The global geography of fish diadromy modes

Aim: Geographical gradients in resource production are likely to translate into macro-ecological patterns in the biodiversity of migratory organisms but few studies have addressed this question at a global scale. Here we tested a hypothesis based on uncoupled latitudinal gradients in marine and freshwater primary productivities aimed at explaining where (e. g. at which latitude) and at which stage of the life cycle (larvae: amphidromy, juvenile: catadromy, adult: anadromy) migration from ocean to freshwater occurs (diadromy). Location: Global. Time period: Current. Major taxa studied: Fishes. Methods: We modelled, using multinomial regressions, the proportion of catadromous, anadromous, and amphidromous species in 994 river basins as a function of freshwater and marine net primary productivities (NPP, mgC/m²/day) and additional biotic and abiotic variables. Using data extracted from GBIF we tested whether diadromous, catadromous and amphidromous species differed with respect to the NPP of their marine and freshwater occurrences. Results: Among diadromous species, anadromous species are present in higher proportion when conditions for growth are more favourable in the sea (higher productivity, higher temperature, lower number of competitors) than in freshwaters. The model reconstructs the latitudinal pattern observed in the proportion of anadromous species, including an asymmetry between the northern and southern hemispheres. According to GBIF occurrences, the ratio of marine over freshwater productivity is higher for anadromous species compared to catadromous and amphidromous species. Main conclusions: Our results support the hypothesis stating that migration between ocean and freshwater occurs by choosing the biome that optimizes pre-reproductive growth and is one more example of the importance of primary productivity in shaping large scale community patterns. This result suggests that some diadromous fish populations and species may suffer from anticipated climate change if inter-biome productivity gradients are affected.