Behavior of lake trout sac fry: Vertical movement at different developmental stages

Behavior of lake trout (Salvelinus namaycush) fry is not well understood but could affect the role of predation on recruitment. Movement out of the substrate as sac fry could expose fry to predation. The purpose of this study was to compare vertical movement of lake trout sac fry at different developmental stages, to describe their diel movement patterns, and to examine the distance that sac fry will swim off the bottom. Fry traps set at three heights were used to capture fry moving out of rock substrate in laboratory tanks. Movement out of the substrate occurred ata the sac fry stage, steadily increased as fry advanced developmentally, and peaked at swimbladder filling. Catch of fry was always greatest at night. Few to no fry were captured during daylight hours. Catch of fry was usually greatest in traps near the bottom (17 cm off the bottom), but sac fry were regularly captured in traps set 37 and 57 cm off the bottom during all stages of development. These results show that lake trout sac fry move out of the substrate at night before swimbladder filling and thus, could suffer from predation by fishes that feed at night near the bottom.