A few weeks ago I posted Fish Oil Reduces Heart Rate and Oxygen Consumption During Exercise, which discovered that fish oil somehow reduced oxygen demand during exercise.
Now I found another study that concluded marine-based omega-3 supplementation improved lung function ( Results indicated that consuming omega-3 during 12 weeks training had a significantly positive effect on pulmonary variables such as FEV1, FVC, VC, MVV, FEF25–75, FIV1 (p = 0.001), -The effects of omega-3 supplementation on pulmonary function of young wrestlers during intensive training
FEV1= Force Expiratory Volume in 1 second
FVC= Forced Vital Capacity, or the total amount of air you can forcefully exhale
Finally, there is this study out of the U. of Indiana (Fish Oil Supplementation Reduces Severity of Exercise-induced Bronchoconstriction in Elite Athletes) that found potential benefit for athletes with exercise induced bronchospasm.
As with anything else, more is not necessarily better. (Keep Your Fish Oil Intake Under 3 Grams / Day!
To date fish oil has not been found to improve endurance, however I suspect it may help athletes that report difficulty breathing during exercise.