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PIT Tag Ingestion: the Curious Fate of Shed Tags in Salmonid Hatcheries

Kathleen Hemeon, Ronald Twibell, Douglas Peterson, Benjamen Kennedy, Matthew Piteo

U.S. Fish and Wildlife Service, 1440 Abernathy Creek Rd, Longview, WA 98632

Passive integrated transponder (PIT) tags are commonly employed in salmonid fishery management for the evaluation of fish passage through barriers, smolt survivorship, and hatchery returns. Tagged hatchery-reared salmonids are essential for monitoring conservation and programmatic targets, where models assume unaccounted for tags are primarily the result of fish mortalities. It is known that some PIT-tagged fish will shed tags after implantation, but tag retention is presumed to be high if loose tags are not detected in rearing vessels; however, tag ingestion has been observed in hatchery settings. This study evaluated the ingestion rates of loose tags, ingested tag retention time, effects of ingested tags on growth, and tag collision between implanted and ingested tags for three species of salmonids: Spring Chinook, Coho, and Steelhead. Ingestion varied between species, where Spring Chinook did not ingest any tags, Coho ingested 51% of available tags, and Steelhead ingested 69% of available tags. Steelhead expelled 50% of ingested tags 46 days faster than Coho and, by day 200, 24% of Steelhead and 30% of Coho ingested tags were not yet expelled. Once tags were ingested, growth rates of these treatment fish were not significantly different than fish that did not ingest tags except for Coho mass-standardized growth rates that were slightly lower in treatment Coho than control Coho. Tag collision between an ingested tag and an implanted tag frequently prevented tag detection of both tags and could be interpreted as false mortalities in survivorship models. Future research will use these data in simulations to determine at what implanted tag shedding rate tag ingestion could significantly bias model survivorship estimates of hatchery-reared salmonids.