Mercury (Hg) is an element that can be found naturally in the environment around the world. This element exists in both organic and inorganic forms in the environment. The sources of Hg vary because it is a naturally occurring element and therefore can enter the environment naturally or from either anthropogenic sources. The most prevalent form of organic mercury is methylmercury (MeHg) (Agency of Toxic Substances and Disease Registry [ATSDR],1999) which is also regarded as the most toxic form of Hg (Olson, 2017).
Most human exposures to MeHg derive from the consumption of contaminated seafood, such as fish, shellfish, crustaceans, etc. Bacteria, primarily present in the sediment, convert elemental mercury into methylmercury through a process called methylation (UNEP, 2013). Once MeHg is created in the sediments, it moves via diffusion back into the overlying waters (François M. M. Morel, Anne M. L. Kraepiel, & Amyot, M., 1998). Micro-organisms (like phytoplankton) absorb methylmercury into their bodies via seawater from passive diffusion and dietary uptake (Schartup et al.
, 2018). The micro-organisms will eventually be consumed by larger predators and the metal will slowly bioaccumulate into the tissue of larger predatory creatures that are higher in the food chain over time (François M. M. Morel, Anne M. L. Kraepiel, & Amyot, M., 1998; Schartup et al., 2018). Consumption of such Hg-contaminated food can result in mercury poisoning.
A prominent case of methylmercury poisoning occurred in the early 1950s and affected residents living around Minamata Bay on Kyushu Island, Japan. The consumption of contaminated seafood captured in Minamata Bay resulted in at least 2,264 patients with Minamata disease (MD) as of the year 2000, but as many as 200,000 people are suspected to have MD (Ekino, Susa, Ninomiya, Imamura, & Kitamura, 2007; Ninomiya et al.
, 2005; Tomiyasu et al., 2000). Health effects associated with acute mercury poisoning include decreased functionality inability to hear, taste, and smell, blurred vision, uncoordinated movements, and muscle weakness, (Ekinoa, Susab, Ninomiyaa, Imamuraa, & Kitamura, 2007). Even low doses of methylmercury over time can lead to cardiovascular effects, compromised motor functions, memory loss, and risk of neurodegenerative diseases (Zahir, Rizwi, Haq, & Khan, 2005).
San Diego Bay has a long and rich history. During the 1800s, the Bay contained extensive salt marshes, mudflats, and open waters as well as immense biodiversity of wildlife. (Environmental Health Coalition, 2011). However, in the early 1900s, San Diego Bay was dredged to create man-made channels and its mudflats and salt marshes were filled (Environmental Health Coalition, 2011). Over the years, the health of the Bay deteriorated from neglect, disposal of sewage, accidental spills, illegal dumping, and inadequate enforcement. In 1963 sewage disposal in San Diego ended. However, by 1987, the National Oceanic and Atmospheric Administration (NOAA) ranked San Diego Bay the sixth most polluted bay in the nation out of 50 bays across America (Environmental Health Coalition, 2011). The pollution can be attributed largely to the natural deposition of atmospheric mercury and, secondarily also from hazardous waste discharge from industrial facilities.
Hg is continuously being recycled to and from the atmosphere. Hg is introduced into the environment naturally and anthropogenically. Hg can be aerosolized naturally from processes like volcanism, hot springs, fracturing, and other disturbances to the Earth’s surface, which is naturally embedded with Hg (Gavis and Ferguson 1972; Jonasson and Boyle 1972). Unfortunately, Hg can also be emitted from anthropogenic sources like the combustion of fossil fuels (coal) or solid waste incineration (Pacyna and Keeler 1995; Petersen et al. 1995; Selin, 2009). Once produced, Hg enters the atmosphere and will return to the Earth’s surface via precipitation (Pirrone et al.1995; (UNEP,2013). Additionally, between the early 1900s to 2012, the City of San Diego, the National Steel and Shipbuilding Company Shipyard facility (NASSCO), San Diego Marine Construction Company,1 Campbell Industries (Campbell), the BAE Systems San Diego Ship Repair Facility (BAE Systems), San Diego Gas and Electric (SDG&E), the United States Navy, and the San Diego Unified Port District (Port District) have permitted or contributed to the discharge of waste to the Shipyard contaminated sediment sites resulting in the accumulation of pollutants in the bay sediment. Most discharges contained mercury as well as nickel, copper, silver, arsenic, polychlorinated biphenyls (PCBs), polynuclear aromatic hydrocarbons (PAHs), and more contaminants (CRWQCB, 2018).
A study conducted in 1990, showed that fish in San Diego Bay contained elevated levels of mercury (as well as arsenic and polychlorinated biphenyls) (San Diego County Department of Environmental Health, 1990). Despite this, in a survey conducted in 2005, many fishermen (61%) reported that they continue to consume fish caught from the Bay (Environmental Health Coalition, 2005). In a recent fish consumption advisory notice from the OEHHA (2018), they have advised that fifteen species of fish from the San Diego Bay should be consumed with strict limits and actions. Of the fifteen species, the leopard shark contained the highest level of mercury (1018 ppb) and the diamond turbot contained the least amount of mercury (22 ppb) (OEHHA,2018). In July of 2018, OEHHA issued an updated fish advisory for San Diego Bay based on levels of both mercury and PCBs in fish. Based on mercury levels, the advisory cautioned “When consuming fish from the San Diego Bay, women ages 18-45 and children ages 1- 17 should not eat barred sand bass, Pacific chub mackerel, sharks, shiner perch, spotted sand bass, top smelt, or yellowfin croaker.”
It is believed that low-income individuals, as well as homeless populations around the Bay, are dependent upon the fish and shellfish caught from San Diego Bay as their primary food source (Chad Loflin, personal communication, May 26, 2017). Therefore, it is increasingly important to consider the possible health risks that could affect these populations that may be consuming Hg-contaminated fish and shellfish from San Diego Bay.
In recent years, the Pacific oyster (Crassostrea Gigas), was introduced into San Diego Bay during the late 1990s (Crooks, 1998). These oysters originated from Asia but have successfully invaded North America, South America, Europe, Africa, and Australasia (Ruesink et al. 2005). They are typically found adhered to rocks in shallow subtidal areas with depths no greater than 15 meters (National Introduced Marine Pest Information System (NIMPIS), 2002). C. gigas are filter feeders, indicating that they ingest protozoa, bacteria, detritus, larvae of other invertebrate animals, and a variety of diatoms that are present in their environment (Nehring, 2011). Currently, there hasn’t been official documentation of San Diegans harvesting and consuming these oysters from San Diego Bay, but there is evidence of scarring, this is an indication of oysters being harvested (Chad Loflin, personal communication, July 31, 2018). Although several monitoring programs; such as the San Diego Bay Regional Harbor Monitoring Program (Port of San Diego) (San Diego Water Board (SDRWB, 2017), Bight Regional and Regional Harbor Monitoring Programs (SCCWRP) (SDRWB, 2017), and City of San Diego Shallow Water Habitat Survey (SDRWB, 2017), have documented the levels of mercury in fish San Diego Bay and the associated health risk from such fish consumption, there is a dearth of studies conducted on shellfish consumption, especially on oysters. Part of the reason for this is because these oysters are not native to the San Diego Bay, and invaded only recently. However, the current utilization of San Diego Bay for recreational and subsistence harvesting of shellfish from San Diego Bay, in combination with a lack of data regarding levels of Hg in the Pacific oyster from the Bay, has created a pressing need for information on both the levels of mercury in these shellfish and the associated risk for consumption.