Smaller fish pack better nutrition, lower mercury and are better for the environment, study finds

Smaller fish species are more nutritious, lower in mercury and less susceptible to overfishing, a Cornell University-led research team reveals. They highlight that, when it comes to fishing sustainably and economically, smaller species that are more common and less expensive are also more nutritious and lower in mercury. 

The team’s study, published in One Earth, was conducted in the Amazon River, but the findings have implications for biodiversity conservation and public health across the globe as large fish species populations decline worldwide.

“Fish can provide a nutritious source of food at lower environmental cost than other animal-sourced foods, but fish can also potentially expose consumers to mercury contamination, and overexploitation of oceans and rivers harms biodiversity,” says first author Sebastian Heilpern, a Schmidt AI in Science postdoctoral fellow in the College of Veterinary Medicine.

Larger fish have higher mercury risk

The researchers analyzed the nutritional value, mercury content, price and abundance of 59 fish species in the Amazon River.

Worldwide, larger fish species tend to have the highest levels of mercury because the element accumulates in their tissues as they live longer and eat other mercury-contaminated organisms. 

The problem is especially acute in the Amazon because of the expansion of poorly regulated gold mining that uses mercury to separate gold from river sediments.

The Amazon is one of the most biodiverse regions in the world. “Roughly 2,500 species of fish exist in the Amazon River, and about 100 different species are sold for consumption,” says Heilpern.

“Helping people get the best nutrition from limited resources is a key priority, especially as population growth and climate change put even more pressure on natural resources.”

Global implications

Though this research was conducted in the Amazon, many findings are more broadly applicable. For example, though humans tend to prize larger fish species for cultural reasons and easier preparation, globally, larger fish have higher mercury levels and are more vulnerable to human pressures.

Larger species, such as goliath catfish, tuna and salmon, are more likely to take long migratory routes, highlight the study authors. Especially for freshwater species, these routes can be interrupted by dams and other habitat interruptions that further jeopardize reproduction.

“Smaller fish are less susceptible to overfishing, in part because they have faster reproduction cycles, which may also make them more nutritious because micronutrients like iron and zinc fuel cell metabolism and growth,” Heilpern notes.

The researchers advocate that public health messaging should incorporate a more holistic view of the nutritional and environmental impacts of eating a variety of fish.

“Human food systems are a huge driver of biodiversity loss in the environment. At the same time, biodiversity in our food systems sustains the nourishment that humanity depends upon,” says Heilpern.

“My hope is that this information can help provide guidance and identify solutions so that our food systems can become more sustainable, for the benefit of human health and ecological systems.”

In previous research, a US study proposed that probiotics can potentially block mercury absorption in the gut. The authors of the paper say that with the help of probiotics, the human gut could be harnessed to block the absorption of toxic metals while supporting the nutritional intake function of iron and other minerals.