Otolith microchemistry highlights the importance of extensive connectivity for conservation of an iconic migratory fish in a large tropical river basin

Migratory fishes exert important influences on tropical river food webs, but these species are often most vulnerable to environmental change. Movement patterns of fishes in tropical rivers are also generally poorly understood, hindering conservation efforts.Common snook, Centropomus undecimalis, depend on connected coastal habitats for reproduction and growth and can migrate extensively up rivers when not hindered by barriers to movement. In southern Mexico, they are frequently captured 300 river km or more upstream from the delta of the Usumacinta River. The Usumacinta is a large river system with no mainstem dams, providing an expansive network of connected river, stream and wetland habitats which form a much larger migratory domain that exists in many other systems where common snook have been studied.To assess whether fish captured in different zones were associated with different natal habitats or distinct migratory patterns, variation in common snook otolith chemical signatures was examined in the Usumacinta River from the Gulf of Mexico coast to as far as 600 river km upriver.Otolith microchemistry was useful for characterizing migratory histories of individuals, but there was no clear evidence that fish captured in different river zones used different types of natal habitats. Based on lifetime Sr:Ca in otoliths, a diverse array of movement patterns was evident in the Usumacinta system, with 97% of common snook showing evidence of freshwater habitat use.Prevalent use of riverine habitats by common snook far from the coast reinforces the need to preserve connectivity in the Usumacinta River and other undammed systems supporting migratory species. The natural flow regime and lack of barriers allow for longitudinal and lateral connectivity in this system, providing pathways for migratory species to move extensively and have access to an array of habitats, including productive floodplain lagoons.