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Sharkbites Saturday

Do heavy metals like mercury and arsenic impact the health of great white sharks?

“Great White Shark” by Elias Levy is licensed under CC BY 2.0

Sharks are considered meso or apex predators, meaning they have a high ranking in the food chain and therefore do not have many natural predators. However, being in the upper levels of the food web means that substances like toxins released into the environment by humans can bio-magnify, meaning over each step in the food chain (plankton to small fish, small fish to bigger fish, etc) toxins start adding up (figure 1). Unfortunately for sharks who are at the top of the food chain, they can become victim to this magnification and store large amounts of toxins like heavy metals in their tissues and organs. This process has been documented in many large marine fish species such as sharks, tunas, and swordfish for a wide variety of toxins, notably heavy metals like mercury, where in many cases warnings are issued to reduce consumption by humans.

A recent study from Merely et al. 2019 set out to study these accumulating toxins in one of the largest predatory shark, the great white shark, by determining the base levels of these toxins as well as assessing if the levels of the 14 tested heavy metals related to each other, size, and sex of the sharks sampled. The authors aimed to answer questions like do larger sharks have higher levels circulating in their blood than small sharks? Do the measured health indicators change in relation to blood levels of these heavy metals? And how do the amounts of heavy metals measured in this study relate to other fish?

Figure 1. An illustration of bio-accumulation, the build up of toxins within one species over time, verses bio-magnification, where toxins build up through the food web.
Image courtesy of www.blue-growth.org

To answer these and more questions, scientists collected blood and body measurements from 43 white sharks at various locations throughout South Africa. The blood was analyzed for a variety of health metrics including the amount and type of white blood cells, a key part of the immune system, as well as other blood plasma ions typically tested in a routine blood exam we get at the doctor’s office. Most importantly, the blood samples were analyzed for 14 heavy metals including commonly concerning metals to human health such as mercury and lead, as well as the micro-nutrient metals of selenium, manganese, iron, and zinc that could defend against the negative effects of other metals.

The outcomes of this study were extremely interesting: white sharks have very high levels of many heavy metals compared to other vertebrates; however they don’t seem affected by it, at least not by the health standards measured in this study. For comparison, the levels of mercury and arsenic circulating in the blood of captured white sharks were above the threshold of toxicity for humans. Other metals though, such as lead, were below human toxicity standards. There was no difference in heavy metal concentrations when comparing male and female sharks.

Image courtesy of www.medicalnewstoday.com

Even more surprisingly, body size was not related to heavy metal concentrations either, where we would expect that high heavy metal loads would decrease the health and therefore size of the animal. This was unexpected, as typically larger sharks have been consuming more prey over their longer lifetime, meaning they bioaccumulate more metals (figure 1). Not only were there no negative effects found, but copper concentrations and body size were positively correlated, meaning the more copper a shark had circulating in its blood, the better the body condition of the shark. The authors hypothesized that copper may be protecting against oxidative stress, a type of stress where free radical ions and antioxidants are out of balance in the body. Therefore copper could have been leading to healthier sized sharks. The study did not find a relationship between heavy metals and immune function, however only some sharks were tested for this and other types of testing are needed to confirm this finding.

Overall, this work provided new and interesting findings that white sharks in this study do not seem to experience traumatic negative impacts on their health from heavy metals, particularly mercury and arsenic at levels that are considered toxic to humans. However, more research is needed to fully understand how this species can cope with these high concentrations. We cannot yet say that sharks are fully immune to these toxins, there could be other effects that simply were not measured by this study. However, this does serve as a very interesting continuation to many other studies of toxicity in sharks and will surely be followed up by more work in this field. It is important to note that these high circulating levels of heavy metals in white sharks do illustrate high bio-magnification in the South African ecosystems where these sharks feed. In the future, these blood markers could be useful to gauge the amount of heavy metals within the food web and ecosystem.

 

The article is based on the following peer-reviewed literature:

Merly L, Lange L, Meÿer M, Hewitt AM, Koen P, Fischer C, Muller J, Schilack V, Wentzel M, Hammerschlag N. 2019. Blood plasma levels of heavy metals and trace elements in white sharks (Carcharodon carcharias) and potential health consequences. Mar. Pollut. Bull. 142:85–92. doi:10.1016/J.MARPOLBUL.2019.03.018.

 

 

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