Alligator Gar in Texas' Coastal Zone: Insights from Otolith Microchemistry and Genetic Analyses

The free-flowing reaches of many Texas rivers support some of the best remaining populations of Alligator Gar Atractosteus spatula in the world.  However, the coastal estuaries and bays connected to these rivers also contain an abundance of Alligator Gar.  Recent research suggests individual use of both fresh and saltwater habitats and the potential for distinct river and bay stocks; the outcomes of which have important implications for population and habitat management for Alligator Gar in coastal systems.  We used otolith microchemistry and genetics to describe spatiotemporal patterns in movement and habitat use and to understand population structure of Alligator Gar in the Guadalupe River-San Antonio Bay system.  Otolith strontium:calcium (Sr:Ca) ratios of 150 Alligator Gar collected throughout the 300-km river-bay continuum revealed distinct patterns in salinity exposure including life-long river residence, transience (fish utilizing both river and bay habitats) and life-long estuary/bay residence.  Sr:Ca profiles from otolith cores indicated Alligator Gar likely utilize river and low-salinity, estuarine habitats for spawning and nursery habitat.  Complementary genetic analyses will determine if observed differences in habitat use influence genetic stock structure.  The results of our study will be instrumental in defining appropriate geographic scales for management of Alligator Gar in coastal river systems.