minecraft-assets/assets/minecraft/loot_tables/gameplay/fishing/fish.json

{
    "pools": [
        {
            "rolls": 1,
            "entries": [
                {
                    "type": "item",
                    "name": "minecraft:fish",
                    "functions": [
                        {
                            "function": "set_data",
                            "data": 0
                        }
                    ],
                    "weight": 60
                },
                {
                    "type": "item",
                    "name": "minecraft:fish",
                    "functions": [
                        {
                            "function": "set_data",
                            "data": 1
                        }
                    ],
                    "weight": 25
                },
                {
                    "type": "item",
                    "name": "minecraft:fish",
                    "functions": [
                        {
                            "function": "set_data",
                            "data": 2
                        }
                    ],
                    "weight": 2
                },
                {
                    "type": "item",
                    "name": "minecraft:fish",
                    "functions": [
                        {
                            "function": "set_data",
                            "data": 3
                        }
                    ],
                    "weight": 13
                }
            ]
        }
    ]
}