A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors

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.

Low mercury, cadmium and lead concentrations in tuna products from the eastern Pacific

“…The highest values found here is in canned tuna samples containing bigeye tuna, specie that made roughly 16 % of total catches in the same period. In a recent study, Houssard et al. (2019) in the central and western Pacific reported that bigeye had higher Hg content than in yellowfin and albacore, even sometimes above acceptable consuming limit.…”

“…In relation to the few samples with above limit Cadmium that were detected on skipjack products; it has recently been reported: 1) diet of the tuna is a driving factor on the type of element accumulation ( Houssard et al., 2019 ), 2) Skipjack is merely an epipelagic species like Yellowfin, which has been found to feed on small fish, squids, phytoplankton, etc. ( da Silva et al., 2019 ), 3) Phytoplankton up take Cadmium is similar to phosphate ( Morel et al., 2020 ), and 4) Cadmium has a nutrient like vertical distribution that resembles much the dissolved organic/inorganic phosphate ( Ormaza-González, 1990 ; de Baar et al., 1994 ) and correlate linearly with a high r 2 .…”

“…but also on the water column where they live ( Gworek et al., 2016 ). Recently, Houssard et al. (2019) have demonstrated that even though the fish size is the main explanation or reason for Hg content, oceanographic conditions like thermocline depth is an important fact that contributes, while the trophic level is of minor influence as well as the oceanic primary activity.…”

“… Kumar (2018) reported that geographical origin of the tuna rendered important Hg variations and suggested long-term monitoring. Recently, Kumar (2018) and Houssard et al. (2019) are reporting a dependency on the depth of thermocline (that affects the depth of the DOM); and not only that but also the depth of the main fish habitat; for example, mesopelagic fish (living deeper >300 m) tend to have up to 4 times higher Hg than the epipelagic fish (<200 m deep).…”

A representative sampling of tuna muscle for mercury control

“…Based on the indications reported in this study on the corrected sampling procedure for Hg determination in Tuna, the aim is to verify the different Hg distribution in the various carcass sites. In addition to ecosystem characteristics that contribute to the variability in Hg concentration in aquatic food webs, also correlated with species (Camilleri et al , 2018) and latitude (Houssard et al , 2019 ; Lavoie et al , 2013 ), there is a further factor related to the mechanisms of accumulation and distribution of Hg in fish tissues (Balshaw et al , 2007 ). It is known that Hg has a strong binding property to the sulfhydryl (SH) groups and that cysteine is by a large margin the most likely candidate as the predominant biological thiol, though it is likely part of a larger protein (Balshaw et al , 2008 ; Bradley et al , 2017 ; De Almeida Rodrigues et al , 2019 ; Harris et al , 2003 ; Itano and Sasaki, 1983 ; Kumar, 2017 ).…”