Fish like Salmon are a rich source of Omega-3 fatty acid, one of the primary types of essential fatty acids (EFA's). Essential fatty acids are considered "essential" because they are the main structural components of the body's cell membranes, but cannot be synthesized by the body and must be obtained from the diet.
Salmon oil supplements are available as liquid oil, and salmon is also an ingredient in a number of grain-free dry foods, like Taste of the Wild, as well as some canned, wet dog foods. There are also a variety of "salmon bites" dog treats.
A 2013 study on cats found that salmon oil supplementation has "the ability to modulate the immune system of healthy cats possibly resulting in a greater ability to fight infection and disease."