DOI: 10.5061/dryad.b8gtht7hc
title: Mercury isotope clocks predict coastal residency and migration timing of hammerhead sharks

description: 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.
description: 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<sup>−1</sup> dw (dry weight). Repeated measurements of a
 tuna flesh homogenate (BCR-464, Institute for Reference Materials and
 Measurements, 5.24 ± 0.10 μg·g<sup>−1</sup> 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<sup>−1</sup> dw. Another 20 mg muscle
 sample was then diluted into 3 mL of pure bi-distilled nitric acid
 (HNO<sub>3</sub>), 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
 (H<sub>2</sub>O<sub>2</sub>), 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<sup>−1</sup> using an inverse aqua regia (3:1
 HNO<sub>3</sub>: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
 <sup>202</sup>Hg 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).
description: Details of shark samples along with their Hg isotope values
 (Δ<sup>199</sup>Hg, δ<sup>202</sup>Hg,
 Δ<sup>200</sup>Hg, Δ<sup>201</sup>Hg,
 Δ<sup>204</sup>Hg) are presented in an EXCEL file
 (.csv).