Air breathing in the Arctic: influence of temperature, hypoxia, activity and restricted air access on respiratory physiology of Alaska blackfish (Dallia pectoralis)

Abstract

Abstract The Alaska blackfish (Dallia pectoralis) is an air-breathing fish native to Alaska and the Bering Sea islands, where it inhabits lakes that are ice-covered in the winter, but enters warm and hypoxic waters in the summer to forage and reproduce. To understand this species’ respiratory physiology under these conditions, and the selective pressures that maintain the ability to breathe air, we acclimated fish to 5°C and 15°C and used respirometry to measure: standard oxygen uptake (SṀO2) in normoxia (19.8 kPa PO2) and hypoxia (2.5 kPa), with and without access to air, partitioning of SṀO2 in normoxia and hypoxia, maximum ṀO2 and partitioning after exercise, and critical oxygen tension (Pcrit). Additionally, the effects of temperature acclimation on haematocrit, haemoglobin oxygen affinity and gill morphology were assessed. SṀO2 was higher, but air breathing was not increased, at 15°C or after exercise at any temperature. Both 5°C- and 15°C-acclimated fish increased air breathing to compensate and fully maintain SṀO2 in hypoxia. Fish were able to maintain SṀO2 through aquatic respiration when air was denied in normoxia, but when air was denied in hypoxia SṀO2 was reduced by approximately 30-50%. Pcrit was relatively high (5 kPa) and there were no differences in Pcrit, gill morphology, haematocrit or haemoglobin oxygen affinity between temperatures. Overall, Alaska blackfish depends on air breathing in hypoxia, and additional mechanisms must thus be utilised to survive hypoxic submergence during the winter, such as hypoxia-induced enhancements in blood oxygen carrying capacity and binding, behavioural hypoxia avoidance and metabolic rate suppression.