Effects of Age at Treatment on Oxytetracycline Mark Formation in Larval Northern Pike

Abstract

Oxytetracycline (OTC) immersion treatments were evaluated at the Waterville State Fish Hatchery (Waterville, Minnesota) for producing fluorescent marks on larval Northern Pike Esox lucius otoliths. A pilot marking trial conducted with 7-d-posthatch (dph) larvae resulted in poor mark efficacy and prompted further investigation on the treatment of younger larvae and modification of the treatment by use of osmotic induction to promote better mark formation. Northern Pike treated within 24 h post-hatch had significantly higher mark efficacies and marked otolith intensities than did Northern Pike treated at later life stages. Similar to the pilot marking trial, immersion of 7-dph fry in OTC solution resulted in poor mark efficacy, in which only 38% of the inspected fry possessed visible marks and most of the marks that were visible appeared faint. Simple immersion of newly hatched fry in OTC solution resulted in 91% marking success, and 65% of the visible marks appeared either clear or intense. The poorer mark efficacy on 7-dph fry suggests that differences in mark formation were more dependent on contemporaneous physiological processes than on otolith size. Osmotic induction prior to OTC immersion had a much weaker effect on mark intensity than did age-class of treatment in the study.

Received November 12, 2013; accepted March 30, 2014

ACKNOWLEDGMENTS

We thank Douglas Dieterman and John Krenz for their contribution to this research. A special thanks goes to the Minnesota Department of Natural Resource fisheries staff: Jack Lauer, Marc Bacigalupi, Scott Mackenthun, Steve Shroyer, Peter Muggli, and Nancy Morris. We also thank David Staples for his advice on statistical analysis.