Regional fidelity vs dispersal of capelin (Mallotus villosus) in the Gulf of St. Lawrence, inferred from otolith chemistry.

Capelin (Mallotus villosus), a key forage fish in the Northwest Atlantic, links zooplankton to predators including commercial fishes, seabirds, and marine mammals, yet its life-cycle connectivity in the Gulf of St. Lawrence (GSL) remain poorly understood. Between 2022 and 2024, otoliths from 927 individuals collected during and after spawning were analysed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and seven trace elements (Li, B, Mg, K, Zn, Sr, Ba) were used to discriminate three chemically distinct regions within the GSL. Early-life regional origin was inferred by classifying otolith core chemistry, treated as an integrated late-larval to early-juvenile signal rather than a strictly natal marker, against a discriminant model trained on otolith-edge reference signatures. Assignment of core signatures to regions indicated limited cohort-level effects, in accordance with the low interannual variability in regional otolith edge signature. A combination of widespread dispersal and patterns consistent with regional fidelity was found, with contrasts among regions in terms of fidelity vs interconnection. Fish sampled during spawning were more often reassigned to their inferred early-life region than post-spawning fish, a regional-scale homing-like pattern consistent with regional spawning fidelity. This coexistence of both dispersal and regional fidelity patterns likely contributes to a portfolio effect that may buffer the population against environmental variability and localised reproductive failures.