Oxytetracycline and thermal marking of alewife (Alosa pseudoharengus) otoliths

Alewives (Alosa pseudoharengus) are anadromous clupeids native to the east coast of North America. With their population in decline, there is increasing interest in releasing hatchery-reared alewives for stock enhancement. As a result, techniques are needed to produce long-lasting marks to identify stocked fish. The objective of this investigation was to evaluate the efficacy of marking alewives using oxytetracycline immersion and thermal marking techniques. Oxytetracycline marking trials consisted of immersing alewife larvae in three concentrations of oxytetracycline (200, 400, and 600 mg/L) for two durations (4 and 6 hours). Sagittal otoliths were removed and examined for marks with an epifluorescence microscope after 158, 232, and 350 DPH in October 2015, January 2016 and May 2016. Exposure to higher concentrations and longer durations in the immersion bath resulted in a higher percentage of fish with visible marks. Oxytetracycline marks were present on 100% of the otoliths sampled from the 6-hour immersion in 600 mg/L. Alewife larvae and juveniles were exposed to various temperature change regimes to produce thermal marks on the sagittal otoliths. All of the juveniles survived the thermal marking but the larvae experienced 100% mortality during the marking procedure. Otoliths were examined for thermal marks after 142, 271, and 349 DPH in October 2015, February 2016 and May 2016. Exposure to warm water temperatures for 24 hours created wider translucent increments between the opaque bands on the otoliths. There did not appear to be a difference in the lightness of the increments or darkness of the bands between the 25ºC: 18ºC or 28ºC: 16ºC treatments. The results of this study indicate that the optimal marking procedure for mass-marking alewives for stock enhancement is immersing larvae in 600 mg oxytetracycline/L for 6 hours