If you landfill over the area below the trees, fish will spawn in the landfilled area

Probably should make it so fish only spawn in water and also prevent landfilling the last edge of tiles on the map

Preventing land-filling edge of map would always allow fish to spawn around the island. Could make the map 2 tiles more wide and prevent building in that last tile, filled with water, that way fish always have a place to spawn.

Also, might want to check before spawning trees and gardens that the patch of sand/dirt that it's growing on is not covered with a building, and/or with any entity.

Also check if the tile is covered with brick.

Honestly I would rather find a way to prevent landfill (and I guess brick?) from being placed on the water tiles - I specifically disabled landfill production here, but I guess you managed to find a way of getting landfill in-game anyway.