In my experience potato doesn't suit all dogs - I know Burns had problems with their Potato recipe when it first came out (I think it was something to do with Calcium content but not sure!)
If you like Salmon and want a natural food then Whites Premium do a great new recipe which isn't as expensive:

Ingredients: Fresh Salmon, White Rice, Oats, Peas, Salmon Oil, Hydrolysed Salmon Liver, Sunflower Oil, Seaweed, Tocopherol Rich Extracts, Minerals and Vitamins

Analysis: Protein 18.5%, Oil 9%, Fibre 2%

Check it out at www.pet-pantry.co.uk