Evaluation of large-scale marking with alizarin red S in different age rainbow trout fry for nonlethal field identification

Fry of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), was subjected to one-hour and four hours immersion in Alizarin red S (ARS) bath 150 mg · L–1. The experiment involved seven age groups (40, 50, 60, 70, 80, 90, and 100 days) and was conducted to estimate the minimal age of salmonids for mass marking with ARS bath enabling subsequent effective, simple field nonlethal identification, based on fin rays checking. The fish were examined at the ages of 200 and 300 days. The results showed a high retention level of ARS traces in caudal fin rays ensured satisfactory visibility and quick detection. A success rate of marking detection was >90% at 200 and 300 days of age and the fish were immersed in ARS solution from 60 days of age (685°D). This treatment provided better results in fish bathed for four hours. Recognition of marks using a laser pointer and protective glasses was successful even in fish, with a 3.5–5.0 times length increase compared to the marking time. The results indicate a high potential for ARS marking and its field identification for juvenile salmonids, which can significantly expand the possibilities of field experiments.