Increasing salinity decreases water solubility, such that the oceans can dissolve about 20 percent less oxygen than fresh water of the same temperature. Any other solutes in water tend to decrease the solubility of gases, particularly non-polar gases like oxygen. Temperature and pressure are the other major factors in solubility, with increases in temperature reducing gas solubility and increasing the solubility of most other solutes.
Oxygen is poorly dissolved by water in the first place; carbon dioxide, for instance, is a far more soluble gas. Water molecules are highly polar and so have only a very weak attraction to the highly non-polar oxygen molecules. The sodium and chloride ions of dissolved salt attract water molecules much more strongly and reduce their affinity for oxygen even further. Because of the very poor ability of salt water to hold oxygen, water-breathing animals must have very efficient respiration and low oxygen requirements, or both.
This problem is exacerbated in warmer waters, which can become extremely oxygen-deprived under certain conditions. This results in a situation where warm waters, despite their benefits for mostly cold-blooded marine animals, do not necessarily support more life. Indeed, some of the larger predatory fish, such as tuna and even great white sharks, are partially warm-blooded to allow them to operate better in cold waters where their food sources are prevalent.