Otolith-derived growth variations in red mullet (Mullus barbatus) reveal spatial heterogeneity under present and future climate scenarios

Understanding how environmental variability shapes growth in marine fish is essential for predicting population dynamics under climate change. This study investigates spatial differences in the growth of red mullet (Mullus barbatus) across three neighbouring regions of the South Adriatic (GSA 18) and North-Western Ionian (GSA 19) Seas (central Mediterranean). Otolith increments widths were analysed to quantify life growth, while environmental drivers were assessed using diverse oceanographic variables, including sea surface temperature, bottom temperature, oxygen concentration, salinity, and chlorophyll concentration. Generalized additive models were fitted to identify the environmental and spatial factors influencing growth under both contemporary and projected climate conditions. Growth exhibited pronounced spatial heterogeneity, with distinct localized hotspots in both the eastern and western portions of GSA 18, reflecting fine-scale hydrographic variability. Temperature emerged as the primary environmental driver, with optimal growth estimated around 18.5°C. Forecasts under representative concentration pathway climate scenarios revealed a non-linear response: a mid-century enhancement in growth followed by a decline by 2099, linked to warming beyond the thermal optimum. These results underscore the significance of local environmental conditions in shaping growth trajectories and highlight the potential vulnerability of red mullet populations to future climate change. Otolith chronologies, combined with spatially explicit modelling, offer an effective framework for detecting environmental impacts on growth. Accounting for spatial and climate-related variability in growth may enhance the accuracy of current assessment models and support the development of climate-ready fisheries management strategies in the Mediterranean.