Toxic Sharks

One weapon in the arsenal of those opposed to hunting sharks for flesh and fins is a food-safety argument - shark meat contains high levels of many dangerous environmental chemicals.  Indeed the significant pollution of the much of our water supply means that many aquatic food sources carry chemical pollutants from their environment.  Chemicals concentrate over time in fish, stored in fat and muscle and liver, and so longer-lived animals carry higher levels.  Toxins concentrate as they move up the food chain as well, with predators assuming the chemical burdens of their prey.  As some of the largest and longest-lived ocean predators, sharks are in a position to have the highest levels of poisons.

A new study has shown, in at least one species of shark, just how high those levels can be.  Burger et al analyzed 19 species of fish caught in coastal New Jersey waters for levels of methylmercury (MeHg), a chemical that causes significant human toxicity, and is particularly dangerous for the children’s developing brains.  They found the highest levels of MeHg in large predatory species - shortfin mako shark (Isurus oxyrinchus), Atlantic bluefin tuna (Thunnus thynnus), and striped bass (Morone saxatilis).  Levels of MeHg in mako reached 1.8 parts per million (ppm), while those in bluefin averaged around 0.5 ppm.  The US Environmental Protection Agency has stated the people should not eat fish with MeHg levels greater than 0.3 ppm -- mako shark MeHg levels were 6 times the recommended safe level for human consumption, and bluefin levels nearly twice the safe limit.

The toxicity of MeHg in the body is offset to some degree by levels of Selenium (Se), as these two chemicals compete for binding to key enzymes.  So fish carrying high levels of MeHg, which also have high levels of Se, can be considered (somewhat) less toxic.  Burger et al measured Se levels in their fish, and found higher than average levels of Se in bluefin tuna, which might help to offset the toxicity of this fish.  (Though consuming large amounts of tuna is still not recommended.)  Mako however had relatively low levels of Se, magnifying the dangers of eating this fish.

The levels of specific environmental contaminants are very region-specific, and MeHg in shark meat will vary with species and location.  Their position as apex predators means sharks will nearly always have higher body burdens of toxic chemicals, however, and MeHg is only one of many dangerous pollutants that accumulate in fish.  If the shark finners, and their customers, won’t stop taking sharks for the sake of sharks, perhaps they will consider doing so for the sake of their children?

The paper is: Burger, J, Jeitner, C, and Gochfeld, M. (2011) Locational differences in mercury and selenium levels in 19 Species of saltwater fish from New Jersey. Journal of Toxicology and Environmental Health A, 74:863-874.

It can be found at: