IONIC AND RESPIRATORY REGULATION IN RAINBOW-TROUT DURING RAPID TRANSFER TO SEAWATER

Ionic and respiratory parameters of large rainbow trout were measured during freshwater to seawater transfer. Branchial Cl- efflux increased immediately on transfer and urine excretion of Cl- increased after 14 h. Branchial Na+ efflux did not start to build up until 4-5 h after transfer and there was no increase in urinary Na+ excretion. These differences are discussed. The acid/base balance showed no major changes but arterial P[partial pressure]O2 fell greatly on transfer but by 5 h had returned to 82% of the normal level. After 24 h arterial PO2 had decreased markedly again and this continued to the end of the experiment. On seawater transfer O2 consumption fell gradually for 5 h and then rapidly increased back to the freshwater value. Possible explanations of these observations are discussed in relation to gill morphology and function during the transfer period.