Habitat use and trophic structure of Atlantic tarpon (Megalops atlanticus) inferred from geochemical proxies in scales

Atlantic tarpon, Megalops atlanticus, are highly migratory euryhaline predators that occupy different habitats throughout life. Atlantic tarpon are known to inhabit oligohaline waters, although the frequency and duration of movements across estuarine gradients into these waters are poorly known. This species supports over a two billion dollar industry within the Gulf of Mexico and is currently listed as vulnerable by the International Union for the Conservation of Nature (IUCN). Analysis of trace element and stable isotope compositions of growth increments in fish scales is a non-lethal method for reconstructing migrations across estuaries in vulnerable species. We analyzed Atlantic tarpon scales from the Texas coast to validate this method using inductively coupled plasma mass spectrometry (ICP-MS) for trace elements and isotope ratio mass spectrometry (IR-MS) for stable isotope ratios. Multiple scales and otoliths were taken from the same individual to confirm the consistency of elemental and isotopic uptake within the same individual and between structures. Results show that scale Sr/Ca and δ13C are effective proxies for salinity, while increases in δ15N are consistent with known trophic shifts throughout life history. Patterns of elemental concentrations and isotope values across scales within an individual were consistent with each other. Scale and otolith transects contained the same overarching trend with comparable shifts in elemental concentrations across growth increments in the two structures. Migratory contingents, or groups within distinct populations that exhibit different patterns of habitat use and movement across salinity gradients, were identified. The distribution of contingents indicated that migratory behavior is highly variable, with some, but not all fish transiting estuarine gradients into oligohaline waters. Yet, the majority of individuals sampled exhibited early life residency in oligohaline waters. This work demonstrates the use of low salinity habitats by Atlantic tarpon. Our validation of the methods for analyzing scales will provide novel opportunities to monitor fish migrations across salinity gradients.