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

Abstract

Evidence of smolting was studied in Danish hatchery‐reared brown trout Salmo trutta L. Twenty‐four hour seawater (SW) challenge tests (28‰, 10°C) at regular intervals showed that maximal hypo‐osmoregulatory ability developed within a 3–4‐week period in March and April. The improved ability to regulate plasma osmolality, muscle water content and plasma total [Mg] developed asynchronously, indicating that developmental changes in the gill, the gastrointestinal system and the kidney may not necessarily concur during smolting. Gill Na+, K+‐ATPase activity peaked in April at the time of optimal hypo‐osmoregulatory ability. Na+, K+‐ATPase a ‐subunit mRNA level in gills was unchanged from January until April, but decreased 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 February 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, Jv, in the middle intestine fluctuated throughout the spring. In the posterior intestine, Jv was low until late March, when it increased fivefold until early May. Drinking rate in fish transferred to SW for 24 h surged during spring. Na+, K+‐ATPase activity in the pyloric caeca was elevated from March until May, and increased in response to SW transfer in June, suggesting a hypo‐osmoregulatory function of the pyloric caeca. Plasma GH levels surged in FW trout during spring, concurring with the increase in gill Na+, K+‐ATPase activity and SW tolerance, but peaked in May when gill Na+, K+‐ATPase activity and SW tolerance were regressing. GH levels were generally low in SW‐challenged fish, and there was no consistent effect of 24‐h SW exposure on GH levels. In wild anadromous trout, gill Na+, K+‐ATPase activity varied seasonally as in hatchery‐reared fish, but peaked at higher levels suggesting a more intense smolting in fish living in their natural environment.