Detection of Visible Implant Elastomer in Fingerling and Yearling Rainbow Trout

Visible implant elastomer (VIE) was evaluated for marking yearling rainbow trout Oncorhynchus mykiss. The rate of tag detection after the yearlings had been at large for 35 months was also compared with the detection rate found in a subsequent year‐class that had been marked as fingerlings and had been at large for 29 months. Fish were marked in the postocular adipose eyelid tissue and on the ventral surface of the lower jaw and examined in the dark under ultraviolet light. Detection rates for both year‐classes ranged from 29% to 33%, based on the proportion of fish with two detectable marks. Because the marks were not easily recognized, we believe the detection rates were overestimated. We conclude that detection of VIE at the sites we chose was problematic for the strain of rainbow trout we marked. More favorable long‐term detection rates in other species suggest that poor detection rates may be unique to heavily pigmented strains of rainbow trout.