I think the problem is essentially that the mod can not create its required json files during the booting process, however it's set to do so which results into it crashing.
this can be fixed by using an earlyer version of the fishing overhaul and following these steps:
Download this earlyer version of the mod:
http://forums.stardewvalley.net/threads/unofficial-mod-updates.2096/post-22834
Boot up the game so it generates fish.json, fish Traits.json, treasure.json and the config.json
Close the game
remove all files except the 4 newly generated json files
place the updated mod files into the folder
edit the config and set "AllowFishOnAllFarms" to true (not sure if this is required for modfish but a user on nexus did this to add fishes to modded areas, also might wanna increase "HudMaxFishTypes" to like 20 or so, while you're at it)
boot up the game and load your save untill you're ingame.
equipt the fishing rod, tab out of the game and delete fish.json and fish Traits.json.
tab back into the game and press F5. this lets the mod recreate those files (which is the reason why it crashes during the boot, it seemingly has no issues creating them while the game is running) I'm running SVE + More Fish, and they both work. some of the SVE changes dont seem to work perfectly like catching ghostfish in the immersive farm area, but everything else works as usuall (more fish are capturable and show up in every area with their capture chance)
