Just caught one of these for a 0.1 skill up at lvl 19 fishing. So the skill cap is incorrect and may be in the mid to upper 20's. I have no idea what this thing is good for. I'm a Hume, so I can't eat the thing, plus there are no listings for crafting atm.
Used Halcyon Rod + Crayfish Ball.
Sell to NPC for around 475gil in Jeuno.
Ratio on catches with Crayfish Balls:
50% Yellow Globe
42% Gurnard
8% Cobalt Jellyfish
(these results were according to one stack of crayfish balls before changing to Lugworms.)