I find Frilled Shark to be surprisingly easy to catch. Despite all the HQs needed for the triple-mooch, the catches are 100%, 100%, 75%, and unclear (40%+ - hard to datamine due to weather and time restrictions, but it sure feels like ~75%). If you assume a 20% chance to HQ, that's a 0.2*0.2*0.2*0.75*0.75 = 0.45% chance per cast. So, on average, ~200 casts for a catch. If each cast takes on average 5 Eorzean minutes (15 seconds - a high estimate), that's 12 casts per hour, 10 hours per window (food expires after 10 Eorzean hours anyway), for 120 casts per window. So, that gives about a 50% chance per window to catch one. The windows are pretty common, from what I've seen. I often catch two per window.