The ontogeny of drinking in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Drinking was measured by the rate of uptake of tritiated dextran in fresh water by the alevins and fry of rainbow trout at various stages of development during a 40‐day post‐hatch period. Drinking increased almost S‐fold during the initial 16 day yolk‐sac stage. Drinking rate increased most between 16 and 23 days, the transitional period between yolk‐sac absorption and first feeding. The maximum weight‐specific drinkingrateof 3.24μlg−1 h−1 recorded for 40‐day‐old fry was higher than previously recorded for adults. Abrupt transfer from an adaptation temperature of 10 to 19° C increased drinking significantly (Q10= 1.2), but sudden transfer to 1 1‰ salinity sea water caused a substantial fall in drinking rate in 23‐day fry. A 24‐h period of adaptation to 1 1‰ and 15‰ salinity restored drinking to a rate similar to that in fresh water. The water drunk by 31‐day fry fed to satiation was initially higher than by unfed fry, but drinking rates subsequently fell below the control level. The results are discussed in terms of osmoregulation and the uptake of dissolved or suspended substances by the intestinal tract for the purpose of immunization.