We take breathing for granted. And especially we take the availability of oxygen for granted. For air-breathing animals, things are relatively simple on the physics. If there is adequate ventilation, the air is comprised of about one-fifth oxygen, and only things like altitude really impinge on how well that can be utilized. The relevant principle for we air-breathing sorts is partial pressure, and for everybody but folks on mountains and those flying at high altitudes, it simply isn’t a matter of much thought or import.
Once one goes aquatic, though, things are different. Oxygen tension is highly dependent upon a number of factors, including salinity and temperature. Of the non-biotic factors, temperature is the most important. And temperature is the thing at issue when we are talking about climate change. Relatively small changes in temperature can trigger fish kill situations, though for most people large scale death of fish is most commonly associated with biotic anoxia through agents like algal and dinoflagellate blooms.
The research linked above looks at the abiotic issue of declining oxygen tension due to increasing temperature. And that, in turn, is linked to climate change.
Scientists confirm computer model predictions that oxygen-depleted zones in tropical oceans are expanding, possibly because of climate change
An international team of physical oceanographers including a researcher from Scripps Institution of Oceanography at UC San Diego has discovered that oxygen-poor regions of tropical oceans are expanding as the oceans warm, limiting the areas in which predatory fishes and other marine organisms can live or enter in search of food.
As the title says, this is yet another aspect of habitat loss. Where oxygen tension drops, fish may either have to leave or die, and over broad enough areas, leaving just isn’t an option. The study also discusses how low-oxygen tension waters can be carried into coastal areas, creating problems for all sorts of organisms dependent upon a continuous supply of oxygen in the water. As global warming progresses, the regions of low oxygen tension enlarge and more often are carried to coastal areas. This adds yet another stress to already decimated fisheries.
Oxygen is not omnipresent in aquatic and marine environments. As the temperature goes up, the places where enough oxygen can be found in the water for fish and other species of commercial interest goes down. Though the physics is more complex than for air-breathers, the situation just comes down to managing to keep in places where the oxygen stays high enough, all the time. It just will be getting tougher as the heat rises.