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Study: Researchers Using PIT-Tags Should
Account for Tag-Loss, Tag-Induced Mortality

by Staff
Columbia Basin Bulletin, October 9, 2009

A recent study on the effects of PIT tags on the survival, growth and behavior of hatchery spring chinook salmon indicates that tag-induced mortality and tag loss is substantial.

Such effects of implanting these tiny 12 millimeters-long passive integrated transponder tags into the bellies of juvenile fish must be considered by researchers tracking the impacts of the federal Columbia River Basin hydropower system upon salmon and steelhead listed under the Endangered Species Act, says the study.

In the Columbia River basin, researchers have tagged millions of hatchery fish, and some wild fish, for a variety of studies aimed at gaining more knowledge of the life history of ESA-listed salmonids. PIT-tag research is central to determining the survival of these fish during downstream migration through the hydro system and how many return as adults to spawn.

"Smolt-to-adult recruit survival (SARS) of PIT-tagged fish was significantly lower than that of non-PIT-tagged (NPT) fish because of tag loss and reduced survival, resulting in an average underestimate of SARS of 25 percent," write the authors of the article recently published in the North American Journal of Fisheries Management.

"After correcting for tag loss, we estimated PIT tag-induced mortality to be as great as 33 percent with a mean of 10.3 percent over all brood years," says the study.

As for tag loss, the "average loss of PIT tags was 2 percent in juveniles before release and 18.4 percent in recaptures returning 6 months to 4 years after release. Adult tag losses were not significantly correlated with age of return, indicating that the majority of PIT tag loss had occurred within the first 6 months after release."

The study also noted that "lengths and weights of PIT-tagged adults were less than those of NPT fish of the same age. There was no significant difference between migration timing of PIT-tagged and NPT adults within the upper Yakima River," where the hatchery spring chinook were released.

"Given the widespread and increased use of PIT tags, and their use in calculating critical estimators related to salmonid life history of Endangered Species Act populations, the effects of using PIT tags must be quantitatively considered under actual study condition and, if necessary, accounted for," says the article's authors.

The researchers and authors of "Effects of Passive Integrated Transponder Tags on Smolt-to-Adult Recruit Survival, Growth, and Behavior of Hatchery Spring Chinook Salmon" are Curtis Knudsen of Oncorh Consulting, Mark Johnston of the Yakama Nation, Steven Schroder of the Washington Department of Fish and Wildlife, William Bosch and David Fast of the Yakama Nation and Charles Strom of the Yakama Nation's Cle Elum Supplementation and Research Facility.

The study was funded by the Bonneville Power Administration. The article can be found at

To nail down tag-loss and tag-induced mortality, the researchers engaged in a "double-tag study" in which fish were tagged with both PIT-tags and coded wire snout tags "to test the assumptions that tags are not lost and do not affect post-release survival, behavior, or growth."

By double tagging, researchers could measure the number of fish returning with the coded wire snout tag, but missing the PIT-tag, thereby calculating tag-loss. Beginning in 1998 and continuing until 2002, 37,000-40,000 age-1 hatchery-origin spring chinook each year were marked with double tags at the Clem Elum Supplementation and Research Facility. Non-PIT-tagged fish were also marked for identification, allowing survival comparisons between PIT-tagged and non-pit-tagged fish in the release study.

How do the tagged fish shed a passive integrated transponder buried inside the body cavity? That isn't known yet, said researcher Knudsen, but speculation focuses on the possibility the transponder is "expelled as a foreign body during smoltification." He said most lose their tags the first 6 to 8 months after release into the Yakima River.

And the reasons for the high mortality rate induced by PIT-tags? That too is uncharted territory.

Knudsen said it is likely the implanted tag is creating some kind of stress for the fish. He said if you compare the situation to a human it would be like implanting a tag "8 inches long in a person 6 feet tall."

The researchers in the article stress the need for double tagging when using PIT-tags so any results are not skewed due to tag loss and tag-induced mortality.

"Understanding the performance of PIT tags or any tag or mark under proposed study conditions," they say, "is critical to ensure that the most appropriate tag for a given situation is applied and that the results can be interpreted correctly in a broader perspective. If tag loss indeed occurs, true survival of tagged groups will be underestimated, but this can be corrected for if the loss has been estimated by using a double-tag study design.

"When tag-induced mortality occurs or if growth and behavior are affected by tagging, investigators should only cautiously extrapolate study results to the remainder of the untagged population."

Study: Researchers Using PIT-Tags Should Account for Tag-Loss, Tag-Induced Mortality
Columbia Basin Bulletin, October 9, 2009

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