Being a 50 fisher, I can answer this. Though this information may be slightly flawed, this is what I've been told, and from what I've seen, it appears to be accurate. Gathering affects the time you have to reel in a fish before it'll take your bait and swim away; so by having a higher gathering stat, you improve the amount of time you have to hit your reel in button. Perception seems to have a smaller effect on HQ items. What it does instead is increases the max size of a fish/sea thingy. HQ stuff is determined by how big the fish you reeled in, is. So by increasing the perception stat, thereby increasing the max size of a fish you can reel out, you indirectly increase the chances of getting a HQ fish, albeit not by much.