Instream Evaluation of Passive Integrated Transponder Retention in Brook Trout and Brown Trout: Effects of Season, Anatomical Placement, and Fish Length

Abstract

We examined effects of tagging season, anatomical placement of tags, and fish total length (TL) at tagging on 2-month retention of passive integrated transponder (PIT) tags in 181 brook trout Salvelinus fontinalis (99–302 mm TL) and 709 brown trout Salmo trutta (122–511 mm TL) in small streams of southeastern Minnesota. A lower percentage of brown trout retained their tags in spring (56%) than in fall (68%), but season did not affect tag retention in brook trout. Retention rates varied with anatomical placement for both species. For brook trout, retention was 70% when the PIT tag was placed into the body cavity using an insertion point that was posterior to the pelvic fins and 100% when the tag was placed into the dorsal musculature. Among brown trout, PIT tag retention was 56% for body cavity placement (posterior insertion) and 95% for dorsal musculature placement. Tag retention was not associated with fish TL for brown trout and was only weakly associated with TL of brook trout. Although reported retention rates of PIT tags implanted into the body cavity of salmonids have often exceeded 90%, our retention estimates were considerably lower. This discrepancy can be partially attributed to differences in laboratory versus field studies. However, most previous studies of salmonids have used body cavity insertion points that were anterior to the pelvic fins, whereas we used a posterior insertion point; this was probably the primary factor responsible for the difference in results. We caution against the assumption that retention of PIT tags is universally high, and we recommend the estimation of PIT tag retention before or during marking studies. We also provide a list of factors that should be considered and evaluated when designing PIT tag studies, especially tag retention studies.