More than 3 billion people around the world rely on fish for food. Fish are a favored source of proteins and healthy fats in highly recommended diets, like Mediterranean and Nordic.

But new research suggests that as with all things, too much good fish could also be a bad thing.

A large, long-term study of almost 500,000 people, found people who eat more fish than the equivalent of half a can of tuna a day were 22 percent more likely to contract a malignant melanoma.

"Melanoma is the fifth most common cancer in the [US] and the risk of developing melanoma over a lifetime is one in 38 for White people, one in 1,000 for Black people, and one in 167 for Hispanic people," explains Brown University dermatologist Eunyoung Cho.

It's important to note that this doesn't at all mean we should avoid eating fish. This study shows a trend, not an underlying cause, which means researchers have not directly demonstrated that eating more fish increases your risk of skin cancer. Also, even if there does prove to be a direct link, the benefits of eating fish would still likely outweigh total avoidance.

However, such a strong link within a big sample size, that makes sense in the wider context of our current environment, does beg for further investigation.

"Although the results are from a cohort study, which means they are observational and hence do not imply causation, they cannot be ignored," says University of Newcastle dietitian Clare Collins, who was not involved in the study. "The role of contaminants that may be present in some fish needs to be considered."

It is well established that toxins in our environment, including those that we know directly cause cancer like heavy metals, build up through the food chain.

For example, Mercury emitted through industrial processes like burning coal finds its way into our waterways where microbes break it down into methylmercury.

This is taken up by plankton and ends up accumulating in the tissues of the shrimp that eat those plankton, then the fish that eat the shrimp, and so on, getting more concentrated the higher up the food chain it goes. This is known as biomagnification.

"We speculate that our findings could possibly be attributed to contaminants in fish, such as polychlorinated biphenyls, dioxins, arsenic, and mercury," says Cho.

"Previous research has found that higher fish intake is associated with higher levels of these contaminants within the body and has identified associations between these contaminants and a higher risk of skin cancer."

The researchers, led by Brown University epidemiologist Yufei Li, used data from the USA NIH-AARP Diet and Health Study, from participants recruited between 1995 and 1996. They collated this with the National Death Index and state cancer registries and found the risk of melanoma was 22 percent higher in those who ate around 43 grams of fish a day compared to those who ate the median amount (around 3 grams per day).

This link was linear, meaning the amount of tuna consumed increased the cancer incidence, and it was consistent across several demographic and lifestyle factors after also considering other risks like mole count, hair color, history of severe sunburn, and sun-related behaviors.

The intake of fish was only calculated at the start of the study though, so this may have changed over the participants' lifetime though.

These findings in no way reduce other well-established causes of skin cancer.

"It is critical that we don't confuse or cloud the prevention message," CEO of Melanoma Institute Australia Matthew Browne cautioned in a comment about the study. "The scientific evidence is clear – sun exposure is the single biggest risk factor for developing melanoma."

But as levels of these contaminants increase thanks to intensifying land use and even climate change (mercury concentrations in some waterways has been increasing as rainfall increases) this potential cause of skin cancer shouldn't be neglected. Li and colleagues call for further investigation.

This study was published in Cancer Causes & Control.