Experimental Evaluation of Adult Spring Chinook Salmon Radio-Tagged during the Late Stages of Spawning Migration

We conducted an experiment to assess the effects of radio-tagging on adult spring Chinook salmon Oncorhynchus tshawytscha during the late stages of their spawning migration. Fish were captured at Roza Dam, Yakima River, Washington (river kilometer 208). Each fish had a radio tag inserted into the stomach via the esophagus and was then transported to a raceway and held for 50 d to assess survival. In the same raceway we also held a control group that was handled but not tagged and a group with externally attached transmitters. Survival of fish with gastrically implanted radio tags was low (10%), while 70% of controls and 90% of fish with externally mounted tags survived to the end of the experiment. Postmortem dissections revealed that after implantation, some radio tags had migrated from the stomach position to the posterior digestive tract and peritoneum. Fish from all treatments exhibited some level of tissue degeneration of the gastrointestinal tract. Gastric implantation should be used with caution, particularly for fish at vulnerable life stages. For spring Chinook salmon in the Columbia River basin, tagging with gastric implants should be conducted early in the freshwater migration to improve survival and reduce tag effects.