Facultative oligohaline habitat use in a mobile fish inferred from scale chemistry

Reconstructing fish movements is critical to understand the diversity of habitats required to sustain mobile species. Chemical constituents in otoliths have been invaluable for the field of fish migration ecology to track natal origins and reconstruct lifetime movement patterns. However, alternative non-lethal structures, such as scales, are preferred for imperiled species to avoid mortality. We analyzed 29 individual scales from highly migratory and vulnerable Atlantic tarpon Megalops atlanticus (hereafter referred to as tarpon) in the Gulf of Mexico to identify migrations across salinity gradients and associated trophic shifts using paired measurements of elemental (Sr/Ca) and isotopic (δ13C and δ15N) proxies. Although tarpon can inhabit freshwater, the specific patterns of facultative oligohaline habitat use are unknown. Individual scale-based salinity and diet histories were highly variable, with 4 contingents identified depending on the presence and sequence of movements. Scale salinity proxies (Sr/Ca and δ13C) indicated that tarpon spent on average 42 ± 34% of their scale-based life histories within oligohaline habitats. Transhaline movements were accompanied by shifts in δ15N that indicated putative trophic shifts between marine or estuarine and oligohaline food webs. Oligohaline habitat use is common yet individually facultative for tarpon. This information is critical to devise sustainable fisheries management plans that account for the full range of diverse habitats used by this species throughout its life. Chemical analyses of scales have the potential to be broadly informative about migrations and trophic interactions in species where lethal methods must be avoided.