the oceans, which absorb 93% of the Earth’s energy imbalance. The oceans have been warming at an accelerated rate since the 1960s, studies show. Scientists say that extreme weather events appear to threaten marine mammal populations in their existence. If people are to save these populations, they have to make precise plans how the frequency of such events can be kept at a minimum.