There was an issue that rose in Italy more or less a decade ago about this very subject. I don't know if the following statement was based on real data or not, but some preservationist groups warned the population about imported tuna cans. They said that more than half of those cans didn't contain real tuna but other kind of aquatic species, mainly sharks and dolphins.
This fact caused a strong reaction from the population and had a very wide echo inside the local medias. The selling of canned tuna dropped for several months despite the efforts by the local producers to put a stop to these uncontrolled rumors.
In particular, the reference to dolphins was striking for the local consumers. Even if, for what I remember, no real proof was ever showed about this being the truth.
Edited: Bruconero on 7th Mar, 2013 - 2:40am