Much of the fish on supermarket shelves in Europe comes from developing countries which can export more than 90% of their catch.
Swapping fish for meat to help combat climate change risks exacerbating hunger in Africa, from where fish is increasingly exported to wealthy nations instead of providing key vitamins to malnourished local people, experts have warned.
Some consumers in rich countries are shunning meat in favour of other forms of protein, including lentils and fish, in order to reduce the amount of planet-warming greenhouse gases emitted by intensive livestock farming.
But popular fish such as sardines and mackerel are sourced from African countries that export most of their nutrient-rich catch instead of selling it to their own populations, said a paper published in the journal Nature.
A shift in diets would "serve to ... worsen the food and nutritional security of already vulnerable people in places such as West Africa, Asia and the Pacific", said Christina Hicks, the paper's lead author.
The global fishing industry is worth $166 billion, and much of the fish on supermarket shelves in Europe and China comes from developing countries such as Namibia and Kiribati, which can export more than 90% of their fish catch.
The study found that across much of the tropics, especially in sub-Saharan Africa, some of the most nutritious species of fish such as anchovies are found in countries where citizens suffer from a lack of essential vitamins and minerals.
Yet "foreign fishing, illegal fishing, subsidies, prices, and trade all act to divert much-needed nutrients away from those in need," said Hicks, a professor at Britain's Lancaster University.
Globally, more than 2 billion people suffer from a deficiency of micronutrients such as iron, zinc, and vitamin A essential for the functioning of human bodies, experts say.
In Namibia, almost the entire population is estimated not to have an adequate intake of vitamin A, while in Mauritania, the same applies to nearly half of its people.
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Even a small portion of the catch from their waters could go a long way towards combating malnutrition-related diseases in millions of people within 100 km (60 miles) of the sea, Hicks said.
One way forward is to reform international fishing policies so local governments require companies to divert a small portion of their catch into programmes for malnourished children, Hicks said.
In Mauritania, for example, foreign fishing makes up over 70% of the fish caught, much of which are highly nutritious species but are processed in-country to be used in aquaculture abroad, she said.
Countries could replicate projects under way in Bangladesh and Uganda where fish heads, bones and tails that are usually binned by factories are turned into fish powder that can be added to meals to boost nutrition, Hicks said.
Globally, fish consumption is at an all-time high of 20.2 kg (44.5 lb) per person, according to the United Nations Food and Agriculture Organization.