How much xp per hour can I expect fishing rocktails at level 90 fishing on RuneScape?
Here is a Rocktail Fishing Guide. The author, with 99 fishing and a granite lobster, was able to catch between 110 and 141 rocktails per hour.
I know you get 380 xp per fish, but I don't know how many you can catch in an hour. If you get one at least every thirty seconds, you get 45,600 xp in that hour. If you can get 3 every minute (or one every 20 seconds), you get 68,400. So, probably between those two numbers.
I get all of my runescape information from Zybez.net, but their fishing guide doesn't say much about fishing rocktails.