Researchers followed 500 couples in Michigan and Texas for one year, asking them to log their seafood consumption and sexual activity in daily journals. Couples had 39% higher odds of having sex on days when both partners ate seafood, the study found.

By the end of the year, 92% of couples who ate seafood more than twice a week had conceived, compared to 79% of couples who ate less fish and shellfish. The association between seafood and fertility remained even after accounting for how frequently couples had sex.

“While an increase in sexual activity might be one behavioural mechanism linking seafood intake to higher fecundity, it is not the only mechanism,” said lead study author Audrey Gaskins, a nutrition researcher at the Harvard T. H. Chan School of Public Health in Boston.

Doctors generally advise adults to eat at least two servings a week of oily fish like salmon, mackerel, herring, and tuna that are rich in omega-3s, which are linked to a lower risk of heart disease and stroke.

But women who are pregnant or trying to conceive are told to eat no more than three servings a week of fish to avoid exposure to mercury, a contaminant that can cause birth defects and may be highly concentrated in shark and swordfish, as well as mackerel and tuna.

At the start of the current study, researchers asked couples how often over the past 12 months they ate canned tuna, as well as crab, fish, and shellfish caught in local waters or in unknown locations.

Men who ate more seafood during the year before the study tended to have sex more often than men who ate less, but there wasn’t a meaningful difference for women, the study found.

Couples’ odds of conception also didn’t appear to be influenced by how much seafood they ate during the year before the study, researchers report in the Journal of Clinical Endocrinology and Metabolism.

How much seafood people ate also didn’t seem to be influenced by factors like income, education, exercise, or body weight.

The study wasn’t a controlled experiment designed to prove whether or how seafood might influence sexual activity or fertility. It also wasn’t clear what types of fish people ate, which could influence their risk of mercury exposure.

“All fish are not equal,” said Tracey Woodruff, director of the Program on Reproductive Health and the Environment at the University of California, San Francisco.

“A lot of sardines or anchovies are good. They’re very small and there’s not a lot of contaminants,” Woodruff, who wasn’t involved in the study, said in a phone interview. “Tuna gets complicated because that can have more mercury in it.”