The respiratory development of Atlantic salmon. II. Partitioning of oxygen uptake among gills, yolk sac and body surfaces.

During post-hatch development of Atlantic salmon (Salmo salar L.), O2 uptake partitioning changes from primarily cutaneous to primarily branchial. Over 80 % of post-hatch O2 uptake was cutaneous, with the yolk sac responsible for 33 % of total O2 uptake. The well-vascularized yolk sac was a less effective gas exchanger than the unperfused skin of the body, suggesting that oxygen delivery is by direct diffusion to the tissues. Branchial O2 uptake increased quickly as gill lamellae developed, contributing 60 % of total O2 uptake before the completion of yolk resorption (body mass 0.2 g) and increasing to 69­81 % in fish weighing over 0.3 g. The area-specific O2 uptake of the skin decreased through development as skin thickness increased, while that of the gills increased from 0.10 µg h-1 mm-2 to 0.23 µg h-1 mm-2. Partitioning of O2 uptake of the skin and gills changed in concert with changes in the partitioning of the anatomical diffusion factor (ADF, mass-specific surface area per unit diffusion distance) between skin and gills, which changed from more than 95 % to less than 10 % cutaneous; thus, ADF is a useful rough indicator of oxygen uptake potential. Caution should be used in predicting oxygen uptake potential from ADF, however, because O2 uptake per unit diffusion barrier of the yolk sac was less than half that of the general body surface, and O2 uptake per unit diffusion barrier of the gills changed dramatically over development.