You can use content patcher and change the crab pot recipe, but I believe after learning a certain profession it changes the crab pot recipe again which might be done by code instead of a content patcher.
To create a content Patcher Mod
Create a folder of your mod "[CP] NameOfYourContentPatch" then create 2 files inside of this folder include 2 files (manifest.json, content.josn) and also include 1 folder called "Assets" and add a file in assets called "Fish.json"
Folder structure should look like this
[CP] NameOfYourContentPatch
content.json
manifest.json
assets
Fish.json
Manifest.json should include what is listed below and edit the name author and unique ID what what you want for it
Later you can add update keys which is used by mod managers or SMAPI to determine when you have a newer version out. For update key inside the brackets you can include for example
["Nexus:ReplaceThisYourNextModID"] when you create a mod on nexus mod it creates a URL that has a number
https://www.nexusmods.com/stardewvalley/mods/32012 32012 would be the nexus mode ID and so if this was your mode you can update your manifest updatekeys to : "UpdateKeys": ["Nexus:32012"],
Content.json you can copy the code below and save, which shows we plan to modify and "Edit" a crafting recipe for Crab Pot
Next you can then edit the text "Value": "388 40 335 3/Home/710/false/Fishing 3/" and edit this.
"388 40 335 3" section controls the crafting recipe required items where -> [FirstItemID] [number of First ItemID needed] [SecondItemID] [Number of Second ItemID needed] and so on.. and so in this actual code we know we need
item 388(which is wood) amount of 40, and itemid 335(iron bar) amount of 3. You look up item id online Here: https://mateusaquino.github.io/stardewids/
You can also choose to edit the out section which currently contains 710
"710" which is the output item and item amount in this case 710 is crab pot, you can change this to "710 4" to give you 4 crabpot when you craft instead of 1 or change the item complete by swapping item ID with something else.
Last you can copy and paste these into the Fish.json
You'll need to look at the fish data above to see what you'll want to change above you'll notice that all the trapped fishes like crab
"717": "Crab/trap/.1/684 .45/ocean/1/20/false",
contains the /trap/.1/ after the first slash, and so I suspect if you modify any of the other fish values after first slash to "trap" the fish becomes trappable, and the number after the 2nd slash i suspect would be the %chance that the fish can be caught in that trap.