Ugh, I had trouble with this one too. :c
I don't think it's possible to catch one off the island coast, so fish in the village ocean. And I'm pretty sure you can only catch it past 9 PM. It's about the size of a horse mackerel or dab.
I think I've caught it...I'm having more trouble getting the stringfish than anything. ;_;
Figures, out of all the river fish, Merengue would ask for one that's worth 15,000.