Ingestion Of Pit Tags By Hatchery-Reared Juvenile Steelhead And Subsequent Detection At Main-Stem Dams In The Columbia River Basin

During the past 30 years, millions of hatchery-reared Pacific salmon Oncorhynchus spp. have been implanted with PIT tags to estimate their survival and track their migration through the extensive system of hydroelectric dams in the Columbia River basin. The mark-recapture models that are used for these analyses assume that the tags are not lost, making it important to understand retention rates for PIT tags and the fate of shed tags. To understand the potential fate of shed PIT tags in a production hatchery, we distributed 12-mm PIT tags in two traditional cement raceways and the two circular tanks of a partial reuse aquaculture system (PRAS) containing juvenile summer steelhead O. mykiss. We then monitored the detection of these PIT tags at the time that the fish were released as smolts and at main-stem dams during their migration. The probability that a loose PIT tag was inside a steelhead smolt at release ranged from 0.200 to 0.519 across the raceways and tanks, confirming that the fish consumed tags during hatchery rearing. Detections at six dams-including one located 1,208 km from the release site-and up to 36 d after smolt release confirmed that some ingested tags remained in the steelhead during their migration. The estimates of apparent survival for smolts with ingested and implanted PIT tags suggested that some ingested tags were being excreted during migration through the Columbia River basin. The results highlight the need to reevaluate assumptions about the fate of shed PIT tags in steelhead production hatcheries and ensure that biologists and managers have reliable estimates of tag retention.