Thermal Marking of Otoliths in Lake Trout Sac Fry

Abstract
The ability to distinguish hatchery-produced lake trout Salvelinus namaycush from their wild counterparts is essential for evaluation of restoration efforts in Lake Superior. However, when early life history stages are stocked, conventional external marking procedures are inappropriate. The use of thermal marks in otoliths has become an accepted procedure for fry of Pacific salmon Oncorhynchus spp., but knowledge of the technique with lake trout fry is limited. This study adds to existing knowledge of thermal marking with insight into factors that affect lake trout. Several thermal marking regimes with lake trout sac fry were evaluated by varying the number, range, duration, and spacing of temperature pulses. Pulses produced individual bands on otoliths that together constituted the mark. Mark visibility varied within treatments, but in general, the highest quality marks were achieved when fry were subjected to pulses of 10°C or higher for 8 h or more on alternate days. These marks were visible o...