Where are the -real- Fishing improvements?

[Magistrella]

So by that logic, Botany and Miner should not have the higher yield abilities because they can make a retainer that harvests large sum of items for them.

No, by that logic its more efficient to use a FSH retainer because you only can fish a certain species out of a pool of many - unlike BTN and MIN that can target what they want to get specifically. The same way it works irl, yknow?