Does otolith chemistry indicate the natal habitat of Newfoundland capelin Mallotus villosus?

Identifying natal origins is important for understanding population structure (e.g. connectivity), but is often difficult to determine for fish with high dispersal potential. Natural tags, such as otolith microchemistry, however, provide the potential to tackle this challenge. Capelin Mallotus villosus is a key marine forage fish that spawns in two distinct habitats in coastal Newfoundland, beach (intertidal, or above low tide zone) and deep-water (15-40 m) habitat, where eggs remain throughout incubation. Capelin are predicted to occupy deep-water habitat as temperatures increase at beaches; however, the contribution of deep-water spawning habitat to recruitment is unclear. Here we test whether otolith chemistry differs between field-reared capelin eggs/larvae at beach relative to deep-water habitat on the northeastern Newfoundland coast during July-August 2011-2013. Throughout experimental rearing, we monitored temperature, salinity and trace metals in ambient water. Ambient trace metal concentrations were homogenous across rearing habitats, but beach-reared larval otoliths had significantly lower Ba/Ca, Sr/Ca, Mn/Ca, and Mg/Ca relative to those reared in deep-water. Temperature was consistently higher at beach relative to deep-water rearing habitat (~ 3-7 °C) throughout the experiment, and the partition coefficients of Sr/Ca in larval capelin otoliths (DSr) were negatively related to temperature (log DSr = − 0.026 T (°C) + 0.907). As salinity and ambient trace metal concentrations did not vary consistently between habitats, these findings suggest that rearing temperature primarily influenced otolith chemistry. Overall, this study reveals the potential to use otolith chemistry in adult capelin to determine the natal (egg) rearing habitat and, thus, the relative contribution of each spawning/rearing habitat to recruitment in future investigations.