I agree with Mike for the most part- however, I'm going to play Devil's Advocate.
Mike states that on his server, the fish used in alchemy sell particularly well. However, on Uldum, the price of the same fish is considerably lower. However, it's still worth the time for me. Why? Let me do a chart:
Server: Duskwood Uldum
Stonescale Eel 18-22G 11-14G
Oily Blackmouth 2G 1-1.5G
Firefin Snapper 1.5G 40-60s
However, the Fire Oil that I can make from those Firefins sells at a much higher rate (I believe it was 1.8-2G per stack). The Blackmouth Oil is much the same. Stonescale eels tend to sell much faster than the Stonescale oil, but the potions that it is used in go for a good price. Thus, especially for alchemists, it's worth it to fish. Also, before level 50, it's a decent income- there's a minimum of bother (for example, when mining or gathering herbs, you often have to fight your way to each resource, while when fishing you can fish in the same area, or move up and down the coast to find schools of fish).
Now that I'm almost 60 there are other ways to make money, but fishing is relaxing to me, and it helps bring in money. It's always good to keep your options open, and keep an eye on your server's market. Note supply and demand for items. Sometimes it's obvious (like gathering professions- on established servers there's almost always a high demand for crafting materials at the auction house). Other times, it's less obvious (such as deviate delights selling better for Leawond than they do for Mike). If you want some other free advice, just check the link in my sig.
Edited, Aug 30th 2006 at 10:59am EDT by Wondroustremor