Time series of otolith growth patterns reflects oxygen depletion tolerance in an abundant mesopelagic fish species

Mesopelagic fishes play a key role in marine ecosystems worldwide and are considered a potential future resource for fisheries. It is therefore crucial to fully understand their long-term growth drivers and ability to respond to ongoing global changes. In the last decade, reduced water exchange due to the warming of the North Atlantic Ocean has caused oxygen depletion in the basin water of Masfjorden, a deep Norwegian fjord hosting a large biomass of mesopelagic fishes. Semi-enclosed estuarine systems such as fjords, characterised by strong ecological and physical gradients, provide unique conditions to study the effects of environmental change on marine populations. Drawing on this natural infrastructure and a long-standing annual survey, we aimed to describe general patterns of variation and assess whether low-oxygenated waters affect the growth of one of the key mesopelagic species in the North Atlantic, the glacier lanternfish Benthosema glaciale (Myctophidae). We collected biological samples and environmental information spanning several years before, during and after a prolonged hypoxic period. Using otolith annual increments as a proxy for individual growth in a mixed-effects model framework, we examined growth variability in relation to internal (individual-specific) and environmental factors. Adult fish demonstrated tolerance to the measured environmental fluctuations, with minimal changes in growth trajectories. While ocean deoxygenation poses a major challenge to marine regions at a global scale, the resilience of B. glaciale to mild hypoxia suggests promising prospects for population persistence. • Otolith macrostructure reveals drivers of growth variation in an important myctophid. • Slow juvenile growth leads to faster growth in adult Benthosema glaciale . • Young individuals grow faster under mild deoxygenation-lower light conditions. • Older age classes appear tolerant to mild hypoxia and water-darkening.