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First identification of probable nursery habitat for critically endangered great hammerhead Sphyrna mokarran on the Atlantic Coast of the United States

Great hammerheads (Sphyrna mokarran) have undergone significant global population declines and were assessed as Critically Endangered by the IUCN in 2019. Identification and protection of critical habitat, particularly during early life stages, is considered imperative for species conservation and management. We report the first identification of a probable nursery area for young‐of‐the‐year and juvenile great hammerheads off the Atlantic coast of the United States. Notably, these animals were all encountered within highly human‐impacted marine habitats near Miami, Florida. The capture of nine individuals under 200 cm total length in a 10‐month period supports the hypothesis that this area represents a primary or secondary nursery habitat for great hammerheads.


| INTRODUCTION
The concept of "nursery areas" in marine science dates back more than a century, and most broadly, it has been used to refer to areas where juveniles are found in higher densities, avoid predation more successfully, or grow faster than in other areas (Beck et al., 2001). In recent years, shark-specific definitions have emerged to apply to larger-bodied, mobile elasmobranch species. Heupel, Carlson, and Simpfendorfer (2007) described three key criteria for identification of shark nursery habitat: (1) juvenile sharks are more commonly encountered in that habitat than elsewhere, (2) sharks remain in the area for extended periods, and (3) the area is used as a nursery repeatedly across years. Despite the clarity of these criteria, identifying nursery habitat for many shark species remains a challenge (see Heupel et al. 2018).
The great hammerhead shark is the largest member of the family Sphyrnidae, yet data on nursery habitat for this species in US waters remains limited and largely speculative. Great hammerheads can be found in coastal and pelagic tropical and warm temperate waters circumglobally (Castro, 2010;Compagno, 2001). Due to species-specific physiological characteristics (reviewed in Giles et al., 2014) and frequent interactions with fisheries as a target species and as bycatch, great hammerheads have faced significant population declines throughout their range. In late 2019, they were categorized as Critically Endangered by the IUCN (Rigby et al., 2019). Great hammerheads were also considered for listing under the US Endangered Species Act (ESA) in 2014, but were not listed based on a lack of species-level data available to infer population trends with high confidence, highlighting the conservation and management difficulties presented by identification across hammerhead species (Miller, Carlson, Hogan, & Kobayashi, 2014). Great hammerheads have been the subject of recent conservation concern and rule-making in Florida (e.g., in relation to land-based shark fishing) and are protected from commercial and recreational harvest in Florida waters (see Miller et al., 2014;Gallagher & Klimley, 2018;Shiffman et al. 2017;Shiffman 2020). Studies of great hammerhead movement ecology make clear the difficulty of conserving adult animals based on their large-scale migrations (which often move through international waters, creating regulatory and management barriers) and their intermittent philopatric attachment to particular sites (Guttridge et al., 2017;Harry, Macbeth, Gutteridge, & Simpfendorfer, 2011;Techera & Klein, 2011).
Although there is stronger evidence for philopatric behaviors associated with residency and site fidelity than reproduction in great hammerheads, natal philopatry is well-known in some other shark species, and the quality of available nursery habitat can exert an important influence on juvenile survivorship (Chapman, Feldheim, Papastamatiou, & Hueter, 2015;Heupel & Simpfendorfer, 2011;Heupel et al. 2018). Size at birth for great hammerheads is reported to be between 66 and 71 cm TL with growth rates approaching 40 cm within the first year in the Western Atlantic and Gulf of Mexico (Harry et al., 2011;Piercy, Carlson, & Passerotti, 2010). A lack of neonates in nearshore habitats has been used to argue great hammerheads primarily use offshore habitats for pupping (Harry et al., 2011;Hueter & Tyminski, 2007), although pupping has never been witnessed. Evidence suggests that protecting not only neonates but also older juveniles can play an important role in population recovery for threatened shark species (Brewster-Geisz & Miller, 2000). The discovery of a highly urban-impacted, previously unidentified area of importance to young-ofthe-year (YOY) and juvenile great hammerheads, with the potential to support neonates, may have significance for decisions about conservation and management of the threatened ecosystems of Biscayne Bay.

| METHODS
Biscayne Bay is a large, shallow, oligotrophic embayment, and marine estuary on the southeast coast of Florida, containing interconnected ecosystems including mangrove shoreline, seagrass beds, and coral reefs. These ecosystems support considerable biodiversity including more than 30 endangered species or species of special concern (Cantillo et al., 2000), and more than 100 species important to local recreational and commercial fisheries (Stoa, 2016). Rapid population growth, urbanization, and associated land-use change and anthropogenic pressure has resulted in eutrophication, pesticide and chemical accumulation, changes in salinity, habitat loss, direct damage from propellers and anchors, overfishing, and localized die-offs of seagrasses, mangroves, and corals (Cantillo et al., 2000). The fishing site discussed here encompasses an area of 2.5 km 2 in north-central Biscayne Bay, located less than 7 km from downtown Miami (Figure 1).
From June 2018 to January 2020, nine juvenile great hammerheads were captured via scientific longline in Biscayne Bay, Florida. Fishing took place as part of a research program deploying both longlines and drumlines, however, all juveniles were captured on longlines, on 13/0 or 15/0 circle hooks. Captured individuals were measured, sexed, tagged with mark-recapture tags, and sampled (<5 mm fin clip, <4 mm white muscle biopsy, <4 ml blood via caudal venipuncture) before release. Research effort over this period encompassed 80 total fishing days, distributed throughout the year with the greatest effort in the summer months.
Sampled individuals were carefully evaluated for species-specific characteristics, especially cephalofoil shape and relative size of the dorsal fin, to ensure identification due to the known risk of confusion with scalloped hammerhead S. lewini, especially at younger ages and smaller sizes (Barker, Frazier, Bethea, Gold, & Portnoy, 2017;Compagno, 2001).

| RESULTS
All great hammerheads captured at this site were < 200 cm in total length (TL), with a mean length of 133.4 cm TL and a median length of 131 cm TL (Table 1). Of five regularly sampled sites off the coast of Miami, this is the only site where great hammerheads <200 cm TL have been encountered. Larger subadult and adult individuals have been caught at other sites around Miami, and information on the reproductive status of mature individuals present in the area is currently under investigation. Individuals under 200 cm TL have been previously classified as sexually immature and under 5 years of age (Castro, 2010;Piercy et al., 2010). In addition to captured individuals, at least two additional hammerheads in this approximate size class have been observed from the research vessel swimming around the longlines in shallow, clear water. Although not included in this analysis, three scalloped hammerheads (S. lewini) in the same size class were also captured at the study site, and all were successfully visually distinguished from juvenile great hammerheads.
Species identifications were confirmed with genetic analysis (see supplementary materials for methodological details). Nine fin clips were analyzed and yielded highquality sequences for either COI or the control region. All BLAST searches to GenBank for samples identified in the field as great hammerheads found a 100% match to S. mokarran.

| DISCUSSION
While our findings remain preliminary, this data suggests we have identified a probable nursery site for great hammerhead sharks which demonstrably meets two of the three criteria for a nursery area, with the third supported by preliminary data and currently under investigation. Little is known about nursery areas for great hammerheads, although some have been tentatively identified on the Gulf coast of the state of Florida based on reported catches of juveniles, and larger juvenile individuals have been captured around the Florida Keys. Great hammerheads of this size (<200 cm) had never been confirmed in coastal Atlantic waters of the mainland United States until 2017, when one individual was identified in South Carolina as a result of genetic analysis (Barker et al., 2017;Hueter & Tyminski, 2007) (Figure 1).
Parturition for great hammerheads is thought to occur during the summer months (Piercy et al., 2010). In the present study, three individuals captured in January or February were < 100 cm TL. Given estimated sizes at birth (66-71 cm) and growth rates (40 cm  year) for great hammerheads, we infer that these individuals were likely YOY. A recaptured individual grew 25 cm (103 to 128 cm) in less than 5 months (146 days) at liberty between August 2019 and January 2020, suggesting the possibility that published growth rates may underestimate growth potential in this habitat (Figure 2). At this time, this site demonstrably meets two of the three criteria for a nursery area described by Heupel et al. (2007) which requires that (a) sharks be more commonly encountered there than elsewhere, (b) sharks remain in the area for extended periods, and (c) the area is used repeatedly across years (Figure 3). In reference to Criteria 1, we have caught no hammerheads in this size class (<200 cm TL) at any other fishing sites, exclusively encountering them at this site. Criteria 2 is supported by our recapture of one individual, but will be further explored in future passive acoustic tagging studies providing improved spatial use data at far higher resolution. This site definitively meets Criteria 3, as juveniles have been encountered there across all four seasons and over a more than 12-month period, from June 2018 to January 2020.
The discovery of probable great hammerhead nursery habitat in a highly anthropogenically impacted environment only miles from downtown Miami, Florida, suggests both the ability of this physiologically delicate species to adapt to survive human impacts and the potential for management improvement. The capture of three similarly sized juvenile scalloped hammerheads further supports arguments that this habitat may represent an important nursery for multiple shark species of conservation concern. Both commercial and recreational fishing are allowed in this area, and within the boundaries of nearby Biscayne National Park. Two of the juveniles captured in this study had hooks from previous recreational capture still embedded in their jaws, and researchers regularly encounter illegally unmarked trap lines at this site. Despite the controversy around limits on marine resource extraction (a 2015 effort to designate 6% of the park area a no-fishing zone failed), there is precedent for speciesspecific regulations associated with critical habitat in this region. For instance, spiny lobsters are protected from harvest pressure within the park-bounded Biscayne Bay/Card Sound Lobster Sanctuary.
Nursery area designation or identification of important habitat for juvenile sharks has occasionally driven conservation action, including designation critical habitat or establishment of marine protected areas (see Heupel et al. 2018). For threatened hammerhead species, state regulations outlawing harvest may not offer adequate protection from harmful sub-lethal effects or mortality associated with recreational fishing capture given their relative intolerance to capture stress. This suggests the establishment of habitat protections or reduction of fishing pressures for hammerheads or their vital prey species within nursery habitats may lead to improved conservation of great hammerheads. identification, conservation, and management of estuarine and marine nurseries for fish and invertebrates: A better understanding of the habitats that serve as nurseries for marine species and the factors that create site-specific variability in nursery quality will improve conservation and management of these areas. Bioscience, 51(8), 633-641.

SUPPORTING INFORMATION
Additional supporting information may be found online in the Supporting Information section at the end of this article.
How to cite this article: Macdonald C, Jerome J, Pankow C, et al. First identification of probable nursery habitat for critically endangered great hammerhead Sphyrna mokarran on the Atlantic Coast of the United States. Conservation Science and Practice. 2021;3:e418. https://doi.org/10.1111/ csp2.418