As a fisher, I hardly fish at all myself anymore. I have a retainer who fishes 90% of what I use to sell. The only time I fish is to get something that my retainer can't get, or to get a fish that I haven't caught yet (your retainer can only get what you have gotten before). But you only need 1, then you can rely on your retainer.
Retainer's can get white coral at lvl 5 (which is probably what I sell most consistently on my server, but it doesn't sell that high). Then bluebell salmon at lvl 16 (sells high, but in bursts). Then lvl 26 Hammerhead sharks, which very often sell well. Then at lvl 41, you can get silver sharks, also good sellers. I tend to pop in some red corals as well which is level 45 for retainer.