First of all, as of July 15, 2019, fish achievements seem to be popping at 150 fish caught. This may change.
Admittedly, I haven't gotten this yet, and may not play long enough to do so, but here are some tips I've found.
Many fish can be farmed at the Lone Star Lake, which has no travel cost so it is ideal for a lot of early fishing and farming for credits. You also don't have to have a larger keepnet since you don't have to pay to return to the lake.
I've caught about 50 crappies so far by standing on or near the pier and used the default telescopic rod that you get with the tutorial. This will take you quite a ways, and work for panfish as well. I have had success using red worms for crappies.
A suggestion: buy the Texas advanced license so you don't have to throw back any trophy fish. You can get fined if you don't throw back bigger fish with only a basic license.
Hope this helps!