Giving teenage boys a fish dinner at least once a week boosts their intelligence, research out today suggests.
Experts found that 15-year-old boys who ate fish regularly scored better on intelligence tests when they reached 18 than those who rarely ate it.
Including fish in the diet was also found to influence a boy's performance irrespective of whether they had a good education.
Researchers from the University of Gothenburg in Sweden questioned 3,972 boys about their fish intake aged 15 and matched this against intelligence scores aged 18.
Of the group, 57% ate fish about once a week, 20% ate it more than once a week and 23% ate it less than once a week.
The intelligence tests included logic and verbal reasoning as well as spatial awareness and geometric perception. One test examined technical and mechanical skills with maths and physics.
The results were combined to give three intelligence scores across all these areas.
The researchers found that eating fish had a "significant impact" on the scores.
Boys who ate fish once a week increased their scores by an average of 6%, while those who ate fish more than once a week increased them by just under 11%.
Writing in the journal Acta Paediatrica, the researchers said: "We analysed the association between reported fish consumption and performance in intelligence tests, adjusting for socio-economic factors.
"The main finding was that frequent fish intake compared with infrequent fish intake at age 15 was associated with significantly higher cognitive performance as measured by combined intelligence, verbal and visuospatial skills three years later.
"The association between fish consumption and the three intelligence scores was similar in both education levels indicating that education did not influence the association between frequency of fish meals consumed and cognitive performance."
Professor Kjell Toren, one of the lead researchers, added: "We found a clear link between frequent fish consumption and higher scores when the teenagers ate fish at least once a week.
"When they ate fish more than once a week the improvement almost doubled.
"These findings are significant because the study was carried out between the ages of 15 and 18 when educational achievements can help to shape the rest of a young man's life.
"A number of studies have already shown that fish can help neurodevelopment in infants, reduce the risk of impaired cognitive function from middle age onwards and benefit babies born to women who ate fish during pregnancy.
"However we believe that this is the first large-scale study to explore the effect on adolescents."
Prof Toren said it was not clear exactly how eating fish influences intelligence.
"The most widely held theory is that it is the long-chain polyunsaturated fatty acids found in fish that have positive effects on cognitive performance," he said.
"Fish contains both omega 3 and omega 6 fatty acids, which are known to accumulate in the brain when the foetus is developing.
"Other theories have been put forward that highlight their vascular and anti-inflammatory properties and their role in suppressing cytokines, chemicals that can affect the immune system."
Another researcher, Dr Maria Aberg, from the centre for brain repair and rehabilitation at the University of Gothenburg, said the link between fish and intelligence held true "regardless of their parents' level of education".
She said the team was now looking at how different types of fish may influence the results.
"But for the time being it appears that including fish in a diet can make a valuable contribution to cognitive performance in male teenagers."