A hierarchical Bayesian model to monitor pelagic larvae in response to environmental changes

European anchovies and round sardinella play a crucial role, both ecological and commercial, in the Mediterranean Sea. In this paper, we investigate the distribution of their larval stages by analyzing a dataset collected over time (1998–2016) and spaced along the area of the Strait of Sicily. Environmental factors are also inte-grated. We employ a hierarchical spatio-temporal Bayesian model and approximate the spatial field by a Gaussian Markov Random Field to reduce the computation effort using the Stochastic Partial Differential Equation method. Furthermore, the Integrated Nested Laplace Approximation is used for the posterior distributions of model parameters. Moreover, we propose an index that enables the temporal evalu-ation of species abundance by using an abundance aggregation within a spatially confined area. This index is derived through Monte Carlo sampling from the approx-imate posterior distribution of the fitted models. Model results suggest a strong relationship between sea currents’ directions and the distribution of larval Euro-pean anchovies. For round sardinella, the analysis indicates increased sensitivity to warmer ocean conditions. The index suggests no clear overall trend over the years.