1. The unnamed fish are the ones called "fish" and stop coming up from fishing completely before you hit GM (around 85-90 skill?). Having been beyond the ability to catch them when HS was released, I don't know how much they weigh now (used to be 1 each before HS), and since the HS release there have been stacking issues (did they ever fix them to stack again, or do we need to report that in the Bug Reporting forum?)
2. The unnamed crustaceans are "Crab" or "Lobster". I believe these all weigh 1 stone, unlike the named versions.
3. The Quest Fish all weigh 10 stone, and stack with others of their name. Yes, Pike is a named (shallow water) fish. There are 12 named shallow fish for quests, 18 named deep fish for quests, 12 named dungeon fish for quests, and 12 named crustaceans for quests (6 crab, 6 lobster).
4. The Quest crustaceans all weigh 5 stone, and stack with others of their name.
5. The Rare and Legendary fish & crustaceans (the recipe ones, and the ones that can be mounted) all have variable weights, which rarely happen to coincide with 10 stone (and ones of this type over 20 stone display their fisher, date caught, and their weight). They DO NOT stack. They tend to be colors that make those that complain about neon weapons and dyes go into gibbering fits.
6. Standard Big Fish, Red Herring and Mud Puppy (the latter two quest big fish from the Underworld) cut up into normal fish steaks based on their weight.
7. The Recipe fish cut up into steaks that retain their fish type, also by weight.