Understanding the Mercury Cycle in the Ocean
To understand why does tuna have mercury but not salmon in comparable amounts, it is first necessary to grasp how mercury gets into the marine ecosystem. Mercury is a naturally occurring element, but industrial activities like coal-fired power plants and mining have significantly increased its presence in the environment. Once released into the air, mercury settles into oceans and waterways. In the water, microorganisms convert it into a highly toxic organic form called methylmercury.
This methylmercury is then absorbed by tiny organisms, like plankton, at the base of the marine food chain. From there, it begins its journey up the food web through a process known as bioaccumulation, where organisms absorb the contaminant faster than they can excrete it.
The Role of Bioaccumulation and Biomagnification
Bioaccumulation is the initial buildup of mercury within an individual organism's body over its lifetime. The concentration of mercury within that organism can increase over time, especially since fish excrete methylmercury very slowly. However, the real divergence between tuna and salmon's mercury levels is explained by biomagnification.
Biomagnification is the process by which a toxin becomes increasingly concentrated in the tissues of organisms at successive trophic levels of a food chain. Simply put, when a larger fish eats a smaller, contaminated fish, it absorbs all the mercury from its prey, compounding its own mercury burden. As a fish moves up the food chain, its mercury levels become exponentially higher.
- Tuna's Predatory Nature: As large, long-lived predatory fish, tuna sit high on the oceanic food chain. They consume many smaller fish that have already accumulated small amounts of mercury. This repeated ingestion of mercury-contaminated prey causes high concentrations to build up in their muscle tissue over a long lifespan, especially in larger species like albacore and yellowfin tuna.
- Salmon's Lower Trophic Level: Salmon, particularly those from cleaner waters or those that are farmed, typically feed on insects and smaller aquatic organisms, placing them much lower on the food chain than tuna. Their smaller size and shorter lifespan mean less time and fewer predatory meals to accumulate high levels of mercury.
Comparing Mercury Levels in Tuna vs. Salmon
The stark difference in mercury levels is evident in data from the U.S. Food and Drug Administration (FDA).
| Feature | Tuna (Albacore) | Salmon (Fresh/Frozen) |
|---|---|---|
| Trophic Level | Higher (predator) | Lower (feeds on smaller prey) |
| Lifespan | Long, allows for more accumulation | Shorter, less time for accumulation |
| Average Mercury Level (ppm) | 0.350 ppm | 0.022 ppm |
| Primary Diet | Consumes many smaller fish | Eats smaller organisms, insects |
| Size | Large, older specimens have higher levels | Generally smaller and shorter-lived |
| Biomagnification | Significant effect due to high-level predation | Minimal effect due to lower position in food chain |
Canned Tuna vs. Fresh Tuna
The mercury content in canned tuna can also vary significantly based on the species used. Canned light tuna, which is often made from the smaller skipjack species, typically contains lower mercury levels than canned albacore or "white" tuna. This is because skipjack are smaller, younger fish and haven't had as much time to accumulate mercury as the larger albacore tuna. This highlights that even within the 'tuna' category, not all products are equal regarding mercury exposure.
Making Safer Seafood Choices
Consumers can minimize their mercury intake while still enjoying the nutritional benefits of fish by being mindful of these differences. Both the FDA and the EPA provide guidance on safe seafood consumption, particularly for vulnerable populations like pregnant women, nursing mothers, and young children. They generally recommend choosing a variety of fish and prioritizing those lower in mercury. Fish is a rich source of omega-3 fatty acids and other important nutrients, and for most people, the benefits of eating fish outweigh the risks of mercury when moderate consumption guidelines are followed.
Conclusion
The fundamental reason for the difference in mercury levels between tuna and salmon is a matter of biology and trophic position. Tuna's role as a large, long-lived predator high on the food chain, combined with the process of biomagnification, causes it to accumulate significantly more methylmercury than the smaller, shorter-lived salmon. This ecological dynamic explains the disparity in their mercury content and underscores the importance of being an informed consumer when it comes to seafood choices. By understanding the science behind biomagnification, consumers can make healthy decisions that balance nutritional benefits with potential health risks.
Food Standards Australia New Zealand offers additional, detailed advice on mercury in fish.
