THE RESPIRATORY DEVELOPMENT OF ATLANTIC SALMON .1. MORPHOMETRY OF GILLS, YOLK-SAC AND BODY-SURFACE

During development from larva to juvenile in Atlantic salmon, Salmo sa lar, there is a change in the anatomical potential for gas exchange am ong gills, body skin and yolk sac as the larvae resorb yolk, grow and develop gills. Newly hatched Atlantic salmon have poorly developed gil ls but do have a high skin area to mass ratio and a large well-vascula rized yolk sac. Cutaneous surfaces accounted for over 95 % of the tota l area available for respiration in newly hatched Atlantic salmon (bod y mass 0.032-0.060 g). The branchial contribution to total area increa sed rapidly, however, so that by the end of yolk absorption (body mass 0.19-0.23 g) it constituted 22 % of the total area and overtook cutan eous surface area between 5 and 6g wet body mass. Harmonic mean diffus ion distance across the skin increased through development from 20 mu m at hatch (14 mu m across the yolk sac) to 70 mu m in an 11g fish. Di ffusion distances across both the filaments and lamellae of the gills decreased through development, from 3.7 to 2.4 mu m for lamellae and f rom 14.5 to 10.8 mu m for filaments. The total anatomical diffusion fa ctor (ADF, mass-specific surface area per unit diffusion distance) rem ained constant over early development and appeared to be higher than i n adult fish. The distribution of ADF changed over early development f rom 50% yolk sac, 42 % body surface and 8 % branchial in newly hatched fish to 68 % branchial and 32 % cutaneous at the end of yolk resorpti on. Generally, early post-hatch development of gills, ADF and some cut aneous surfaces showed high mass exponents, After yolk resorption (bod y mass 0.2g), however, these coefficients were lower and closer to uni ty. The change in scaling at the end of yolk resorption in this study may reflect the completion of larva to juvenile metamorphosis in Atlan tic salmon. Comparison between our data and values in the literature s uggests that the timing of gill development is related more to develop mental stage than to body size.