Key Contaminants Found in Herring
Like all marine life, herring can be exposed to a range of environmental toxins. Due to their position lower on the food chain, they tend to have lower levels of some bioaccumulating toxins compared to larger predatory fish. The primary contaminants of concern include heavy metals, persistent organic pollutants (POPs), and microplastics.
Heavy Metals
Heavy metals are a class of environmental pollutants that can accumulate in fish tissue. Herring typically carry low levels of mercury, significantly less than larger predatory fish like swordfish or bigeye tuna. However, other metals and metalloids are also found:
- Mercury (Hg): Present in trace amounts. Herring has one of the lowest mercury levels among common fish species. A 2015 study on Norwegian spring-spawning herring found mean mercury concentrations of 0.04 mg kg$^{-1}$ wet weight, well below EU maximums.
- Arsenic (As): Found primarily in organic forms within herring, particularly concentrated in the roe as arsenolipids. While often considered less toxic than inorganic arsenic, some arsenolipids can be harmful. A study on Norwegian herring found total arsenic levels significantly correlated with fat content.
- Cadmium (Cd) and Lead (Pb): While generally present in low concentrations in finfish muscle, these heavy metals can be found, particularly in fish from polluted areas. Studies have detected varying levels, but often within permissible limits for consumption.
Persistent Organic Pollutants (POPs)
POPs are fat-soluble chemicals that persist in the environment and accumulate in the fatty tissues of animals. Since herring is an oily fish, they are susceptible to accumulating these compounds.
- Dioxins and Polychlorinated Biphenyls (PCBs): These are particularly relevant for herring from the Baltic Sea. Historically, dioxin and PCB levels were high enough to prompt EU consumption restrictions on larger Baltic herring. However, thanks to stricter pollution laws, levels have been dropping steadily over the past decades. Despite the improvements, larger, older fish still contain higher concentrations.
- Polybrominated Diphenyl Ethers (PBDEs): These flame-retarding chemicals have also been tracked in herring populations. Monitoring programs in areas like Puget Sound have shown high levels in some localized stocks, while others remain low.
Microplastics
Herring, as filter feeders, are known to ingest microplastics (MP), predominantly in the form of synthetic fibers. A 2019 study on Baltic herring found MPs in about 22% of the fish examined.
- Risk of contaminant transfer: While some initially feared a 'Trojan horse' effect where microplastics would transfer POPs to the fish, a 2019 study on Baltic herring found no correlation between the amount of ingested microplastics and the concentration of POPs in the muscle tissue. This suggests that microplastics contribute negligibly to the overall toxin burden compared to other environmental exposure routes.
Factors Influencing Contaminant Levels
Several factors determine the level of toxins a particular herring carries. Consumers can use this knowledge to make more informed choices.
- Geographic Location: The fishing ground significantly impacts contaminant exposure. For example, dioxin and PCB levels are a specific concern for Baltic Sea herring, whereas Pacific or Norwegian herring may have different profiles.
- Fish Age and Size: As toxins like POPs and heavy metals bioaccumulate, older and larger herring tend to have higher concentrations of these contaminants than younger, smaller individuals.
- Fat Content: Because POPs are fat-soluble, the fat content of the fish is a key variable in accumulation. Processing methods that reduce fat, such as smoking, may alter toxin concentrations, though specific data on mercury is less clear.
- Food Chain Position: As a lower-trophic-level fish, herring inherently accumulates less mercury through biomagnification compared to larger, longer-lived predators.
Herring vs. Other Fish: A Toxin Comparison
This table illustrates the average mercury levels (in parts per million) for herring and other commonly consumed fish, based on FDA monitoring data.
| Fish Species | Average Mercury Level (ppm) | Typical Food Chain Position | Primary Toxin Concern | Remarks |
|---|---|---|---|---|
| Herring | 0.084 | Low | PCBs/Dioxins (Baltic) | Low mercury, high omega-3s |
| Canned Tuna (Light) | 0.128 | Medium | Mercury | Higher mercury than herring |
| Canned Tuna (White/Albacore) | 0.35 (varies) | Higher than Light | Mercury | Mercury levels vary by type |
| Salmon | 0.022 | Low to Medium | PCBs/Dioxins (Wild vs Farmed) | Lower mercury, varies with source |
| Swordfish | 0.995 | High | Mercury | High mercury content |
Is Herring Safe to Eat?
For most of the population, including herring in a balanced diet is safe and offers significant health benefits. The low mercury content is a key advantage over larger predatory fish. Concerns about POPs, particularly in Baltic herring, are managed through size limits and declining pollution levels. Studies often conclude that the benefits of nutrients like omega-3 fatty acids outweigh the risks from contaminants for most adults.
However, specific populations should exercise caution:
- Pregnant and breastfeeding women: Need to be mindful of fish consumption to minimize toxin exposure to the developing fetus or infant. While herring is a better choice than high-mercury fish, recommendations for fish from specific polluted waters, like the Baltic, may apply.
- Vulnerable populations: For those who consume large quantities of fish from specific, heavily polluted regions, or groups with higher overall exposure, the risk-benefit balance may differ.
Additionally, preparation and storage are important. Improperly handled herring can lead to histamine toxicity, a form of food poisoning. Fresh, properly stored, or safely pickled herring is the best option.
Conclusion
Herring is a nutritious oily fish, prized for its high omega-3 fatty acid content. While the question of what toxins are in herring is valid, the actual risk to consumers is generally low. The primary contaminants to be aware of are persistent organic pollutants (like dioxins and PCBs) in herring from certain polluted waters (especially older, larger fish) and the potential for histamine in poorly stored fish. The mercury level in herring is notably low compared to most popular fish choices. For most individuals, the significant health benefits of regular, moderate herring consumption outweigh the minor risks associated with contaminants. Choosing herring from well-managed fisheries in less polluted areas, or opting for smaller individuals, can further minimize exposure to these substances, while still providing essential nutrients. For specific guidance, especially for vulnerable groups, consulting with national food safety authorities like the FDA is recommended.
