Otolith shape and microchemistry reveal fine-scale population connectivity in the myctophid Benthosema glaciale (Reinhardt, 1837) along a complex seascape.

Myctophid fishes constitute a major component of the world’s ocean biomass and are seen as a potential resource for commercial exploitation. Within this family, the glacier lanternfish Benthosema glaciale (Reinhardt, 1837) is the dominant North Atlantic species and plays a key role in pelagic food webs in open waters, deep coastal regions and fjords. Previous studies employed genetic markers to evaluate population structuring across the species’ broad distribution range, detecting heterogeneity in shallow-silled fjords. However, genetic methods cannot reconstruct ontogenetically determined movements, and research on alternative methods is lacking. Here, we examine whether otolith features can resolve coastal connectivity in B. glaciale. We collected individuals from a coastal site and a range of fjord types along the complex Norwegian west coast to capture topographic and environmental characteristics that could give rise to population structuring. We show that otolith shape coefficients and temporally-resolved microchemical signatures reveal consistent and significant patterns of variation supporting the presence of subpopulations within small geographic distances. Contrary to earlier hypotheses, we propose that oceanographic dynamics affecting early life stages have a greater influence on shaping coastal mesopelagic populations than topographic barriers. This study presents the first evidence of the effectiveness of otolith morphometrics and microchemical analysis in addressing ecological questions in this abundant myctophid, and highlights their potential to provide valuable information for future management and conservation strategies.