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 on fry otoliths at 40× magnification, and individual bands were visible at 100×. Higher magnification was needed to distinguish individual bands when fish were marked with consecutive-day pulses. As fish grew older, 200× magnification was required to distinguish consecutive-day bands. Precision in preparing otoliths for examination was also a factor governing mark visibility, but the better marks were visible with less preparation. Marked fish are currently being maintained in our hatchery until maturity, and mark recognition has been 100% for up to 7 years. Nearly 1.5 million marked lake trout sac fry were released on a reef in Lake Superior during the 3-year period 1994–1996, and mark evaluation of lake trout captured in this location will be attempted in the future.