Fish has traditionally been thought of as healthy because it is a great source of lean protein, minerals like selenium, and omega 3, which is important for brain health and also anti-inflammatory.
However, it is safe to say that fish and seafood are not what they used to be 70 years ago before chemical production ramped up exponentially, especially in the US.
Significant chemicals discharged from urban, industrial, and agricultural sources have entered water bodies; our oceans and waterways are now contaminated with:
It is no surprise then, that these chemicals have found their way into seafood, and the higher up the food chain (i.e. the bigger the fish) the higher the level.
This depends on your bio-individual nutritional needs, toxic load, detox capacity, and stage in your life. For example, pregnant women and children, or anyone with an already high toxic load should probably be more cautious than a healthy adult who is already taking steps to reduce their environmental exposure.
If you do not consume fish, you do need to be cognizant of omega-3 intake from other sources or even supplements if indicated.
It is important to note that we are usually talking about non-lethal concentrations of these toxicants when it comes to fish and seafood - i.e. one serving is not likely to harm you, however, consistent, low-level exposures can accumulate over time if your detox capability is poor, or you have a high toxic load already.
What are the common contaminants found in fish? And what can you do to reduce your risks?
1) Mercury - this is a naturally occurring heavy metal in low levels, however, the majority of mercury in our environment now is a result of coal burning and mining activities. Mercury bio-accumulates up the food chain, meaning, when the bigger fish eat the little fish, the concentration of mercury in their bodies increases.
Mercury is a neurotoxin  and can affect our mitochondria (which produce energy for cellular function). Low-level, long-term exposure from eating contaminated fish in the context of poor detox capacity may affect our nervous system, energy levels, and brain health.
Children and pregnant women are most at risk from the health impacts of mercury exposure because methylmercury readily crosses the placenta, breast milk, and in small children, their brains are still developing and may be more susceptible to toxic levels.
What is toxic to me, however, may not be toxic to someone else. This is why I believe studies have NOT shown consistent, conclusive findings in this area. We all have different toxic loads and different detox capabilities.
Two people can ingest the same fish with the same mercury concentration but have very different outcomes.
Someone with a low toxic load, and actively supporting their detox organs will likely be less affected. But we do know this - gestational mercury exposure seems to be correlated with adverse neurological development in babies  whereas fish consumption was found to be beneficial. i.e. in someone with good detox capabilities, the benefit of consuming fish and the ingestion of healthy DHA/EPA outweighed the presence of mercury. So, context matters.
Someone with impaired gut health, poor liver function, or constipation will likely have a higher blood level of mercury when they ingest contaminated fish, than someone who has optimal detox pathways.
What to do:
2) PCB/Dioxins - these are potent endocrine disrupters (mess up our hormones) and have been associated with insulin resistance and type 2 diabetes, polycystic ovary syndrome, fertility issues, and even certain cancers like breast cancer. It does not break down easily, so it can build up in our bodies and has been found in human breast milk - hence the importance of pregnant/nursing moms taking more caution.
What to do:
(source: Mozaffarian D, Rimm EB. Fish Intake, Contaminants, and Human Health: Evaluating the Risks and the Benefits. JAMA. 2006;296(15):1885–1899. doi:10.1001/jama.296.15.1885)
3) PFAS - also known as 'forever chemicals' because they stick around in the environment 'forever', this family of chemicals is now widespread in the environment due to its extensive use, release, and disposal in cookware (non-stick coating), water/stain proofing, for the last 2 decades. As a result, the CDC biomonitoring program has found that 97% of Americans had PFAS in their bodies - a frightening statistic given its widespread health implications.
PFAS has been found  to be associated with fertility, thyroid issues, high cholesterol, liver/ kidney issues, certain cancers, and more.
What to do:
Despite headlines, fish and seafood are NOT the primary sources of PFAS (not yet anyway).
We (hopefully) drink around 2 L of water per day, making contaminated water one of the main sources of PFAS, followed by cookware, and indoor air/dust from furniture, carpet, and clothing treated with PFAS.
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 Branco V, Aschner M, Carvalho C. Neurotoxicity of mercury: an old issue with contemporary significance. Adv Neurotoxicol. 2021;5:239-262. doi: 10.1016/bs.ant.2021.01.001. Epub 2021 Feb 2. PMID: 34263092; PMCID: PMC8276940.
 Kim B, Shah S, Park HS, Hong YC, Ha M, Kim Y, Kim BN, Kim Y, Ha EH. Adverse effects of prenatal mercury exposure on neurodevelopment during the first 3 years of life modified by early growth velocity and prenatal maternal folate level. Environ Res. 2020 Dec;191:109909. doi: 10.1016/j.envres.2020.109909. Epub 2020 Jul 12. PMID: 32871452.
 Oken E, Wright RO, Kleinman KP, Bellinger D, Amarasiriwardena CJ, Hu H, Rich-Edwards JW, Gillman MW. Maternal fish consumption, hair mercury, and infant cognition in a U.S. Cohort. Environ Health Perspect. 2005 Oct;113(10):1376-80. doi: 10.1289/ehp.8041. PMID: 16203250; PMCID: PMC1281283