Otolith trace element composition reveals spatial and ontogenetic structuring in the glacier lanternfish Benthosema glaciale (Reinhardt, 1837)

Abstract. Detecting habitat use in mesopelagic fishes is hampered by a lack of detailed surveys. Otolith chemistry can fill this gap by providing lifetime chronologies of the environmental conditions experienced by individuals. We analysed otolith transects of six trace elements with known environmental or physiological sensitivity in the dominant North Atlantic myctophid, the glacier lanternfish Benthosema glaciale. Fish were sampled across a complex fjord–shelf system where topographic and hydrographic conditions create vertical and horizontal structuring. Element:Ca ratios changed with age in all six elements. Overall, Sr increased through life, while Ba declined during juvenile stages and rose again in adults. These paired trajectories align with a shift from early exposure to surface-influenced waters toward longer residence in deeper water masses as fish grow. Mg and Mn were highest early in life and declined thereafter, suggesting ontogenetic control linked to growth and physiology. P increased from around the age of maturation, while B showed a weaker age structure. Spatial patterns were dominated by a contrast between the coastal site and the fjords, with additional element-specific differences among fjords. Individual fish also differed persistently in their overall elemental levels and in how their chemical trajectories changed with age. Several elements covaried after accounting for age, site and individual structure, most strongly Sr with Ba, suggesting shared incorporation processes or exposure histories beyond their common ontogenetic and spatial trends. Our results indicate that otolith chemistry in B. glaciale records life-stage-specific information on water-mass exposure, providing a retrospective window on habitat use where direct observation is impractical. More broadly, they highlight the value of this approach for linking biological and biogeochemical processes in deep and vertically structured systems.