\nted in Great Britain THE OXYGEN TRANSPORT SYSTEM IN TROUT (SALMO GAIRDNERI) DURING SUSTAINED EXERCISE

1. The capabilities of the oxygen transport system of rainbow trout in supplying the increased oxygen demands in exercise, in a water tunnel at 9-10-5 °C, have been investigated by increasing the velocity of water flow, with a 1 h period between increments, up to the maximum swimming speed (critical velocity, UCIlt). 2. At Ucrlt, NOi was elevated above the resting level by 7-5 times. The logarithm of iVo was linearly related to the swimming speed expressed as a proportion of?7cr|t- ^ increased in almost direct proportion to the increase in iVOi. 3. Heart rate rose slightly at half UCIlt and reached a maximum, i-6 times the resting rate, as £/crlt was approached. Ventral and dorsal aortic mean blood pressures rose by 60 % and 20 % respectively at Ucni while their pulse pressures doubled. Central venous pressure was virtually unchanged. 4. Pa0 fell slightly during exercise but Ca O, was unaffected. On the other hand P f O i halved and Q O i fell from 3-17 (S.E. = 0-3) too-6(s.E. = 0-7) mmol/1. Cardiac output increased by about 3 times resting values. 5; The results are discussed and an attempt is made to estimate the maximum capabilities of the components of the oxygen transport system in sustained exercise