Mercury Contamination in Billfish in the Tropical Eastern Pacific: a common feature in apex predators in the marine environment
Nelson Ehrhardt and Mark Fitchett, University of Miami Billfish Research Initiative and Central American Billfish Association (CABA)
The eastern tropical Pacific, which encompasses an area including oceanic ecosystems off the Central American Isthmus, is prone to naturally occurring mercury contamination and pollution from man-made sources. Volcanoes, which are quite prevalent in the region, are also major natural sources of gaseous and particulate mercury emissions that may lead to high concentrations in remote areas with major environmental implications (Witt et al., 2008). Scientists from Oxford University found that a single volcano in Nicaragua (Masaya) emitted 7.2 metric tons of mercury per year (which is more than total man-made emissions from most industrialized nations) with local mercury fluxes comparable to notoriously polluted cities such as Detroit and Tokyo (Witt et al, 2008). Transport of mercury emissions from volcanoes in Central America to the tropical eastern Pacific is facilitated by the strong prevailing winds from the Caribbean Sea to the Pacific through the lower passes in Mexico (Tehuantepec), Nicaragua, Costa Rica and Panama. Natural mercury sources are not only limited to volcanoes, but are found within the ocean basin as well. Some of the highest mercury emissions are found from hydrothermal vents along the eastern Pacific (Bostrom and Fisher, 1969) and have yielded high concentrations of mercury and other heavy metals in invertebrates, including vent clams off Mexico (Ruelas-Inzunza, 2003).
Mercury originating from industrial Asian sources have been shown to circulate to the eastern Pacific in approximately two years via biophysical processes and is then converted into the potent neurotoxic methylmercury form within prominent oxygen depleted conditions in the eastern Pacific (Sunderland and Strode, 2009).
While mercury concentrations in lower trophic levels may not have direct deleterious effects on biota within those trophic levels, mercury is fat soluble and accumulates dramatically in higher trophic levels up the food chain (Bloom, 1992). Consumption of apex predators (i.e. tunas, sharks, billfish, etc.) by humans provides a health risk due to exposure of such bioaccumulated heavy metals which have adverse health effects in the nervous, vascular, and renal systems.
Studies on mercury toxicity in dolphin tissues captured within the eastern tropical Pacific not only found that specimens captured between Mexico and Peru exhibited particularly high levels of mercury in tissues and organs – but those animals captured south of Mexico had significantly higher mercury concentrations in their tissues and organs (André et al., 1990), which is reason to believe other top predators in the waters south of Mexico also have equal or greater levels of mercury in their body tissues than those in higher Latitudes.
Scientific studies have found that billfish species in the eastern tropical Pacific Ocean contain high levels of mercury. A recent study carried out in Mexico has shown that there might be health risks in consuming sailfish and striped marlin (Soto-Jimenez et al, 2010). It has been prior noted that marlins have a particularly high tolerance for the heavy metal and have shown to have accumulated high levels in their tissues (Pelletier 1985).
Mexican scientists found that an average serving size of consumed sailfish per week places children, women, and even adult men at risk of health hazards due to mercury toxicity in levels beyond internationally recommended provisional tolerable weekly intakes (Soto-Jimenez et al, 2010). Mean mercury concentrations in sailfish and striped marlin far exceed any known regulatory limit of metals in fish and seafood (Soto-Jimenez, 2010).
These recent findings should not be of sole concern in Mexico, but within the Central American community that consumes sailfish and other billfishes and sharks. Sailfish have been shown to migrate regularly within and offshore coastal areas in search of prey species (Miyabe and Bayliff 1987 Figure 1; Ehrhardt and Fitchett, 2006 Figure 2) with a frequency to travel across international boundaries within the Central American and Mexican community at least once every 12 days (Prince et al, 2005, Figure 3). Because of this, it can be assumed that sailfish sampled in the Mexican study are very well within the same mixed stock of sailfish captured in fisheries within waters off the Central American Isthmus. Furthermore, as top trophic predators, such as billfishes, progress in size and age they bioaccumulate higher concentrations of mercury and other heavy metals in their organs and tissues. Data from Soto-Jimenez (2010) was used here to portray the bioaccumulation of mercury in larger sailfish (Fig. 4). Although the regression in figure 4 is not significant from a statistical stand point, there are higher risks associated with the consumption of larger individuals. This is reason for greater concern for those consuming sailfish captured off the coasts of Central America, Panama, and South America because animals in these regions are much larger in size relative to those sampled in Mexico by Soto-Jimenez (2010). Stripe marlin also exhibit transoceanic migrations seasonally and throughout their lifespan (Figure 5) and a specimen captured in Mexico is presumably within the same mixed stock of a stripe marlin population found throughout the eastern Pacific Ocean.
Therefore, insurmountable evidence shows that not only is the oceanic ecosystem off Central America susceptible to high levels of naturally-occurring mercury exposure and also anthropogenic mercury contamination, but top predators that reside in the region must naturally contain high levels of mercury in their organs and tissues by bioaccumulation. While billfishes exhibiting high levels of mercury were captured off Mexico, it is highly likely, due to migratory patterns, that billfish captured off Central America and Panama has equal or potentially greater concentrations of toxic mercury in their tissues and organs. This is particularly concerning for sailfish which migrate within coastal areas in the region as they progress in age and size and making them more available to coastal artisanal longliners that catch them and land them for human consumption. The recent findings by Mexican scientists on mercury concentrations in billfishes along with known anecdotal information on both mercury sources and migratory patterns of billfishes should be reason for immediate and responsible human health concern and elicit an urgent need to examine mercury concentrations in consumable billfishes captured regionally within Central American fisheries.
Figure 1. Spatial stratifications of size (age) groups of sailfish in the tropical eastern Pacific Ocean.
Figure 2. Dynamic spatial distributions of sailfish size population structures (data from Ehrhardt and Fitchett 2006)
Figure 5. Spatial distribution of stripe marlin life traits in the Pacific Ocean. (Miyabe and Bayliff 1987).
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This summary report was prepared for FECOPT, an institution in Costa Rica concerned with the potential risks of billfish consumption by humans.
9 May 2011