450 is the BASE max fishing skill. There are many additional items and gear which will add to your base fishing skill. The most common are lures. There are also hats, boots, fishing linea, a cooking recipe, an enchant, ramdom potion drop, fishing poles, etc which can add up to get you over the base max skill.
Right now, the most skill you could possibly get is 662. However, this would require a random potion drop and some dedication to the fishing profession, even more than the hours it took to level up to 450. : )
One of the best sources for WoW fishing information is by far www.elsanglin.com. This is THE source IMO for fishing and fishing related topics. Good Luck!