1.6 introduced two amazing features to make mad money from fishing.
Pic related - it's the first Summer, I didn't ship all the fish (I couldn't smoke all of them in time), and I'm making 20kg/day, even being bad at fishing.
The bait maker is unlocked by Fishing Lv6. It requires 3 iron bars, 3 coral, 1 sea urchin. You put a fish there, and get 5~10 pieces of bait targetting that fish.
But why would you waste fish to get more fish, instead of using bug meat for the bait? Because you can target the most lucrative fish - like I did with sturgeon there, if I used normal bait I'd be getting carps instead.
Then there's the fish smoker. The recipe is from Willy's and it costs 10kg. You need 10 hardwood, 1 river jelly, 1 sea jelly, 1 cave jelly.
The river and sea jellies are not a big deal to get. The hardwood will likely require luck in the mines or an upgraded axe. Cave jelly is a pain, you need to fish in the mines for a while. (Ideally at Lv100, but Lv20 does the trick.)
But oh wow. It's worth. It doubles the price of the fish, but it keeps quality. Check the iridium quality sturgeon, it's 1200g! Lucrative fish tend to be unruly, but if you fish at good spots you can reliably get gold quality fish; 1.5 from gold quality * 1.5 from angler profession * 2.0 from smoking = 4.5 times the original price.
It does require coal but once you got the cash rolling you can buy it from Clint, that's what I'm doing.
The desert and quarry are also good places.