Assuming you're still pregnant (or breastfeeding), there have been several studies pointing to positive benefits for you and the baby from O3 supplementation. Fewer birth complications, better for the baby's brain, etc.
It's great that you eat salmon regularly and you might as well continue, but my take is, it's easiest to have a steady intake using the fish oil.
As for the study, 5% of a mouse's diet is a lot! Much more than we could possibly ingest. When you compare that with studies of actual humans and how much it's helped them, I tend to put more credence in the latter.