Great question, illustrating the whole point of codeshares -- you are the customer of the selling airline and not the operating airline. The amount of fuel surcharge is determined by the codeshare that you are taking. So the QF codeshare on BA will have a different fuel surcharge amount than BA (depending upon which month this is, it will be either higher or lower than BA -- QF has high fuel surcharges). On your AA/QF example, AA has a lower fuel surcharge but charges a higher base fare; often the AA fare is USD 200 higher than QF on the same flight, but after fuel surcharge the total is almost the same (within USD 30). I find "fuel surcharge" to be a deceptive business practice -- it obviously has nothing to do with buying the fuel for the flight, that is covered in the ticket price, instead it is just a "profit surcharge" meant to increase revenue and operating profits and is completely unrelated to the cost of fuel for the flight in question.