Comparison of Large and Small Visible Implant Tags: Retention and Readability in Hatchery Brook Trout

—Retention and readability of small (2.5 3 1.0 3 0.1-mm) and large (3.0 3 1.5 3 0.1mm) visible implant (VI) tags were compared among different size-classes (211–470 mm total length, TL) of hatchery brook trout Salvelinus fontinalis. Among smaller size-groups (,400 mm TL), retention of large tags (63–86%) 100 d after tagging was less than retention of small tags (89–97%). Overall retention of large tags was much better among larger size-groups ($400 mm TL) than among smaller sizes (97% versus 74%). Insufficient postorbital adipose tissue in smaller size-groups probably caused the poor retention of large VI tags. Retention was different between the two workers who inserted large VI tags. Readability of large and small tags was not affected by fish size at tagging. We recommend that large VI tags be used to mark brook trout longer than 400 mm TL because they are retained better. Fish with small tags from a previous study were subsequently monitored (and reported here) to assess retention and readability between 354 and 454 d after tag insertion. Over this period less than 3% of small VI tags were lost, but readability declined from 95% to 74%. In studies that extend beyond a year, small VI tags may have to be extracted to be read.