Cover the stain with talcum powder. Talcum powder absorbs oil and will pull the stain out if it isn't too deep.
Let the powder sit on the stain overnight.
Dampen a clean cloth with water.
Wipe the powder away.
Vacuum the couch to remove any residual powder.
Let the sofa sit near an open window to air out any fish oil smell. Dabbing the spot with vinegar may also help neutralize the smell without harming the fabric. The vinegar smell will dissipate as it evaporates.