The Fitzroy just had a major flush go through it, it is a long way from the mouth where the salinity levels will be better. I suggest working a low tide and first 1/2 of the incoming down towards the mouth with a drift fish. You will come across sections of water that are obviously more salty and there is where you will find fish. They will hang in these saltier zones and each tide will bring them closer to the CBD. You just have to get onto the salinity levels they can tolerate easily.