As was posted by someone earlier, many items were once disenchantable at the start of this game but all that was changed.  So much of the bad data here dates back to then and that's partly why you can see some really odd things in their database showing that they disenchant into something.  For one reason or another they don't remove most of the erroneous data on here.
It is white, not gray, which means it is a "useful" item. The only "use" I have discovered for it food for hunters' pets. It can not be held in the off-hand and is not a reagent for a crafted item.
Caught one in Hinterlands. Didn't know what to do with it, so I put it on sale in AH. Somebody bought the thing! Caught a 37 lb Redgill at Bloodvenom River Outpost. Thrott says you can hold it in off hand. Why? lol....think I'll try to sell it also.
If it's such a worthless item, why can't you sell it to a vendor?  Even trash fish sell to vendors for a bit of copper.  It just doesn't make any sense to me.  At first, I thought it could be for a recipe.  Then, I asked around... no one heard of it.  Then, I tried to sell to a vendor... vendor's won't buy it!  I don't know what to do with it... I'll probably just toss it.
People open clams and stuff and their Anonymous reporting program misreports the contents as being "disenchanted" from other items.  It also happens with herb pouches, which is why you see all sorts of faction NPC's listed as dropping Morrowgrain.
No, you can't disenchat the Grouper.  Just as you can not disenchant many things that Anonymous lists.  I'm an artisan Enchanter and to be sure I just failed to disenchat a 40 Pound Grouper.  So I know what I'm talk'n about.  I'm not sure why Anonymous always list items this way.  But it almost always list "Disenchants into" on all items which is inaccurate.  Only Green items or better can be disenchanted.
I believe the reason it is listed as being disenchantable is that white items could once be disenchanted back in the day. In any case, they aren't now, so it's a moot point.
No idea what this fish is for but I just caught one. Doesn't look like it's useful for anything but feeding the pet. ^_^
It's just the way thottbot works It says that you can disenchant Lockboxes and things in thottbot disenchant means get out of it somehow, not always disenchant
You can fish any of the of the offhand fish anywhere. These items are for vanity and serve no real purpose other than "lol lewk guys i gots me a fishie!"
Actually, I believe that items show up as "disencahntable" because of an error in the data-gathering mod, or at least a version of it.
Basically, if you tried to DE something and couldn't, Anonymous would think that the next item you got was from "disenchanting" it.  Which is why clam meat shows up pretty often for most things that cant' be DE'd.