The good reputation of fish as a healthy dietary choice is increasingly at odds with the risks involved in its consumption. Scientists have been raising the alarm for decades about the bioaccumulation of heavy metals —as the very serious poisoning recently revealed by singer Robbie Williams reminds us— but new studies also warn of the growing presence of microplastics and, especially, parasites in fish. The pollution of the seas and global warming, both the result of the unsustainable growth of our modern civilization, jeopardise one of the pillars of healthy eating, key to preventing the so-called pandemic of heart disease.
Last December, Robbie Williams confessed that he had almost died from mercury and arsenic poisoning after following a diet based mainly on fish and shellfish (which he ate twice a day). The British pop singer’s revelation to the media is another wake-up call about this problem for the general population, whose consumption of raw fish has grown, though without reaching the extremes experienced by this celebrity musician, thanks to the globalisation of sushi and poké as culinary trends.
In the case of heavy metals, which are assimilated through the diet and accumulate in the tissues, different studies have reported that their concentration has increased significantly in recent decades in many of the most consumed species. For example, in the case of Atlantic cod, researchers have observed a 23% increase in the concentration of mercury, and for bluefin tuna the rise is estimated to be double that of cod. This is despite the fact that since the 1990s, emissions of heavy metals into the environment have declined by up to 30%. In addition, over that time period the ubiquitous contamination of microplastics has also led to their increasing presence in fish species and to the study of possible implications for human health.
THE STRANGE CASE OF PARASITES
But if anything had provoked the food safety crises associated with fish consumption, it was the presence of parasites such as anisakis, and the reality is that in recent years a dangerous increase in their presence and concentration has been detected in commonly consumed marine species. Moreover, some of the conclusions reached by scientists are shocking. A study published last year by researchers at the University of Washington on parasitic worms present in the fish species used to prepare sushi, over the last 40 years, puts the increase in anisakis at 283%. What has changed since then?
The factors identified as responsible for this threefold increase in fish contamination are the “usual suspects” linked to human activity and its impact on the marine environment: mining wastewater discharges, intensive agriculture and industry, as well as massive emissions of greenhouse gases and their influence on the warming of the atmosphere and oceans. In addition to these, on this occasion there is another unexpected factor that turns out to be key in the case of parasites: society’s greater awareness of the need to protect and preserve marine ecosystems, which emerged towards the end of the 20th century.
THE UNEXPECTED CONSEQUENCES OF PROTECTING ECOSYSTEMS
In 1972, the Marine Mammal Protection Act was passed in the US, which has allowed the recovery of the decimated populations of many of these species, such as seals, sea lions and sea otters. As one of the authors of the study, Dr. Chelsea Wood, explains to OpenMind: “In this case, we think that the rising abundance of ‘sushi worms’ might reflect the rising abundance of their marine mammal hosts.” These parasites can reproduce in the gut of marine mammals —something they cannot do in accidental or transient hosts such as fish and humans— and their larvae are released into the water through their faeces. In this way they enter the food chain, through which they reach fish species and, finally, humans.
However, experts insist that these risks are still outweighed by the benefits of eating fish, and are therefore acceptable except for certain at-risk groups such as pregnant women and infants. In the words of Chelsea Wood, “the consequences of parasitic infection for humans are serious, but not usually life-threatening. It’s never fun to throw up or have diarrhoea, but most cases of anisakiasis never even require hospitalisation.”
All it takes is to follow a balanced diet (unlike what Robbie Williams was doing) and, in the case of parasitic worms, avoid eating raw or undercooked fish. But how to reconcile this with the recent boom in foods such as sushi or poké? We should follow the expert’s advice: “First, don’t make homemade sushi. Sushi chefs are trained to recognise and remove these worms. Also, when I eat sushi in restaurants, I order nigiri and cut it in half. The worms are large and easily recognizable; the likelihood that you will encounter one is low, but if it’s there, it should be pretty easy to see. Finally, I never eat sushi from freshwater fish. The anisakis (which are marine fish parasites only) can’t reproduce inside people, but there are several freshwater parasite species that can!”
TAKING MEASURES NOW FOR LONG TERM HEALTH
The same logic that explains the increase of parasites in fish applies to increased heavy metal pollution, despite the reduction in their emissions. According to the authors of the study, the answer lies in the measures taken to avoid overfishing of small species, on which the large, commonly consumed fish feed. Small fish absorb limited quantities of heavy metals, but as these fish become more abundant, the species at the top of the food chain eat a greater quantity and end up accumulating high concentrations of toxins. Thus, the recovery of herring stocks (due to the ban on their overfishing) is key to explaining the 23% rise in mercury detected in Atlantic cod.
As we come to terms with this paradoxical and discouraging rebound effect and its explanation, the reassuring part is that, if we continue working to prevent the dumping of heavy metals into the water, in the long term we will also end up minimising their presence in the food chain. That is why we should not give up on protecting and preserving the marine environment. After all, the oceans help us to see that our own health is inextricably tied to the health of the planet.
Miguel Barral
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