The study doesn't prove that such habits strengthen bones, but researchers believe that the combination of different oils in the fish protects bones from losing mass over time.
"We think omega 3 fatty acids from fish help to prevent" bone loss, said lead researcher Dr. Katherine Tucker, a professor at Northeastern University.
Her group looked at surveys, collected in the 1980s and 90s, of the eating habits of more than 600 seniors who lived in Framingham, Massachusetts.
Measurements of the bone density in their hips were taken 4 years apart.
Women who ate three or more weekly servings of dark fish, such as salmon or mackerel, had smaller amounts of bone loss 4 years later, compared to women who ate less fish.
Men who ate dark fish or tuna at least three times per week also had less bone loss than other men.
Fish are rich in the omega 3 fatty acids, EPA and DHA.
Looking further into what people ate, the researchers broke down how much of both omega 3 and omega 6 fatty acids people were getting in their diet.
They found that it's not just the omega 3s that are involved in bone density.
High levels of an omega 6 fatty acid, called arachidonic acid, was linked to less bone loss in women - but only when women also consumed higher levels of omega 3 fats.