Isolating the influence of ontogeny helps predict island-wide variability in fish otolith chemistry

For marine fishes of commercial interest, defining how individuals vary in certain attributes, through ontogeny, and across space and time, can help expose the structure of harvested populations and guide their management. The chemical markers locked within otoliths are, in principle, ideal for such applications, providing life-long, bio-environmental records for individuals and populations. However, substantial uncertainty still surrounds how intrinsic (i.e., within-individual) and extrinsic (i.e., environmental) processes shape chemical incorporation into otoliths, constraining our ability to predict marker variability when sampling is patchy. To tackle these issues, we measured otolith elemental (Li, Mg, Ca, Mn, Zn, Sr, Ba) and stable isotopic (delta C-13, delta O-18) concentrations from juvenile Atlantic herring (Clupea harengus) captured in Icelandic nursery sites, and built models to isolate ontogenetic (i.e., age-, growth-related) from environmental influences on otolith chemistry. Otolith Li:Ca, Mg:Ca, Mn:Ca and Sr:Ca declined with age within sites. Age slopes differed among sites for Li:Ca and Mg:Ca, and Sr:Ca was lower in larger fish within an age-class. Individual-level variation (i.e., within site, within age-class) was high for all markers, our models highlighting the importance of temperature and salinity (or proxies these represent) in explaining population-level delta C-13 and delta O-18 trends. Age- and year-specific predictions for each marker, at each site, generally accorded well with empirical observations, providing inference on island-wide heterogeneity in otolith chemistry across the juveniles' full distributional range. Such 'isoscapes', generated from mechanistically-focused models as presented here, might benefit investigations of population structure for other exploited species, particularly where sampling limitations hamper fishery-management efforts.