Ionic and respiratory regulation in rainbow trout during rapid transfer to seawater

Summary 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 however 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 fell greatly on transfer but by 5 h had returned to 82% of the normal level. After 24 h arterial had decreased markedly again and this continued to the end of the experiment. On seawater transfer oxygen 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.