Migratory fishes are widespread and critical for the functioning of distantly interconnected ecosystems, and they are gravely imperilled by global river fragmentation. However, the lack of a comprehensive review of migratory fish studies using the most accessible natural biogeochemical tags has hampered the improvement of research representativeness and gap identification and thus fish conservation and associated ecosystem management. Here, we conducted a global synthesis on this topic. A total of 904 studies were included after screening 19,642 documents, and 10,824 extracted sampling points successfully revealed distribution imbalances in multiple dimensions, such as ecosystems (inland 48% vs. marine 52%), major fishing areas (primarily in the Northern Hemisphere), migration categories, taxonomic groups, biogeochemical tags, and tissues. Less than one fifth of migratory fishes have been studied, usually using otoliths and point/line scanning. Bulk element: Ca ratios were examined more frequently than isotopic ratios (64% vs. 36%), led by Sr/Ca, Mg/Ca, Ba/Ca and 87Sr/86Sr, δ13C, δ18O, respectively. The mean values for these elements detected in fish otolith cores were significantly different across migratory categories, but all showed considerable variation. The findings indicate that migratory fishes are likely to have more diverse and complex life histories than commonly thought. The identified knowledge gaps should therefore be filled to promote the application of biogeochemical tags and the conservation and management of migratory fishes.

The datasets and R scripts that support the findings of this study are openly available at https://github.com/Otoliths/FishBT. Additionally, we have developed an online database (named FishBT, Fish Biogeochemical Tags; https://fish-ecology.shinyapps.io/fishbt) providing detailed, integrated biogeochemical data on migratory fish species all around the world. The database is an open, updatable and free platform for data query and visualization tools on migratory fish studies using biogeochemical tags.