Exposure to seawater stimulates lipid mobilization from depot tissues of juvenile coho (Oncorhynchus kisutch) and chinook (O. tshawytscha) salmon.

Tissue lipid content and lipolytic enzyme activity was determined in selected tissues of coho salmon,Oncorhynchus kisutch, at various developmental stages (freshwater parr, freshwater smolt, seawater smolt, and seawater stunt) and in tissues of coho salmon and chinook salmon,O. tshawytscha, exposed to seawater periodically during smoltification. Among developmental groups, total lipid concentration of liver and dark muscle was highest in freshwater (FW) parr. Lipid concentration in both liver and dark muscle was significantly lower in FW smolts, seawater (SW) smolts and SW stunts; no difference was observed among these groups. Alterations in lipid composition were reflected in depot triacylglycerol lipase activity. FW smolts, SW smolts and SW stunts displayed significantly higher lipase activity than FW parr in each of the tissues examined (live, dark muscle and mesenteric fat). Early in smoltification (March, April), exposure to seawater results in enhanced lipid depletion from liver, dark muscle and mesenteric fat, both 30 and 60 days after exposure, compared to FW controls. This depletion was accompanied by increased liver (March and April) dark muscle (March) and mesenteric fat (March) lipase activity. Later in smoltification (May), salinity-induced alterations in lipid metabolism were not observed. These results indicate that exposure to seawater stimulates lipid depletion in juvenile salmon and that the depletion can be explained, in part, by increased depot lipase activity. Furthermore, these data confirm that metabolic dysfunction is associated with stunting.