Retention of T-bar anchor tags by adult steelhead during their upstream migration

Abstract Objective T-bar anchor tags can be used to obtain recapture data from anglers, directly estimate exploitation, and evaluate population dynamics. However, their use by biologists to study anadromous salmonid fisheries is limited. Two hurdles to adoption include the functional difficulty of tagging large anadromous salmonids using conventional tagging equipment and a lack of information on tag loss by large anadromous salmonids and how it changes over time. As such, our objectives were to (1) describe a T-bar anchor tagging system modified to study adult steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) and (2) present an instantaneous tag loss model for steelhead that allows estimation of tag loss over time. Methods First, we developed a modified tagging system by tagging hatchery-obtained steelhead carcasses and live, resident Rainbow Trout larger than 500 mm using a variety of hardware and tag dimensions. Next, we double-tagged adult steelhead captured at the Lower Granite Dam adult fish trap, Washington, United States. We then used data from 182 recaptured steelhead to fit an instantaneous tag loss model. Last, we investigated whether steelhead tag loss was related to body length. Result Tag loss was generally low within the time period under study (i.e., up to 221 days between release and recapture). The estimated probability of tag loss was 0.034 at release, 0.044 at 1 month, and 0.113 at 8 months. We failed to detect significant differences in tag loss parameters between two data subsets consisting of small (<720-mm) and large (≥720-mm) steelhead. Conclusion T-bar anchor tags are useful external tags for studying adult steelhead during their upstream migration. Because anglers can be used to provide recapture data, T-bar anchor tags may be particularly useful where angler effort is high or direct estimation of fishery exploitation is desired.