Global otolith records reveal ecosystem-dependent climatic associations with migratory fish movement synchrony

Spatial synchrony can destabilize populations by eroding portfolio effects, but global tests of movement synchrony are limited by the scarcity of comparable individual movement records. Here, we synthesized 13,975 otolith edge microchemistry and isotope records from 428 publications covering 248 migratory fish species. First, spatial factors explained more than 92% of decomposed otolith variation in both inland and marine systems, supporting otolith edge chemistry as a broad proxy for recent habitat-use dispersion. We then defined a movement synchrony index (MSI) as the negative residual dispersion of otolith signatures within species-region groups. MSI showed descriptive biological patterns, tending to be highest in larvae and anadromous species, but pairwise differences among life-history stages and migratory categories were not statistically supported. Climatic associations were strongly ecosystem-dependent. In inland waters, mean temperature, temperature variability, and precipitation variability were positively associated with MSI, whereas temperature seasonality, mean precipitation, and precipitation seasonality were negatively associated with MSI. In marine systems, all climatic coefficients were statistically unsupported despite moderate model importance for several predictors. Only one interaction was supported, a positive inland interaction between temperature and precipitation seasonality. These findings indicate that broad climate summaries are more informative for inland than marine movement synchrony and that marine forecasts require regionally resolved oceanographic predictors.