Particularly low grade, highly contentious or excessively vague ingredient we usually recommend avoiding
Fish and fish derivatives in dog food
The EC defines 'fish and fish derivatives' as "Fish or parts of fish, fresh or preserved by appropriate treatment, and derivatives from the processing thereof".
Although broad terms like this attract a lot of criticism, it must be noted that they do not necessarily indicate the presence of low grade ingredients. Fish and fish derivatives for example encompasses all fish products, from the very best to the very worst. For example, some producers of good foods use broad terms in order to not give away their 'secret formula' and some imported foods list very high quality fish as 'fish and fish derivatives' simply because the laws or customs in their home country are different. At the opposite end of the scale, the term can be used for very low-grade fish derivatives including some that are nutritionally very poor. Also, because the type of fish isn't specified, manufacturers using this term are able to change their fish source between batches depending on what is available at the time.
Because it is impossible to know what this refers to, we would suggest assuming the worst and avoiding foods with this ingredient. This is particularly important if your dog is prone to dietary intolerance as identifying and eliminating problem ingredients is impossible unless you know exactly what you are feeding.