As far as I've read, cod liver oil acts to lower blood pressure, to slow coagulation of the blood.
This can be a good thing in normal life, but in a situation where surgery is putting stress on your system, and we've seen many hippies (including myself) have lower blood pressure than normal just after surgery, and where we want healing to happen, the lowering of blood pressure even more, and the lack of clotting of surgical openings would be counterproductive, it seems.
Aspirin has the same effect via a different pathway, in that it reduces the ability for blood to clot, which is necessary to heal the cuts made during the surgery. It's fine afterwards ( my surgeon prescribed it to prevent bad blood clots ), but clotting is necessary to heal, especially under the pressure that the body is in during surgery.
Here's one of the articles that describes the effects of cod liver oil :
http://www.webmd.com/vitamins-supplements/ingredientmono-1040-COD%20LIVER%20OIL.aspx?activeIngredientId=1040&activeIngredientName=COD%20LIVER%20OILI'm sure that afterwards cod liver oil is fine, just that surgery is a special situation.