Otolith Marking of Rainbow Trout Fry by Immersion in Low Concentrations of Alizarin Complexone

Abstract

Experiments conducted on rainbow trout Oncorhynchus mykiss fry at Gaden Trout Hatchery, Jindabyne, New South Wales, Australia, sought to mark fish otoliths by immersion in the chemical alizarin complexone at a concentration consistent with minimal mortalities and clear markings. The fish were immersed in three concentrations of a solution of the chemical (10, 20, and 30 mg/L) for four periods (6, 12, 18, and 24 h). The pH of the solutions was adjusted to 8.8–8.9 with sodium bicarbonate, which increased the solubility of alizarin complexone. Marks were always detectable at all experimental concentrations under reflected light and/or ultraviolet light microscopy. All fry treated at a concentration of 10 mg/L survived for all exposure times. However, treatment and posttreatment mortality (within 28 d after treatment) at 20 and 30 mg/L were substantial over all periods. The treatment for 18 h at a concentration of 10 mg/L was chosen to mark a batch of 10,000 rainbow trout. Otoliths from these treated fish still had clear marks when harvested 157 d after treatment. The successful marking of rainbow trout without mortality using a concentration far lower than previously reported makes this treatment suitable and economical for batch marking of large numbers of small fish.