By far the most read blog post I had last year was the one on the relationship of mercury in Yellowfin tuna with location of capture. It seems that more people is worried about eating them than around their sustainability, legality or the welfare of those that catch it… which is saying something!
Figure 1. Observed spatial variation in mercury concentrations (mg kg−1, dry weight) for bigeye, yellowfin, and albacore muscle samples captured in the Western and Central Pacific Ocean
In that post, based on an article I wrote for the SPC Fisheries news, my colleague Valerie Allain (who is a admirably smart colleague doing a lot of amazing work) advanced the news from a paper by a group she is part off, had a much bigger sample size and was focused on the Pacific.
That paper is out, and the findings are very interesting. I hope they put at ease the worries that many people have about the potential consequences on their health by eating a can of tuna or the occasional sashimi or tuna steak.
I really like that the scope of the paper went beyond of the usual size and explored the Influence of Physiology, Ecology and Environmental Factors on the level in tuna
As usual, I’ll quote the abstract, introduction and part of the discussion (Hg Levels and Spatial Distribution), but please read the original paper, is always better!. (Unfortunately, is paywall protected, so unless you work in rich country and your employer has access to subscriptions or universities, I suggest you contact the authors or use legally grey alternatives such as this one)
Abstract
Information on ocean scale drivers of methylmercury levels and variability in tuna is scarce, yet crucial in the context of anthropogenic mercury (Hg) inputs and potential threats to human health. Here we assess Hg concentrations in three commercial tuna species (bigeye, yellowfin, and albacore, n = 1000) from the Western and Central Pacific Ocean (WCPO). Models were developed to map regional Hg variance and understand the main drivers. Mercury concentrations are enriched in southern latitudes (10°S−20°S) relative to the equator (0°− 10°S) for each species, with bigeye exhibiting the strongest spatial gradients.
Fish size is the primary factor explaining Hg variance, but physical oceanography also contributes, with higher Hg concentrations in regions exhibiting deeper thermoclines. Tuna trophic position and oceanic primary productivity were of weaker importance.
Predictive models perform well in the Central Equatorial Pacific and Hawaii, but underestimate Hg concentrations in the Eastern Pacific.
A literature review from the global ocean indicates that size tends to govern tuna Hg concentrations, however regional information on vertical habitats, methylmercury production, and/or Hg inputs are needed to understand Hg distribution at a broader scale.
Finally, this study establishes a geographical context of Hg levels to weigh the risks and benefits of tuna consumption in the WCPO.
Introduction
Mercury (Hg) is a global contaminant of primary concern to human and ecosystem health. These concerns stem from the physiochemical properties of the different chemical forms of Hg in the environment. Inorganic mercury deposited at the surface of the ocean represents a combination of natural geogenic (volcanic) and anthropogenic (fossil fuel combustion, artisanal gold mining) inorganic Hg sources emitted initially to the atmosphere.
Once deposited in the ocean, a yet unknown fraction of inorganic Hg is naturally converted to the highly persistent organometallic toxin methylmercury (MeHg). The balance between microbial methylation processes and demethylation reactions, dominated by photodegradation in the ocean water column, controls net methylmercury production and availability for the marine food webs.
Depth profiles in the open ocean consistently show that in situ MeHg production is highest in the subthermocline oxygen minimum zone. Further, differences in net MeHg production have been documented across the global ocean1 suggesting that MeHg availability and transfer pathways in marine food webs may also vary spatially.
Human exposure to MeHg occurs primarily through consumption of marine fish species. As MeHg is the dominant form of Hg that biomagnifies in food webs, open ocean pelagic predators (e.g., tuna and billfishes) contain high levels of MeHg, sometimes exceeding food safety guide-lines (e.g., 1 mg·kg−1 of fresh tissue).
Methylmercury in situ net production and availability, food web dynamics, and individual dietary intake and excretion rates are primary drivers of MeHg concentrations in fish tissue. MeHg concentrations typically increase with fish age and size because MeHg is taken-up at a faster rate than it is excreted. Due to the bio accumulative nature of MeHg, an organism’s trophic position can also positively correlate with MeHg concentrations in fish tissues, particularly when a large range of trophic positions are considered.
Species-specific Hg concentration patterns, documented among tuna and other pelagic fish predators, show increased Hg levels with deeper habitats. Increasing Hg levels in deep-dwelling tuna could likely be related to their foraging behavior, as they feed heavily on mesopelagic prey from deep oceanic zones where much of the MeHg is produced. Ferriss and Essington used contaminant models to determine if food chain length and/or baseline Hg variations could explain differences in MeHg concentrations in tuna from two ocean regions in the Central and Eastern Equatorial Pacific.
Data limitations impeded the ability to validate the regional food web models and thus to address the question of the relative importance of these two causal effects. Other studies also reported significant variability of MeHg concentrations in yellowfin tuna between and within ocean basins, but the underlying causes were not investigated.
The main objective of this study is to improve our understanding of regional patterns and drivers of Hg concentrations in three commercial tuna species (bigeye, Thunnus obesus; yellowfin, T. albacares; and albacore, T. alalunga) across the Western and Central Pacific Ocean (WCPO).
The WCPO is a remote oceanic region, exhibiting strong geographical gradients in surface temperature, water column stratification and primary production. These gradients were previously shown to influence tuna distribution, foraging behavior, and trophic position in this region, and as such are likely candidates to affect Hg concentrations. Sub objectives include (i) model Hg at size to standardize the effect of fish size on Hg levels, and (ii) develop generalized additive models to map regional patterns in tuna Hg distributions and identify explanatory covariates among environmental factors (temperature gradient, primary production) and ecological drivers (trophic position).
Finally, an extensive literature review of Hg concentrations in bigeye, yellowfin, and albacore tuna is conducted to discuss broad-scale patterns from the global ocean in comparison to those seen in the WCPO.
Results and Discussion
Mercury Levels and Spatial Distribution.
Mercury concentrations in our tuna muscle samples from the WCPO are relatively higher for bigeye and albacore compared to yellowfin. Similar differences in Hg concentrations between these tuna species were reported in the Northern Central Pacific Ocean and were attributed to their foraging depth.
Tuna species foraging at depth (bigeye, albacore), below the mixed layer and closer to where MeHg net production is the highest, exhibiting higher Hg concentrations than surface foraging predators (yellowfin). About 11%, 1.2%, and 0.5% of the sample data set for bigeye, albacore, and yellowfin in the WCPO region show Hg concentrations exceeding the food safety guidelines of 1 mg· kg−1 (ww) (estimated to 3.3 mg·kg−1 when expressed on a dry weight basis).
Mercury concentrations among the three tuna species are highly variable at the subregional level. Bigeye show he largest spatial variation in Hg concentrations
The highest yellowfin Hg concentrations (∼0.7 mg·kg−1) are encountered south of 15°S, in the southern part of ARCHm and the eastern part of SPSGm, respectively.
Yellowfin from the Western Equatorial Pacific regions (WARMm and PEQD) show the lowest Hg concentrations (∼0.35 mg·kg−1).Finally, despite albacore occupying a more southern subtropical habitat relative to bigeye and yellowfin, a southward increase in Hg is evident. Albacore samples located between 15°S and 25°S exhibit higher Hg concentrations (∼1.2 mg·kg−1), relative to those north of 15°S(∼0.6 mg·kg−1) and those from much southern latitudes (30°S-50°S) in Australia/Tasmania and New Zealand areas (∼0.6 mg·kg−1).
Tuna size appears as the primary factor influencing Hg concentrations within species, accounting for 77% and 42% of the total muscle Hg variations for bigeye and yellowfin respectively when fitted with a power-law relationship. Increasing Hg concentration with fish size indicates that Hg intake exceeds Hg excretion rates over an individual lifetime.
Fish size explains only 34% of albacore Hg variation, but the restricted size range (42−108cm) available in the data set for this species provided less contrast to assess this relationship compared to bigeye and yellowfin. These results suggest that fish size strongly governs Hg concentrations in bigeye, and to a lesser extent for yellowfin and albacore.
Mercury concentrations at the most southern latitudes in Australia/Tasmania and New Zealand areas are lower than those reported in the southern part of ARCHm. The consistency in these size-standardized geographical patterns suggests that spatially driven ecological processes, and/or local Hg sources, MeHg production and/or environmental factors may be important, additional contributing drivers.
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This paper is good news, as gives us in the Pacific the chance to further sustain something we knew, our Yellowfin has very low levels of Hg, but most importantly, could argue for the reduction in the frequency of sampling and therefore the costs for the battled seafood safety Competent Authorities in the region.