Laboratory marking of anadromous rainbow smelt embryos and larvae and the implications for restoration

The decline of rainbow smelt (Osmerus mordax) in Massachusetts throughout the last decade has increased the need to develop practical restoration practices.  Successful marking and subsequent recapture of hatchery stocked smelt is critical to quantifying effectiveness of restoration efforts.  Utilizing recent advances in culture techniques, this study developed methods for marking otoliths in rainbow smelt.  Adult smelt in spawning condition were captured from coastal Massachusetts rivers and strip-spawned in the laboratory.  Buffered oxytetracycline hydrochloride (OTC) was applied using multiple concentration treatments to eyed embryos and newly hatched larvae.  Fish were retained and reared in a recirculating water system in the laboratory and sampled at regular intervals for up to 12 months to examine otoliths for the persistence of an OTC mark under ultraviolet light.  Otoliths from larvae marked with 500 and 1,000 mg/L OTC had the most visible and persistent marks.  Results were used to direct the marking of over 200,000 larvae at a time and up to four million larvae in a season. Marked larvae were stocked into a river with suitable spawning habitat but minimal spawning activity.  Samples from returning adults during subsequent years have revealed a quantifiable contribution to the spawning population.