Heavy Metal Contamination in Marine Fish from Turkish Seas: A Systematic Review of Spatial Patterns, Trophic Transfer, and Human Health Risks
This systematic review synthesizes data from 59 studies (2003–2025) on heavy metal contamination in Turkish marine fish, revealing significant regional variability in pollutant levels, generally acceptable but elevated risks for specific consumer groups, and the need for standardized monitoring to address emerging factors like microplastic interactions.
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The ocean is a vast, interconnected system where life moves in layers, from tiny plankton drifting near the surface to large predators hunting in the deep. In this environment, invisible chemicals released by human activity—such as factories, cities, and farms—can settle into the water and the muddy bottom. Some of these chemicals are heavy metals, elements like mercury, lead, and cadmium that do not break down over time. Instead, they persist in the environment and can be absorbed by living things. When small fish eat contaminated plankton or sediment, and larger fish eat those small fish, the metals can become more concentrated as they move up the food chain. This process means that a fish at the top of the food web might carry a much higher load of these toxins than the water around it. For people who rely on seafood as a major part of their diet, understanding how these metals move through the ocean and end up on the dinner plate is a matter of both ecological health and personal safety.
A team of researchers in Türkiye recently set out to map this hidden landscape of contamination across the country's four distinct seas: the Black Sea, the Sea of Marmara, the Aegean Sea, and the Mediterranean Sea. They conducted a systematic review, which is a method of gathering and analyzing all the available scientific studies on a specific topic to find the bigger picture. By looking at fifty-nine different studies published over the last two decades, they examined the levels of heavy metals found in the muscle tissue of commercially harvested fish. Their goal was to determine where the contamination is highest, which types of fish are most affected, and what this means for the health of people who eat them.
The researchers found that the story of metal contamination in Turkish waters is not the same everywhere; it changes depending on the geography and the local human activity. In the Sea of Marmara, a narrow, busy waterway sandwiched between the Black Sea and the Aegean, the most significant concerns were lead and cadmium. This area is surrounded by heavy industry, dense cities, and constant ship traffic, and the water circulation is restricted, allowing pollutants to build up. Fish living near the bottom in this region, such as whiting and red mullet, showed higher levels of these metals, likely because they pick them up from the contaminated mud where they live and feed.
In contrast, the Black Sea showed a different pattern, heavily influenced by the massive rivers that flow into it from across Europe. These rivers carry runoff from agriculture and industry, depositing metals into the coastal zones. While the overall levels in fish were generally within safe limits, the researchers noted that specific areas near river mouths and ports acted as hotspots for contamination. The Aegean and Mediterranean Seas, which are more open and have different water currents, presented a different challenge. Here, the primary concern was mercury, particularly in large, predatory fish like Atlantic bonito, European hake, and scorpionfish. Unlike lead or cadmium, which tend to stay near the bottom, mercury has a unique ability to change into a form that is easily absorbed by living tissue and accumulates as animals eat one another. As a result, the largest, oldest predators in these waters carry the highest concentrations of mercury, a phenomenon known as biomagnification.
Despite these regional differences, the review offered a reassuring conclusion for the general public: the vast majority of fish harvested from Turkish seas contain metal levels that fall below international safety standards. When the researchers calculated the potential health risks for an average person eating a normal amount of fish, the numbers suggested that the risk of harm from non-cancerous effects was low. However, the picture becomes more complex when looking at specific groups of people. Those who eat fish very frequently, such as families living in coastal communities, or vulnerable groups like young children and pregnant women, face a higher potential exposure. For pregnant women, in particular, mercury is a concern because it can cross the placenta and affect the developing brain of a fetus. The researchers noted that while the fish are generally safe, those who consume large predatory species regularly might need to be more cautious.
The study also highlighted that the ocean is a dynamic place where new factors are beginning to influence how these metals behave. The researchers discussed the growing presence of microplastics—tiny fragments of plastic waste—which can act like sponges, soaking up heavy metals and transporting them through the water. They also considered how climate change, with its rising water temperatures and shifting currents, might alter how metals move and become available to marine life. These emerging factors suggest that the situation is not static; the risks could shift as the environment changes.
Ultimately, the review revealed that while the current state of seafood safety in Türkiye is largely positive, the data is not perfectly uniform. The studies they analyzed used different methods, sampled different fish sizes, and measured metals in various ways, making it difficult to compare results directly from one region to another. This lack of standardization means that while we know the general trends, we need more consistent monitoring to track changes over time. The researchers concluded that continued surveillance is essential, especially as industrial activity and mining expand in the region. By keeping a close watch on the water and the fish, authorities can ensure that the seafood remains a safe and healthy resource for the population, protecting both the marine ecosystem and the people who depend on it.
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