Heavy metal in SA sharks rocks the underwater world
Levels of metals such as mercury in great whites indicate the health of the ecosystem, with implications for humans
Great white sharks swimming off the SA coast have high concentrations of mercury, arsenic and lead in their blood.
The heavy metals are present in levels toxic to many animals but researchers found no apparent negative consequences on several health parameters measured in the sharks.
“The results suggest that sharks may have an inherent physiological protective mechanism that mitigates the harmful effects of heavy metal exposure,” said Liza Merly, lead author of a study published in the journal Marine Pollution Bulletin.
Co-author Neil Hammerschlag said: “As top predators, sharks bio-accumulate toxins in their tissues via the food web from the prey they eat.
“By measuring concentrations of toxins, such as mercury and arsenic, in the blood of white sharks, they can act as ‘ecosystem indicators’ for the health of the ecosystem, with implications for humans.
“Basically, if the sharks have high levels of toxins in their tissues, it is likely that species they eat below them will also have toxins, including fishes that humans eat.”
For the study, 43 great whites were captured and sampled off SA during a 2012 expedition by the non-profit ocean research organisation Ocearch. The expedition was the subject of a National Geographic documentary, Shark Wranglers.
“To collect the samples, white sharks were carefully raised on a specialised platform, while blood samples and body measurements were taken by biologists before the sharks were tagged and released,” said expedition leader Chris Fischer, founding chairman of Ocearch.
The sharks’ blood was screened for concentrations of 12 trace elements and 14 heavy metals, providing the first published account of blood concentrations of heavy metals in wild sharks.
A statement from the University of Miami, where Merly and Hammerschlag work in the department of marine ecosystems and society, said: “The data is instrumental in creating a baseline and reference for levels of heavy metals present in the blood of white sharks in South Africa.
“Considering many populations of large sharks are experiencing declines across the globe, it is important to understand the impact of toxic metals, if any, in this population.
“The possibility that white sharks could have a physiological mechanism that protects them from the harmful effects of metal exposure offers new opportunities for future shark research.”
• Some of the sharks tagged by Ocearch in 2012 still feature in its online shark tracker.
• South Africans involved in the new study include: Lucia Lange and Johann Muller from PathCare VetLab in Cape Town; Michael Meyer from the department of environmental affairs; Michael Hewitt from the University of Cape Town biology department; Pieter Koen from the Western Cape agriculture department’s veterinary services; and Volker Schilack and Mauritz Wentzel from V&M Analytical Toxicology Laboratory Services in George.