People: F. Joel Fodrie, Jeffrey D. Plumlee, Matthew D. Kenworthy, Rachel K. Gittman, Danielle A. Keller, Emma E. Garnett, Leandro Vaca-Pita, and Lindsey A. Carr
Environmental stress is a fundamental driver of population and community dynamics across diverse ecosystems. In coastal aquatic habitats at the transition from freshwater to marine conditions, osmoregulatory (i.e., salinity-related) constraints are key for gauging environmental stress. Within mangrove-dominated creeks on Isla Isabela, Galapagos, we observed what appeared to be intense freshwater-saltwater mixing and rapid salinity transitions correlated with tidal cycles.
Despite these perceived large environmental shifts every six hours, we also observed that juvenile Pacific dog snapper appeared resident within these mangrove creeks regardless of tidal stage and local salinity. Consistent residency within mangrove habitats was confirmed by acoustic tagging and tracking of individual fish. Despite the large shifts in salinities twice daily (between 2-35 practical salinity units), tagged snapper remained in these two creeks during ~93% (averaged across fish) of possible observations in the six hours bracketing each high tide (i.e., high salinities), and ~91% (averaged across fish) of possible observations in the six hours bracketing all low tides (i.e., low salinities).
Dog snapper residency within mangrove creeks, despite potentially energetically demanding salinity variability, may also highlight the overall fitness benefits for juvenile fishes provided by coastal biogenic habitats like mangroves. The structural refuge defining these habitats can increase foraging opportunities and decrease predation risk for juvenile fishes, a pattern corroborated by the high survival rate of tagged dog snapper.