The intestinal role in osmoregulatory preadaptation of freshwater‐adapted (FW) yearling coho salmon during smoltification was examined. Measurements of intestinal net fluid absorption (JV) using an in vitro sac preparation showed that JV of FW parr was significantly less than that of either FW or seawater‐adapted (SW) smolts. From two to four weeks following the springtime thyroxin surge, JV increases in FW coho to a level comparable with that observed for SW smolts. JV remained elevated in FW smolts throughout most of the summer and then decreased in the autumn. Two months after the thyroxin peak, intestinal wet weight per unit serosal surface area increases. Coho stunts resulting from premature transfer into sea water had significantly higher JV than that seen in SW smolts. The elevated JV of stunts is only partially explicable on the basis of differences in body weight between stunts and smolts. Our observations indicate that an increase in intestinal fluid absorption is a preadaptive change associated with smoltification occurring in concert with alterations in renal and branchial osmoregulatory mechanisms. The timing of the increase in JV suggests a phase relationship to the thyroxin surge. This temporal relationship may help to explain the efficacy of using changes in plasma thyroxin levels to predict hatchery‐fish release dates that may optimize eventual seawater survival.
|Number of pages||12|
|Journal||Journal of Fish Biology|
|State||Published - Sep 1982|