There are 3 fish that act as offensive weapons. Both are classed as maces and held in your main hand:
Heavy Fish and Offhands
A wide variety of above-average weight fish may be caught from open water in Azeroth, such as the 10 Pound Mud Snapper. Known types are Catfish, Grouper, Lobster, Mightfish, Mud Snapper, Redgill, and Salmon. Catch rates are around 1 in 100 from the waters of Azeroth. The heaviest fish are Mightfish. I've caught some in Azshara that are almost as heavy as I am!
Most heavy fish can be held (in the offhand), although they do no damage in combat.
Only the two offhand fish found in Outland, Enormous Barbed Gill Trout and Huge Spotted Feltail, can be cooked and eaten. Other offhand fish cannot be cooked or eaten. However they can be fed to hunters' pets.
There are 2 rare off-hand fish that provide a stamina bonus:
These are possible to catch and equip as low as level 5. At lower levels, a +5 stamina bonus is very useful. These fish bind to the angler when caught, so cannot be traded.
And yes, it is possible to equip both a fish main and offhand at the same time! Both fish weapons are exceptionally rare, and will sell at auction simply for their novelty value.
Two very valuable, very rare, rings can be gained by fishing. For the benefit of dwarves, these are conveniently numbered "1" and "2".
This is an ultra-rare catch from inland fishing areas in low-level zones. The bonuses from the ring are almost useless, but it does have exceptional novelty value. It is likely to sell for ten or more gold at the Auction House.
((The 1 Ring is, of course, a shameless reference to Lord of the Rings. The reason I'm confident the ring can only be found in low-level inland fishing grounds - the only place Thottbot and Wowhead record it - is because such areas best reflect the location the famous ring is found in the books.))
Epic quality and "vastly superior to The 1 Ring", The 2 Ring is one of the best rings available for hybrid classes (such as Druids and Shaman). This ring can be found within the Bags of Fishing Treasure, given as rewards for completing the Daily Fishing Quests. The 2 Ring is exceptionally rare: The daily fishing quests had been completed thousands of times before ring was even discovered: Chances are you will never find The 2 Ring. The combination of exceptional rarity and exceptional desirability probably makes The 2 Ring the most valuable traded item in Azeroth.
Cut gems can be socketed to many items of armor.
This gem can be found within the Bags of Fishing Treasure, given as rewards for completing the Daily Fishing Quests. Eye of the Sea can be traded. The gem gives the same +15 stamina bonus as the Solid Empyrean Sapphire, which values it at several hundred gold on the auction house markets.
For a long time The Ashbringer was - quite literally - a legendary 2-handed sword: Rumoured to be caught by fishing, but yet never found. The last page of Nat Pagle's Guide to Extreme Anglin' reads,
"...And so that's where you'll find the legendary sword of the Scarlet Highlord, Ashbringer. Ain't it amazin' what you run into in an ordinary day of fishin'?"
Unfortunately, only the last page of Nat Pagle's Guide to Extreme Anglin' has ever been recovered, along with copies that can't be read anymore. Since the description is in a book like this one - about fishing - people assume Ashbringer can be caught while fishing.
However, Zul'Gurub reveals Nat Pagle to be rather more adventurous than most of us. Nat's Measuring Tape can be found there, along with his Broken Reel. Any preconceptions about the man are finally shattered by Nat Pagle's Fish Terminator - "The inscription reads: Fishin's fer sissies - Nat Pagle". And it can't even be used to catch fish. Oh my! I suppose that's what happens when you drink too much Darkmoon Special Reserve while fishing.
After extensive personal research into the effects of Darkmoon Special Reserve on fishing ability, I now doubt that Mr Pagle ever caught the Ashbringer while fishing. I can't even reel in my line, and certainly cannot tell what type of fish I have caught! The Corrupted Ashbringer is found in Naxxramas. So if the Ashbringer does exist, I doubt it will be found by fishing. It has most recently been rumoured to be seen in Northrend... wherever that is.
- Fishing Gear - Describes the other items of equipment that will improve your effective fishing skill.
- Daily Fishing Quests - Complete walkthrough.
- Ashbringer.com - Lore and sightings of Ashbringer. ((Yep, this sword even has its own website.))
- Also in the Valuable Fish chapter: What's Valuable?, Buff Food, Supplies, Minipets, and Other Valuable Fished Items.