As fish migrate over huge distances in the oceans, the various habitats that they experience at different stages of their lives can disrupt many physiological processes, so some species have evolved subtly altered haemoglobins, optimised to carry oxygen in different environments. Stocks of North Atlantic cod can be divided into haemoglobin type I, haemoglobin type II and a hybrid of the two subtypes, depending on the sequence of their haemoglobin peptide chain. Back in the 1960s, K. Sick found that type II fish predominantly occupied the cold waters of the North Atlantic, while the type I fish were found predominantly in the southern North Sea. Knowing that many environmental factors affect a fish's choice of temperature Maria Petersen and J. Steffensen wondered how fish with the two blood types would respond to hypoxia (). The team selected young cod with both blood types, and allowed them to swim in a shuttle box, where the fish could control the water temperature until they found their preferred temperature. When the fish settled, the team found that fish with type I haemoglobin preferred waters that were almost 8° warmer then the type II fish. Then they reduced the oxygen content of the water and waited to see which temperature the hypoxic fish preferred. When the oxygen levels were low, the type II fish chose water at the same temperature as they had when oxygen was abundant, but the type I fish preferred cooler waters when the oxygen level fell. Petersen and Steffensen explain out that moving to cooler waters would help the type I fish conserve oxygen in hypoxic conditions. But this could also face cod with a serious problem. If global temperatures continue to rise, then type I fish would flourish in the warmer Atlantic waters, but if oxygen levels fell too, the fish would come under threat, unable to retreat to more comfortable climes.
