Are Fish Becoming More Toxic?

Methylmercury is very toxic, and often forms through bacteria from different environments coming into contact with mercury, then in turn people can become exposed to this toxic compound by consuming fish and seafood as many water species end up ingesting this compound. 

Many sea fish also get exposure to this toxic substance through their diet via algae absorbing organic methylmercury that fish eat, which are then absorbed. Then larger fish eat smaller fish that ate the algae, to go onto also accumulate methylmercury; larger fish at the top of the food chain and up accumulating more of this toxic substance, which end up on human plates.

Exposure to methylmercury via shellfish and fish is not a new concern, but some believe levels of this compound present in this very popular staple of cuisines around the globe are increasing; and recent research indicates that approximately 82% of American exposure to methylmercury comes from consuming fish and shellfish. 

As published in the journal Nature, Harvard University researchers suggest that the levels of methylmercury in fish including cod, swordfish, and Atlantic bluefin tuna are increasing, and according to the team the reason for this is the ill effects of global climate change.

"This research is a major advance in understanding how and why ocean predators, such as tuna and swordfish, are accumulating mercury," said senior author Prof. Elsie Sunderland.

30 years of data on the ecosystem of the Gulf Of Maine in the Atlantic Ocean was analyzed to investigate what 2 marine predators, the spiny dogfish and Atlantic cod, were eating from the 1970s to the 2000s. Results indicate that methylmercury levels have decreased in cod by 6-20%; but levels have increased in the dogfish by 33-61%

This contrast was explained by examining what each species was able to eat, noting that in the 1970s their prey herring population had significantly decreased in the Gulf of Maine due to overfishing, meaning these 2 predator species had to hunt for other sources of food. 

Cod started to prey on shads and sardines which have low levels of methylmercury causing the levels to decrease in cod. The dogfish started to prey on squid and other cephalopods which are also predators with high levels of the toxic compound causing the methylmercury levels in the spiny dogfish to increase. In the 2000s herring populations started to increase slowly, meaning the 2 predator fish could return to their former diets and the methylmercury levels returned to former levels little by little with cod levels increasing while dogfish levels decreased.

The researchers don’t believe that the change in predator food source availability is the only factor that has influenced the levels of the toxic compound found in larger fish, as they found it difficult to account for the increasing level in tuna by diet alone. 

To explain the increase in tuna a different connect was found: these fish are migratory and swim at high speeds using a lot of energy, meaning they need to eat more to maintain agility and speed. Another factor also influences how much energy is needed and how much they need to eat, this factor is suggested to be global warming, and according to the team the Gulf of Maine is one of the fastest warming bodies of water in the world.

"These [...] fish eat a lot more for their size but because they swim so much, they don't have compensatory growth that dilutes their body burden. So, you can model that as a function," explains first author Amina Schartup.

"Northward migration of the Gulf Stream and decadal oscillations in ocean circulation have led to unprecedented seawater warming in the Gulf of Maine between a low point in 1969 and 2015, which places this region in the top 1% of documented seawater temperature anomalies," according to the authors.

The warmer the water, the more energy is needed by fish to swim, meaning that they need to eat more smaller fish which makes them accumulate higher levels of methylmercury; Atlantic bluefin tuna has been found to increase in methylmercury levels by as much as 3.5% each year from 2012 to 2017. 

The team was able to create a model which predicted the increase in methylmercury levels in sea dwelling fish by using all of this information; the model predicts that for a 5kg spiny dogfish a temperature increase of 1 degree Celcius in seawater could lead to a 70% increase in tissue methylmercury concentrations, for cod this increase would lead to a 32% increase. 

"This model allows us to look at all these different parameters at the same time, just as it happens in the real world. Being able to predict the future of mercury levels in fish is the holy grail of mercury research. That question has been so difficult to answer because, until now, we didn't have a good understanding of why methylmercury levels were so high in big fish." explains Schartup.

"We have shown that the benefits of reducing mercury emissions holds, irrespective of what else is happening in the ecosystem. But, if we want to continue the trend of reducing methylmercury exposure in the future, we need a two-pronged approach," adds Prof. Sunderland.

"Climate change is going to exacerbate human exposure to methylmercury through seafood, so to protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases," she warns.