Hi, small opening remark, fish are not 'other marine mammals'. Fish and other gill breathing animals are strongly affected by water temperature, irrespective of the origin. First of all, fish, except big tuna, have environment temperature, not a fixed one around 36°C like mammals. The proteins of fish adapted to high latitudes function best in cold or cool waters, where disolved oxygen tends to be much higher than in warm, tropical waters.

As gas exchange in water breathing animals is much more energy intensive than air breathing, fish must expend a large part of their energy budget on breathing. Moreover, gills are surfaces, while the body of fish will grow in volume. At some point, the surface limitation of gas exchange through the gills will not allow fish to transform food into own body cells because of lack of oxygen supply. This is why fish species in the warmer part of their distribution range attain a lower maximum size than in the cooler part of their distribution range. It has also been observed, that e.g. salmon can grow a bit more after spawning (spawning reduces body weight requiring oxygen supply). This gill-oxygen limitation theory (GOLT) proposed by Pauly also explains why we observe polewards migrations of fish species as the ocean surface waters warm. They want to remain within their preferred temperature range - if they can. It means also that tropical regions loose species without replacement because it's becoming too warm (oxygen too low, even without overfishing), while temperate and polar regions gain, at least temporarily. But cold adapted animals may be at risk, e.g. mass mortality of snow crabs in the rapidly warming Arctic. Air breathing marine mammals are not affected by this phenomenon, except indirectly in that they prey species may move as a result of a warming ocean.

Dr Cornelia E Nauen thank you for your contribution to the discussion