I'm just posting this on here for informational purposes since I know many members supplement their dogs' diets with Salmon Oil and it's also an ingredient in many foods and treats (I had no idea how common it was until one of my dogs developed an allergy to fish - it's in about 90%+ of foods and in many treats as well). The research was done on humans, so I don't know if the findings extend to the canine world, but I thought it was something to at least be aware of.
http://www.cnn.com/2013/07/12/health/salmon-cancer-time/index.html?hpt=hp_t5
http://www.cnn.com/2013/07/12/health/salmon-cancer-time/index.html?hpt=hp_t5