If it's reducible that means you can use aetherial reduction on it (something new for gatherers added in HW) to turn it into usually crystals/ clusters and occasionally aethersand (of varying qualities -- though from fish I think it's usually waterborn aethersand or something). You only get aethersands from something with a very high collectibility rating, though I haven't done it on FSH yet. They're used for a few level 60 recipes, but I don't recall if they're worth much anymore.

EDIT: oh, also yes if something is reducible then you can definitely gather it. I'm not sure where you were looking at it or anything, but as I said reduction is something only gatherers can do on collectible items that they gathered.