Changes in intestinal fluid transport in Atlantic salmon (Salmo salar L) during parr-smolt transformation

We examined changes in fluid transport by the intestine of Atlantic salmon (Salmo salar L) undergoing parrsmolt transformation during springtime. In vitro measurements of fluid transport rate (J_v) across non-everted middle and posterior intestinal sac preparations were made in late April and early June 1990 and from February through June 1991 for juvenile smolting fish. Intestinal J_v was also compared between parr- and smolt-stage salmon in both years. To evaluate the osmoregulatory role of the intestine, J_v was measured for smolts adapted to seawater and their cohorts remaining in fresh water. The middle intestine of smolting fish underwent a significant decrease in fluid transport during the springtime, while posterior intestinal J_v significantly increased. Parr-stage fish decreased J_v in the middle intestine during springtime similar to smolts. However, the posterior intestinal J_v of smolts showed a significant increase over the parr around the peak smolt period in both years. Seawater-adapted smolts generally exhibited posterior intestinal J_v approximately double that of freshwater cohorts. A decrease over time shown for the middle intestine, together with the increased J_v in the posterior intestine preceding and after seawater entry, suggests the development of a functional regionalization during parr-smolt transformation, with the posterior intestine taking on increased importance in osmoregulation in seawater.