1. The management of migratory taxa relies on the knowledge of their movements. Among them, ontogenetic habitat shift, from nurseries to adult habitats, is a behavioral trait shared across marine taxa allowing resource partitioning between life stages and reducing predation risk. As this movement is consistent over time, characterizing its timing is critical to implement efficient management plans, notably in coastal areas to mitigate the impact of fisheries on juvenile stocks.

2. In the Mexican Pacific, habitat use of the smooth hammerhead shark (Sphyrna zygaena) is poorly described, while the species is heavily harvested. Given the large uncertainties associated with the timing of out-migration from coastal nursery grounds to offshore waters prior to reproductive maturity, a more precise assessment of smooth hammerhead shark movements is needed. 

3. Photochemical degradation of mercury imparts mass-independent isotope fractionation (Δ¹⁹⁹Hg) which can be used to discriminate between neonate coastal shallow habitats and the offshore deep foraging patterns of late juveniles. Here, we present the application of muscle Δ¹⁹⁹Hg as molecular clocks to predict the timing of ontogenetic habitat shifts by smooth hammerhead sharks, based on their isotopic compositions at the initial and arrival habitats and on muscle isotopic turnover rate.

4. We observed decreases in Δ¹⁹⁹Hg values with shark body length, reflecting increasing reliance on offshore mesopelagic prey with age. Coastal residency estimates indicated that smooth hammerhead sharks utilize coastal resources for two years prior to offshore migration, suggesting a prolonged residency in these ecosystems.

Policy implications: This study demonstrates how mercury stable isotopes and isotopic clocks can be implemented as a complementary tool for stock management by predicting the timing of animal migration—a key aspect in the conservation of marine taxa. In the Mexican Pacific, fishing pressure on shark species occurs in coastal habitats depleting juvenile stocks. Consequently, management decision support tools are imperative for effectively maintaining early life stage population levels over time. The finding that smooth hammerhead sharks extensively rely on highly fished habitats for two years after parturition supports the relevance of establishing a size limit in coastal fisheries and demonstrates how the current temporal shark fishing closure could lack efficiency for the species.

Sharks were collected along the western coast of Baja California Sur (Mexico). Juvenile smooth hammerhead sharks were caught by gillnets and longlines in 2009 and from 2014 to 2018. Sharks (n=102) were sampled at four locations in artisanal fishing camps: Bahía Tortugas, Las Barrancas, San Lázaro and Punta Lobos. For each individual, approximately 1 g of dorsal white muscle tissue was sampled. Samples were transported on ice, stored at -20°C at the laboratory and ultimately freeze-dried prior to transport and further treatments.

Total Hg concentration (THg) was determined in a 20 mg aliquot of each sample using a Direct Mercury Analyzer (DMA-80, Milestone, USA), by combustion, gold trapping and atomic absorption spectrophotometry detection (Observatoire Midi-Pyrénées, Toulouse, France). THg analytical detection limit was 0.005 μg·g⁻¹ dw (dry weight). Repeated measurements of a tuna flesh homogenate (BCR-464, Institute for Reference Materials and Measurements, 5.24 ± 0.10 μg·g⁻¹ dw) tested the analysis's reproducibility and accuracy. BCR-464 measurements (n=9) were reproduced within the confidence limits: 5.30 ± 0.44 μg·g⁻¹ dw.

Another 20 mg muscle sample was then diluted into 3 mL of pure bi-distilled nitric acid (HNO₃), left overnight at room temperature and digested at 100°C for 6 hours in pyrolyzed glass vessels closed by Teflon caps on a hotplate. We added 1 mL of hydrogen peroxide (H₂O₂), left the digestion to continue for another 6 hours and completed the extraction with 100 µL of bromine monochloride (BrCl) to convert solubilized MeHg into inorganic Hg. Samples were diluted to THg concentration of 1 ng·mL⁻¹ using an inverse aqua regia (3:1 HNO₃:HCl with 20 vol.% of MilliQ water). Hg isotope composition was measured at the Observatoire Midi-Pyrénées (Toulouse, France) by inductively coupled plasma mass spectrometry (MC−ICP−MS, Thermo Finnigan Neptune) with continuous-flow cold vapor (CV) generation using Sn(II) reduction (CETAC HGX-200). Mass-dependent fractionation is expressed in δ notation (‰). Sample values are expressed relative to their deviation from the National Institute of Standards and Technology (NIST) SRM-3133 standard and calculated by sample-standard bracketing. Along the procedure, THg in the solution was monitored by the ²⁰²Hg signal provided by MC-ICP-MS. We measured blanks and BCR-464-certified materials following the same procedure as for shark samples. We recovered 84 ± 19% of THg in shark samples and 90 ± 2% in BCR-464 replicates (n=10). Isotopic measurement reproducibility was assessed by analyzing UM-Almadén (n=8), ETH-Fluka (n=8) and the biological tissue procedural standards BCR-464 (n=10).

Details of shark samples along with their Hg isotope values (Δ¹⁹⁹Hg, δ²⁰²Hg, Δ²⁰⁰Hg, Δ²⁰¹Hg, Δ²⁰⁴Hg) are presented in an EXCEL file (.csv).