Iibis wrote: I got 4 coins from 4 kills, but don't know if it's a 100% drop or not.
What luck you have! I can say with certainty that it's not 100% droprate. I fish up quite a few green ones from the garrison each week to restock on bait and the time-duration hat/pole, but I don't always receive a coin from the mob that spawns.
I don't have the numbers or any significant testing to back this so it's purely anecdotal, but I do seem to catch lunkers more regularly with higher fishing (upwards of 1100+).
For me, the best places with the most consistent yield: SMV and Spires of Arak. I don't know if this is a contributing factor or not, but whenever there were multiple people fishing near/in the same waters I saw a better lunker catch rate. Could have just been coincidence too.