Natural tags identify nursery origin of a coastal elasmobranch Carcharhinus leucas

Abstract

Traditional methods for evaluating the importance of elasmobranch nursery habitats have focused on estimating densities or abundances of juveniles within individual nurseries; however, rates of juvenile mortality may vary among nurseries. Thus, abundance may not reflect contribution to adult populations, and contribution estimates require methods to identify the nursery origin of adults. We evaluated the use of natural tags, vertebral chemistry (stable isotope analysis, elemental analysis) and genetic markers, to identify bull shark Carcharhinus leucas nursery origin along the Texas coast (northwestern Gulf of Mexico) at multiple spatial scales. Sharks were most accurately assigned (89% accuracy) to their regional origin (Northern vs. Southern Texas) using a combination of vertebral chemistry and genetic markers. Accuracy decreased when incorporating only one type of natural tag (vertebral chemistry: 58%–84%; genetics: 30%–58%), or when assigning sharks among nurseries grouped into three or four regions (30%–71%; ~100 km spatial scale). Synthesis and applications. We describe a novel method integrating multiple natural tags to identify the nursery origin of a highly migratory coastal elasmobranch, over smaller spatial scales than previously investigated. This framework provides a powerful tool to estimate the relative source (nursery) contributions to adult populations of highly mobile species, which represent production estimates sought in the designation of essential fish habitat and ultimately incorporated into marine spatial planning practices.