DREDGE

Trawl nets are fantastic to make extra money fishing as you are traveling from one island to another. You will only need to specifically use nets to catch three varieties of fauna in the game (Aurora Jellyfish, Anchovy and Scarlet Prawn - the Prawn can be caught in a crab Pot but hard to tell where these are located as they are in the Open Ocean) Otherwise, dropping your Trawl Net whenever you are moving your boat in the water will gather fish as you move so you can sell them off at your next visit to the Traveling Merchant or Fishmonger.

To fit your boat up with a Trawl Net you will need to first upgrade 4 of your Rod spaces to also support a Trawl Net by going into the Dry Dock in Greater Marrow or any of the four Floating Docks operated by the Traveling Merchant, depositing two cloth, one lumber and $125. This will allow the four squares on the left side of your cargo hull to be used for Rods or Trawl Nets interchangeably.  Next, you simply need to buy and install any Net (The first one will be available directly after making this upgrade and costs $250). Don't worry about the price, because using a net adds a useful amount of passive income while you are moving about the map. Trawl Nets just need repairs every now and again, but fish stay fresh in the submerged net and there is a lot of space to fill without any labor on your part.

Make sure to drop your Net in the water when you are traveling anywhere and you should collect 150 items before the end of your time with Dredge.