In all honesty, just use rsbuddy's mousekeys to drop fish trout/salmon until around 60 fishing, then head over to Catherby and do lobbies until 75. You wanna wait till 60 so that way you can actually catch at a decent rate. 75, you will start to do swordfish, then hit sharks at like 90. As far as how many you'd have to fish for 99, not sure, but damn, it's so easy with 5x exp.