Species-specific mercury speciation in billfishes and its

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this knowledge gap. We found low percentages of methylmercury in blue and black marlins (15 ± 7 %)
Fish consumption recommendations compared to swordfish and striped marlin (89 ± 13 %), with no significant differences among ocean regions.
ylmercury (MeHg). In oceans, inorganic Hg can be microbially con generally represent the major chemical form (> 90 %) of total Hg in
proportions (%MeHg) and concentrations in higher-trophic-level Crear, 1976), a billfish commonly consumed in tropical regions,
pregnancy have been associated to negative neurodevelopmental out 2022). In this context, quantifying MeHg concentrations and %MeHg in
for MeHg was first set at 3.3 µg/kg bw by the Joint FAO/WHO Expert (M. indica). We hypothesised that some billfish species – particularly
assumption is now challenged by low %MeHg reported in blue marlin in (Table 1). All individuals were measured to the lowest cm (lower jaw
Hawaii (15 ± 10 %; Shultz and Crear, 1976) and French Polynesia (19 fork length, i.e., from the tip of the lower jaw to the fork of the tail), with
± 22 %; Manceau et al., 2021a). These low %MeHg have been suggested individual sizes per species and ocean region reported in Table 1. For all
to be relatively stable and either not bioavailable, or exhibiting little to position, gold amalgamation, and atomic absorption (DMA-80, Mile
centrations therefore overestimates human exposure to MeHg associated 100 ± 3 % (n = 35) among the different analytical batches. The preci
As pelagic fishes, billfish species display relatively high levels of of a 1 % sodium tetraethylborate (NaBEt4) solution were added to all
centrations (µg.g− 1, ww), and percentage of methylmercury (%MeHg, %) in billfish muscle samples. n gives the number of analysed individuals.
different analytical batches. ± 1 %, respectively. For billfish samples from the western Pacific Ocean,
terials were converted on a ww basis, using a mean water content of 75 screw-cap vials (Savillex, Minnetonka, MN, USA) with 1 mL 65 % nitric
% (Table 1), to be compared to safety guidelines. Percentage of MeHg (% acid (Suprapur, Merck) and 0.25 mL 30 % hydrogen peroxide (Suprapur,
trations, and percentage of inorganic Hg (%iHg) was the difference into 15-mL polypropylene pre-washed tubes. Total Se concentrations
between 100 % and %MeHg. Molar concentrations of total Hg and MeHg were determined at LEMAR and PSO (Plouzané, France) on a sector field
tween molar concentrations of total Hg and MeHg. g− 1 dw) showed an average recovery of 95 ± 2 % (n = 5) and 96 ± 11 %
Except for blue marlins from Mauritius, homogenized freeze-dried converted on a ww basis considering a mean water content of 75 %.
Berlin, USA) to determine the total lipid content expressed in % of dry (µg.g− 1, ww), and BW the consumer body weights, set at 30 ± 20 kg for
2.3. Statistical analyses 1.3 µg/kg bw, i.e., the most restrictive PTWI value based on the latest
software 3.6.1 (R Core Team, 2018). Total Hg and MeHg concentrations We then compared these amounts of fish Q to a mean fish portion to
statistical analyses (Zuur et al., 2010). Analysis of Covariance We considered mean fish portions of 150 g and 75 g for adults and
regions as no regional differences were found in our data (Kruskal- Table 1). Highest total Hg concentrations were measured in blue and
Wallis, p > 0.05 for the two species). black marlins from New Caledonia, with mean values four to nine times
For all billfish individuals, we calculated the quantity of fish (Q, g) at different regions of the global ocean (SI Appendix Table S1).
Hg in fish species through the probable binding to proteins in muscle (1.38 µg.g− 1 ww) in a swordfish from Seychelles (Table 1). Methyl
of total Hg concentrations in our dataset, suggesting no inter-species and global ocean (SI Appendix Table S1). Variability of MeHg concentrations
explanatory variables), with swordfish (89 ± 14 %, all regions com species that do (e.g., blue and black marlin) and do not (e.g., swordfish
bined) and striped marlin (90 ± 8 %) exhibiting significantly higher % and striped marlin) demethylate MeHg would be needed to better un
MeHg than blue (12 ± 7 %) and black (15 ± 5 %) marlins (Table 1; derstand possible in vivo MeHg detoxification pathways in these top
Fig. 3C). Low %MeHg in blue marlin is similar to values previously re marine predators. Among Se forms, selenoneine in particular would
ported in this species from French Polynesia (19 ± 22 %; Manceau et al., deserve being quantified in billfish muscle as it is suspected to be
1. and Hawaii (mean %MeHg values between 13 and 29 %; Rivers responsible for MeHg demethylation and excretion in marine organisms
%; Yamashita et al., 2005). Percentage of MeHg values measured in our 4. Toxicological assessment of billfish consumption
in the literature (68 ± 13 %; Kacprzak and Chvojka, 1976). High % About 13 % of our billfish samples (6 over 45) displayed MeHg
Yamashita et al., 2005) than the ones available in the literature from This number rises up to 50 % (23 over 45) if we compare total Hg
different ocean regions (SI Appendix Table S1). Low %MeHg in blue and concentrations to the MeHg guideline value (Fig. 3A), as commonly
and Hawaii in which %MeHg were found to decrease with increasing Jiménez et al., 2010). This relies on different species-specific MeHg
concentrations in billfish muscle tissues (Fig. 4). There was no signifi and considering mean fish portions of 150 g and 75 g for adults and
consumed occasionally (i.e., less than once a week) by children (52 % of are rarely reported in Hg exposure assessments (e.g., ANSES, 2019; Bille

Methods (brief)

on a large number of samples to cover a broad range of fish sizes. This study calls for additional characterization
1.3 µg of MeHg/kg bw (EFSA, 2012). 2.1. Billfish sample collection
MeHg content, potential risk assessments associated to pelagic predator A total of 45 billfish samples were collected onboard pelagic semi-
atory fishes (Bille et al., 2020; Soto-Jiménez et al., 2010). Compared to Ocean, it corresponds to eight swordfish collected in the Seychelles ar
Minamata Convention (AMAP, Un Environment, 2019). Yet, this general marlins, were collected in the vicinity of New Caledonia in 2021
to result from in vivo MeHg demethylation in blue marlin muscle tissue billfishes, a white muscle sample of around 10 g ww was collected in the
giant petrel seabirds (Manceau et al., 2021b). Contrary to MeHg forms Homogenized freeze-dried billfish muscle samples were analysed for
to be relatively stable and either not bioavailable, or exhibiting little to position, gold amalgamation, and atomic absorption (DMA-80, Mile
Hg-Se complexes. Using total Hg concentrations instead of MeHg con routinely used every 5–10 samples, displaying an average recovery of
advices than necessary. Such biased dietary advices could be detri sample.
mental to the health and wellbeing of food insecure coastal populations To determine MeHg concentrations, 100 ± 10 mg of muscle samples
Western-style diet, while remaining highly recommended to fight non- metal grade, HNO3) at 85 ◦ C on a hot plate during 8 h. Sample aliquots
proteins, omega-3 fatty acids, vitamins, and minerals, and represent a samples before purge and trap gas chromatography atomic fluorescence
Fig. 1. Sample location of swordfish (orange), striped marlin (red), blue marlin (blue), and black marlin (dark grey) in the western Indian and southwestern Pacific
Characteristics of billfish samples, concentrations of total mercury, methylmercury and total selenium. Mean ± standard deviation (minimum − maximum) of
centrations (µg.g− 1, ww), and percentage of methylmercury (%MeHg, %) in billfish muscle samples. n gives the number of analysed individuals.
tissues were analysed in triplicate, with analytical blanks and diluted Vista-Pro ICP-OES at the LIENSs facility (La Rochelle, France) (Bodin
CRM standard solution included every three samples to check MeHg et al., 2017). The two CRMs TORT-3 (lobster hepatopancreas, total Se =

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The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git. When DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.

Commit	Date	Change	Description
b01ec52c	2026-08-04	major	2 sections added
d49e450f	2026-08-03	major	5 sections added; narrative text revised