A Link Between Dolphins, Fishing Practices and Mercury Concentrations in Tuna

A Link Between Dolphins, Fishing Practices and Mercury Concentrations in Tuna

In the eastern Pacific Ocean, pods of dolphins routinely swim with large, mature yellowfin tuna. Some commercial fishing vessels exploit this relationship by setting purse-seine nets on dolphins to catch the large tuna swimming below. Since mercury concentrations in fish increase with the size and age of the fish, and tuna caught in dolphin ‘sets' are generally the oldest and largest tuna, it is reasonable to infer that this ‘dolphin-unsafe' fishing method results in tuna with higher mercury concentrations than tuna caught by other means.

Fishing Boat (C) NOAA

A study conducted by Defenders of Wildlife ("Is Our Tuna Family Safe: Mercury in America's Favorite Fish") found that concentrations of mercury in light canned tuna imported from Mexico and Ecuador were disproportionately higher than levels found in American and Asian brands. Mexico and Ecuador have documented histories of setting their fishing nets on dolphins to catch larger and more mature tuna. This may be one reason why Ecuadorian and Mexican tuna in the Defender's study were found to have higher levels of mercury than tuna from other countries and points to a possible correlation between fishing practices and mercury content that warrants further study.

Dolphin-safe and Dolphin-unsafe Fishing Practices

  • Setting nets, called purse-seines, on pods of dolphins to catch the tuna below has proven deadly for dolphins that often get caught in the nets and drown. Indeed, it is estimated that since the late 1950s, more than seven million dolphins have been killed in purse-seine nets.
  • In 1990, the United States banned American fishing vessels from setting nets on dolphins and instituted the use of the dolphin-safe label on tuna cans to allow consumers to make informed choices about how the tuna they buy is caught.
  • Currently, tuna fishing boats are required by the Inter-American Tropical Tuna Commission (IATTC) to submit weekly reports on the estimated number of dolphins harmed as tuna is caught and to post observers aboard tuna fishing vessels.

Fishing Net (C) NOAA

Ecuador and Mexico Fishing Practices

  • Ecuador and Mexico have the largest purse-seine fleets in the eastern Pacific according to the 2006 vessel registry of the IATTC.
  • Mexico and Ecuador also had the highest number of weeks at sea (International Dolphin Conservation Program 2002) and the highest amounts of purse-seine-caught tuna in the eastern Pacific Ocean in 2004 (IATTC 2005).

Ecuador and Mexico Tuna Fishing Violations

  • In 2004, Ecuador and Mexico submitted only 44 percent and 30 percent of their IATTC required weekly reports respectively and had observer reports for less than half of their weeks at sea.
  • In 2002, 13 Mexican vessels and three Ecuadorian vessels had violations of International Dolphin Conservation Program (IDCP) regulations.
  • In 2002, a Colombian vessel associated with an Ecuadorian tuna company was captured while fishing illegally for tuna in Galapagos National Park. The vessel had 70 dead or injured dolphins in its nets (Reuters 2002).

Mercury Levels in Ecuadorian and Mexican Tuna

  • On average, light tuna originating from Ecuadorian fishing fleets had the highest level of mercury, with an average concentration of 0.754 parts per million (ppm). This exceeds the average mercury concentrations for some fish on the FDA and EPA's short list of high-mercury fish.
  • Mexican tuna also had high average levels of mercury (0.310 ppm) significantly higher than the sampled U.S. light tuna (0.204 ppm) and more than twice the average mercury concentration for light tuna reported by the FDA (0.118 ppm).

These findings are based on independent research and the results of independent testing commissioned by Defenders of Wildlife.