Effect of long-term stress on the smolting of two forms of brown trout (Salmo trutta L.)

One-year old sea trout and land-locked brown trout were stressed from the e nd of July until mid-November by draining the water completely from the tan k for 3 min from 1 to 4 times/week. Smelting status was estimated by monito ring individual migration in a flume tank during the spring of 1995 and by physiological parameters (plasma Cl-, Mg2+ and Na+ concentrations and muscl e water content after seawater exposure). The evaluation of migration from the end of February until mid-July indicated that all the groups started th e downstream migration at the same time, the most intense period being from mid-May until mid-June. There were no differences in the migration activit y between treatments. The physiological parameters indicated that there was only a very slight difference in smelting between stressed and unstressed seatrout (muscle water content was significantly higher in seawater challen ged stressed sea trout) and no difference between unstressed and stressed b rown trout. Sea trout had a better hypoosmoregulatory capacity than brown t rout despite a similar migration regime in the spring. (C) 1998 Elsevier Sc ience B.V. All rights reserved.