Linking fish migration and hypoxia exposure to trophic ecology using natural chemical tags

Natural tags in fish reveal life history information that includes migration pathways and dietary sources. The chronological and geochemical properties of otoliths record changes in fish growth and reflect ambient salinity gradients, allowing reconstruction of coastal and estuarine movements. Stable isotopes of carbon and nitrogen in liver and muscle tissues provide dietary histories over short and long time scales, respectively. In order to apply natural tags, validation studies are needed that examine incorporation dynamics of elements and isotopes, such as the effects of physiology and growth on element uptake and isotope turnover, and describe chemical gradients of dissolved elements over time and space. Along the coast of Texas, dissolved Sr:Ca and Ba:Ca varied predictably with salinity over broad geographic and annual time scales. Tissue isotope turnover experiments revealed faster turnover of liver compared to muscle, offering short- and long-term dietary indicators. Using established estuarine ion gradients and tissue turnover rate estimates, estuarine ingress and habitat residence patterns were examined in Atlantic croaker collected from positive, metastable, and negative estuary types, encompassing the north-south Texas coast. Migration patterns varied depending on local salinity regimes, with strong regional gradients exhibited in both otolith elements and tissue isotopes. Fine scale between bay movements were detected using differences in liver-muscle δ¹³C values as indicators of tissue equilibrium status and time since habitat shift. In the northern Gulf of Mexico, the long-term sublethal effects of seasonal hypoxia on fish populations remain unclear, due to unknown exposure histories. Hypoxic redox conditions promote release of dissolved Mn²⁺ from the sediment, thus otolith Mn:Ca profiles can reflect lifetime hypoxia exposure. Lab experiments demonstrated a minimal influence of physiology on element uptake in response to hypoxia. Field studies in the northern Gulf of Mexico quantified estuarine habitat use using otolith Ba:Ca and coastal hypoxia exposure using otolith Mn:Ca, and assessed isotope trophic niche area using muscle tissue δ¹⁵N and δ¹³C values. Isotope niche area was similar between coastal and hypoxia exposed fish, suggesting trophic resilience of Atlantic croaker to seasonal hypoxia. This research provides a novel multi-proxy approach; using lab and field validated natural tags for linking migration and environmental exposure histories to trophic ecology in a marine fish.