Sources disagree on whether dogs can eat fish safely. K9 Magazine says that fish are a high-quality source of omega-3 fatty acids, while Dr. Karen Becker says that most fish in the pet food industry is preserved using carcinogenic ethoxyquin and should be avoided.
Other concerns with feeding fish to dogs include mercury poisoning and thiamine deficiency. Feeding salmon to dogs may cause salmon poisoning disease, which can be fatal and occurs when a pet ingests the parasite Neorickettsia helminthoeca. These risk factors are highest when feeding a dog a mostly fish-based diet, though the risks depend on the source and type of fish.