Differences in the onset of salinity tolerance between juvenile chinook salmon from two coastal Oregon river systems
Trask River juvenile chinook salmon (Oncorhynchus tshawytscha) rear in an estuarine environment, whereas Rogue River juveniles rear in a riverine environment, suggesting these populations differ in the ontogeny of smolting physiology. To study differences in the onset of salinity tolerance between these populations, I reared individuals from the Trask and Rogue rivers (Oregon) in a common environment and monitored changes in their physiology throughout their first 5–6 months. Trask fish were able to tolerate oceanic salinities at a smaller size and younger age than Rogue juveniles. Fifty days post-first feeding and at < 2 g 65% of Trask juveniles survived in 32 ppt compared with 40% of Rogue juveniles. Trask juveniles also maintained their water balance in 32 ppt seawater relative to freshwater at 50 days post-first feeding compared with 80 days for Rogue juveniles. Plasma sodium concentration indicated that both populations did not competently osmoregulate until approximately 108 days post-first feeding. Although the minimum size and age of salinity tolerance depended on which indicator of tolerance was chosen for study, physiological parameters suggested interpopulational differences in the timing of the onset of salinity tolerance.