The Effect of Water Hardness on Mortality of Zebrafish (Danio rerio) During Exposure to Oxytetracycline.

Marking of fish otolithswith oxytetracyclineand tetracycline is a widely used method to evaluate the effectiveness of stocking operations. Availableprotocols for the labeling of fish specify a number of factors influencing mark quality and potential risk for fish during marking. This study investigates the influence of water hardness on mortality of freshwater fishduring marking with OTC. In order to pursue this question complexation of OTC with Mg2+ and Ca2+ cations was measured spectrophotometrically. Furthermore, zebrafish ( Daniorerio) were immersed in OTC solutions (1200 mg/L; 48 h immersion) combined with varying levels of water hardness (5.5, 15.5, 25.5, 32.5°dH). The amount of OTC-Mg–Ca-complexes was positively correlated to water hardness. Moreover, it could be demonstrated that mortality of zebrafish during marking varied as a factor of water hardness. Highest mortalities occurred at the lowest (5.5°dH) and the highest (32.5°dH) tested levels during marking with OTC.