Mackerel is a fish that is reknowned for it's health benefits. It is reported to help arthritis, prevent heart attacks, and according to Junji Takano, is 1500 times better for you than taking medicine. All of these findings make it a great choice for your dog. Mackerel is a perfect source of protein for animals, and as protein builds body mass it can help your dog gain weight. Similarly, mackerel has a high fat content which makes it ideal for weight gain. It also has the added benefit of making your dog's coat extra shiny.
Buy mackerel. This can be fresh from the fish market, or canned from the supermarket. Be careful what canned mackerel you choose however, as some are full of salt. Simply drain the juice from the can if this is the case. Also, choose one that is high in fat which will help with weight gain.
Mix some mackerel in with your dog's dry food. Give it as a meal.
Feed your dog mackerel once a week, or every other day, depending on how much weight they need to gain. If they don't seem to like the taste, consider putting fish oil capsules in their food instead, as mackerel is a key ingredient in fish oil.
If your dog doesn't like mackerel, or you cannot find it locally, try other "oily" fish. Salmon, tuna, herring, or any fish that is known for being high in omega-3 and omega-6 fatty acids are beneficial.
Do not feed your dog mackerel every day as too much could be detrimental. Remember, dogs need a variety of foods to balance their diet.