Evaluation of Osmotic Induction of Calcein Treatments for Marking Juvenile Walleyes

Osmotic induction of calcein was evaluated as a nonlethally detectable alternative to immersion in oxytetracycline for batch marking 42-58-d old walleyes Sander vitreus. A preliminary laboratory study and five larger-scale pond experiments were conducted to evaluate mark efficacy, mark retention, and tolerance to immersion in solutions of salt at 25 g/L or 50 g/L of water followed by immersion in a solution of calcein at 5 g/L of water. Walleyes became obviously stressed when immersed in the salt solutions, and mortalities immediately following the treatments ranged from 0% to 53%. All surviving walleyes possessed marks visible with a hand-held detector. However, the intensity of marks decreased exponentially on walleyes held in ponds, and only walleyes treated by initial immersion in the salt solution of 50 g/L possessed detectable marks when harvested 17-19 weeks posttreatment. The brightest marks occurred on structures most protected from sunlight. Marks on pelvic scales removed at time of capture also remained visible via epifluorescent microscopy for at least 2 weeks when stored in a freezer. These results suggest that osmotic induction of calcein would be a suitable method of producing nonlethally detectable marks for short-term stocking evaluations if techniques to reduce the stress induced by the marking process could be deployed.