Evaluating long-term retention of calcein marks on Brook Trout otoliths

ABSTRACT Objective Marking fish allows biologists to track individuals or cohorts through time and space and is often a requirement for fishery evaluations; however, the type of mark used may vary. Chemical marking of hard structures with calcein is one method used by biologists, and this technique benefits from the ability to mark many individuals with relatively low time commitment and expense. However, long-term retention is not well understood, and external marks can quickly degrade in sunlight. Our objective was to assess retention of internal calcein marks on the otoliths of Brook Trout Salvelinus fontinalis 926 d after initial marking and to determine whether exposure to sunlight affected this retention. Methods Juvenile Brook Trout were immersed in a calcein solution for initial marking before being placed into one of two outdoor raceways. One raceway was exposed to sunlight for the duration of the study, while the other was covered with a black shade cloth. After 2.5 years of grow out in the raceways, otoliths were removed and polished. Multiple observers then performed a blind examination to assess the presence or absence of calcein marks on otoliths. We calculated inter-reader agreement to assess reliability among readers, and we examined the effect of sunlight exposure on the likelihood of correct identification using a generalized linear mixed model. Results About 72% of otoliths had fluorescent marks, and there was no difference in the probability of identifying marks in fish that were exposed to sunlight and those not exposed to sunlight. Further, agreement was high among the three independent readers. Conclusions Collectively, our results suggest that calcein marks on internal structures may be useful for marking fishes over a length of time necessary to answer fisheries management questions.