Long-Term Retention and Visibility of Visible Implant Elastomer Tags in Brook Trout

Understanding how juvenile brook trout Salvelinus fontinalis from different sources contribute to fishable populations would be facilitated by a batch mark that could be applied to early life stages and be retained and visible throughout a fish's life. We evaluated visible implant elastomer (VIE) as a long-term batch mark for juvenile brook trout in hatchery and lake environments. Visible implant elastomer material was injected into the postocular eye tissue of 2,350 age-0 brook trout. Marked fish were stocked into three lakes, and an additional group was held in the hatchery. Tissue dissections revealed that VIE tag retention was 100% in both environments after 970 d. Tag visibility in hatchery fish examined under indoor fluorescent light was greater than 95% through 585 d, then dropped to 55-70% between 700 and 900 d. Tag visibility in lake fish examined under outdoor sunlight was 50-72% at 400 d and 0% at 959 d. When these same fish were observed in dark conditions with blue-filtered light and amber glasses, tag visibility was 75% for hatchery fish at 970 d and 100% for lake fish at 959 d. The high retention and increased visibility when viewed in dark conditions with blue-filtered light demonstrates that VIE tags are a suitable long-term batch mark for juvenile brook trout.