Passive Integrated Transponder Tag Retention Rates in Headwater Populations of Coastal Cutthroat Trout

Abstract

Passive integrated transponder (PIT) tags have desirable qualities (e.g., unique identification, indefinite tag life, and capacity for remote detection) that make them useful for evaluating survival, growth, and movement of fish, but low tag retention rates can confound data interpretation. Although the effects of PIT tags on short-term growth and survival have been minimal and tag retention rates in laboratory and field studies using only juvenile individuals have been high, tag retention rates for fish at different ontological stages (including reproductively active males and females) remain unknown. We evaluated tag retention rates in wild populations of coastal cutthroat trout Oncorhynchus clarkii clarkii in three catchments of western Oregon using a double-marking approach (i.e., the adipose fin was removed from all fish that were PIT-tagged). Tags were inserted into the body cavities of fish 100 mm or more in length (fork length; range = 100–250 mm). In the study catchments, this size range includes both juvenile and mature fish. Tag retention rates were approximately 25% lower than those reported by previous studies of juvenile salmonids alone. A number of PIT tags were recovered in redds, indicating that mature individuals eject tags during spawning. Although some coastal cutthroat trout retained PIT tags for up to 4 years, others expelled them repeatedly and were implanted with a minimum of three different PIT tags during the same period. These data are concordant with those of other studies in which PIT tags had been inserted into the body cavity of salmonids and the population of tagged fish comprised both juvenile and mature individuals. Therefore, for multiple-year studies, it may be prudent to double-mark fish when PIT tags are to be inserted in the body cavity.