The physiological response of chinook salmon smolts to two methods of radio-tagging

Smolts of hatchery-reared chinook salmon Oncorhynchus tshawytscha were radio-tagged by gastric insertion or surgical implant, and their physiological response was measured and compared to that of control insertion or surgical implant, and their physiological response was measured and compared to that of control fish. Plasma levels of cortisol, glucose, and lactate were measured before tagging and at 3 h, 24 h, 7 d, and 14 d after tagging. Significant increases in concentrations of cortisol, glucose, and lactate occurred at 3 h after tagging in both treatment groups. After 24 h cortisol levels were still elevated in both groups of tagged fish, whereas the levels of glucose and lactate had returned to normal for the surgically implanted fish but still remained higher than those of the controls for gastrically implanted fish. After 7 and 14 d, differences between treatment and control fish were not significant. Body size and physiological response to tagging among fish 14-26 cm showed no correlation. The results show that radio-tagging is indeed stressful for chinook salmon smolts but that the presence of the tags is not chronically stressful because levels of cortisol, glucose, and lactate return to normal (control) levels after few days.