Citation: Shipley, O. N., Lee, C. S., Fisher, N. S., Sternlicht, J. K., Kattan, S., Staaterman, E. R., … & Gallagher, A. J. (2021). Metal concentrations in coastal sharks from The Bahamas with a focus on the Caribbean Reef shark. Scientific reports, 11(1), 1-11.
Did you know that people are more dangerous to sharks than sharks are to people? Many anthropogenic activities (activities done by people) over the last century have contributed to pollution of marine environments. Some of these activities, like industrialization, smelting, and fossil fuel combustion, have increased the amounts of metals entering the water. Some metals, like chromium, copper, and zinc, are required for cellular processes within living organisms to occur. However, many metals can be toxic when they enter living organisms at levels higher than needed for survival. Large predators at the top of the food chain, like sharks, often accumulate higher levels of metals in their tissues because they consume the metals from their prey, who have consumed the metals from their prey, and so on. This process is known as bioaccumulation.
So, why does it matter if sharks have high concentrations of metals in their tissues? Not only are sharks important for the marine ecosystem to function properly, people in many places around the world eat shark meat as a main part of their diet. When they eat the shark meat, they are consuming the metals that have accumulated in the shark’s tissue. Consumption of shark meat containing heavy metals can be toxic to humans and pose a risk of long term health impacts as the metals accumulate in the tissues of those eating the sharks. The Caribbean and South America are two places that shark meat is consumed by people.
Knowing that studies of metal accumulation in sharks have not been done in in the Caribbean before, a team of researchers from the United States set out to complete the first study in order to create a baseline of metal contamination in an effort to inform management plans to reduce the amount of metals entering the marine ecosystem. The authors focused on the Caribbean reef shark because of their high abundance, importance for the Caribbean marine ecosystem, and their high capture rate for consumption by humans. They also included blacknose sharks, bull sharks, tiger sharks, nurse sharks, and lemon sharks. They captured the sharks and collected tissue samples from each shark before releasing them. The tissue samples were then chemically analyzed for metals.
The researchers found that different metals accumulated in higher amounts in different species. The table below summarizes their findings of which shark species had the highest concentrations of concerning metals.
They also found that the concentrations of certain metals were positively or negatively correlated with each other. Different metals come from different anthropogenic activities, so the concentrations of each metal found in the sharks can help indicate the source of the pollution. The correlation between the metals can further help, since some metals are often found together in pollution from certain anthropogenic activities. The behavior of the sharks and the metals most prevalent in their tissues can also help indicate where the pollution is coming from. For example, Caribbean reef sharks are not as migratory as other shark species; they tend to stay in the same general area. Tiger sharks on the other hand are migratory and travel globally. These two species had different metals in their tissues. Caribbean reef sharks therefore are likely get mercury, lead, and arsenic from local activities, while tiger sharks are likely to get manganese and cobalt from pollution that occurs on a global scale. For example, lead and mercury are both found in high concentrations in Caribbean reef sharks and manganese, cobalt, and zinc are found in high concentrations in blacknose sharks.
The researchers also found different levels of metal in the sharks’ tissues at different stages of maturity, likely because their diets change throughout their lifespan. Generally, the levels of metals peaked in sharks as they reached sexual maturity. This could be because as the sharks are growing quickly and they are consuming more prey, including prey that they wouldn’t normally consume as an adult. As juveniles, sharks tend to stay in shallower, near-shore waters. As adults, they venture further out on the reef to deeper water. Other studies have found that higher ocean temperatures can lead to higher uptake of metals into the tissue of sharks. Since Caribbean reef sharks do not migrate and stay in the warmer waters found in the Caribbean, water temperature may also play a role in the higher metal levels found in these sharks.
Overall, the authors concluded that sharks in the Caribbean region, and likely globally, have potentially toxic levels of metals accumulating in their tissues. This represents not only a threat to marine ecosystem integrity, but also public health as people are starting to consume more shark meat. They also concluded that because of the wide variety of metals found within the sharks’ tissues and because of the differences between species, more research is needed to pinpoint the source of pollution so that management plans can be created to ensure the health of marine ecosystems and of people.