Changes in branchial and intestinal osmoregulatory mechanisms and growth hormone levels during smelting in hatchery-reared and wild brown trout

Evidence of smelting was studied in Danish hatchery-reared brown trout Salm o trutta L. Twenty-four hour seawater (SW) challenge tests (28 parts per th ousand, 10 degrees C) at regular intervals showed that maximal hypo-osmoreg ulatory ability developed within a 3-4-week period in March and April. The improved ability to regulate plasma osmolality, muscle water content and pl asma total [Mg] developed asynchronously, indicating that developmental cha nges in the gill, the gastrointestinal system and the kidney may not necess arily concur during smelting. Gill Na+,K+-ATPase activity peaked in April a t the time of optimal hypo-osmoregulatory ability. Na+,K+-ATPase alpha-subu nit mRNA level in gills was unchanged from January until April, but decreas ed in May in parallel with a decrease in the activity of the enzyme. In the middle region of the intestine, Na+,K+-ATPase activity increased in Februa ry and remained high until April. In the posterior region of the intestine, the activity was stable from January until April after which it decreased. In vitro fluid transport capacitity, J(v), in the middle intestine fluctua ted throughout the spring. In the posterior intestine, J(v) was low until l ate March, when it increased fivefold until early May. Drinking rate in fis h transferred to SW for 24 h surged during spring. Na+,K+-ATPase activity i n the pyloric caeca was elevated from March until May, and increased in res ponse to SW transfer in June, suggesting a hypo-osmoregulatory function of the pyloric caeca. Plasma GH levels surged in FW trout during spring, concu rring with the increase in gill Na+,K+-ATPase activity and SW tolerance, bu t peaked in May when gill Na+,K+-ATPase activity and SW tolerance were regr essing. GH levels were generally low in SW-challenged fish, and there was n o consistent effect of 24-h SW exposure on GH levels. In wild anadromous tr out, gill Na+,K+-ATPase activity varied seasonally as in hatchery-reared fi sh, but peaked at higher levels suggesting a more intense smelting in fish living id their natural environment. (C) 1999 The Fisheries Society of the British Isles.