They do list their list of ingredients by volume or weight in the order of from most to least in fish food as well. When I see "fish meal" on the first, it is a big no no for me. I like to see "whole salmon" and "whole krill" on the top lists. Hey, I would eat that too haha !
I have not seen "byproducts" on any fish food.
"Fish meal" is mostly fish skin and organs etc. leftover. No where as "whole", but I doubt you will see "whole pig", "whole cow", "whole chicken", on the labels of dog food and cat food.
Do you also have any thing like "enhanced vitamins" in dog food and cat food? It is a very important part of fish food since in the wild even the predator fish still have diet of veggie when they swallow veggie eating fish whole.