Global warming has caused oceans around the world to rise in temperature making it increasingly difficult for fish to breathe, say scientists at Alfred Wegener Institute for Polar and Marine Research located in Germany.
In the past 50 years, temperatures in the Baltic Sea have risen causing oxygen levels in the water to decrease.
 A study was performed on shallow-water eelpouts, a fish with a large head which looks much like an eel, that showed that the creatures required more oxygen. With the decreased levels of oxygen, the eelpouts are literally gasping for air and causing the number of eelpouts to decrease.
"When you say I get less oxygen in, then I have less oxygen available for aerobic energy and I have less energy for the diverse tasks that a species needs to fulfill in an ecosystem, such as being ready to prey, grow, move and reproduce. It's a general weakening of the individual once it is going beyond its thermal limitation", said Hans Pörtner, an animal physiologist and ecologist.
Although the study only focused on the eelpouts, scientists say that the problem affects all fish and overfishing could occur.
"There is relatively strong evidence that the cod in the North Sea find it too warm to maintain high productivity. When the fishing industry maintains the same high fishing pressure—that has not been a problem before—with the lower productivity, this turns into overfishing," added Pörtner.
Scientists also state that the declining numbers in the eelpouts and fish does not mean that they will become extinct.
"It doesn't mean that the species will go extinct necessarily, but it means they will move. If temperatures are going to change in [the] future, then it will have a major impact on the distribution of animals," said a zoophysiologist at the University of Aarhus in Denmark, Tobias Wang.
Recently, the United States Department of Interior announced that they may add the polar bears to the list of threatened species. Also recently, a Canadian Arctic ice shelf broke away from Ellesmere Island. Both issues are said to have been caused by global warming.
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Warming oceans make it harder for fish to breathe