Investigating post-depositional alteration of trace elements in fish scales using tagged and recaptured wild salmon

Non-invasive methods are needed to enable indirect observation of vulnerable migratory fish populations across their life cycle and spatial distribution. Microchemistry data, such as trace element (TE) concentrations, collected non-invasively from the biomineral in fish scales can be used to characterize fish life history (habitat usage, fish provenance). However, scales are generally underutilized in microchemistry studies, due to conflicting evidence regarding the temporal stability of their composition. Some studies have found TE concentrations from scales are stable and reflect environmental conditions and habitat usage through time, while others have found that TE concentrations can change significantly after deposition in fish which have undertaken migration or spawning. Furthermore, validating TE proxies, such as fish scales, is extremely challenging for vulnerable populations because they have notoriously low survival rates in the wild, making tag and recapture experiments incredibly difficult. This study aimed to directly quantify changes in TE concentrations in scales collected from Atlantic salmon (Salmo salar, L.) at the juvenile and adult life stage. Over 1800 wild Atlantic salmon were tagged with passive integrated tags and, remarkably, 3 fish were successfully recaptured as adults. Individuals were sampled twice in the freshwater as juveniles, and again upon recapture during their return migration as adults. Scales were analyzed for concentrations of Ba, Sr, Mg, Mn, Fe, and Zn, using Laser Ablation Inductively Coupled Plasma Mass Spectroscopy (LA-ICPMS). Mixed effects models found significant changes in TE concentrations, between the juvenile scale and freshwater portion of the adult scale for all elements. Post depositional alteration may be caused by continued crystallization of scale apatite after migration, or scale reabsorption during the pre-spawning period. Future work could investigate the TE signal of fish recaptured at sea, to determine exactly when TE concentrations become modified.