Well that is intentional done in the STORY MODE to make the game easier for no one to block at that section and enjoy the rest of the game, pls check the arcade mode and you will see that trash will spawn at the top too, and you will see some dead fish too.
In the story mode the fish is meant to die to show the pollution.
The fisherman HAS a target probally you miss it but it says that you need to gather 9000$, the upgrades will help you gather this very fast!
You can check it again and try arcade mode you will see there the game is harder and more chalenging.
I will highly sugest you to try arcade mode, some people enjoy that section more than story mode!