reduce waste

To reduce your carbon footprint, think about the foods you eat: Stick to seasonal fruit and vegetables - they will need less watering and artificial heat to grow - and focus on local produce that doesn't require much air travel and cut down on products such as meat that require a lot of water and release a large amount of greenhouse gases in the production process.
So you cooked the fish, ate your fill and still have some left over. Unlike chicken, turkey beef and lamb, ideas for fish leftovers are sparse. One tip is to think of cooked fish liked tinned fish (i.e. tuna, salmon). Suddenly using it up seems much easier.