They're laundering illegally caught fish.
A minimum of 20% of the global fishing catch is associated with illegal, unreported and unregulated fishing. Imagine how many thousands and thousands of tonnes of fish that is. All of that needs to be laundered into supply chains, and it happens through mislabelling fish along the way.
Tied to illegal fishing is slavery in seafood supply chains. We'd be happy to present a submission that we put forward to your colleagues a year and a half ago, who were studying supply chains and responsible enterprises. Slavery within the seafood supply chain is very well documented, so that's another reason you wouldn't want to eat a mislabelled fish.
As well, the last 75% of the mislabelled samples that we collected were for more expensive fish. Most of the time, you're just being ripped off. From a financial standpoint, if you're paying for coho salmon, which is quite a bit more expensive than Atlantic salmon, you want to make sure you get that. If you're paying for Chilean sea bass, you don't want that to be a cheap whitefish.