In my days as a Rav haMachshir [head of a kosher supervision agency], I heard quite a few people say they could buy filleted, skinned fish at the fish counter at their local supermarket, trusting the store management to be honest because of the risk they would run in playing games.
Of course, even if the fish is the right fish, there still are problems from non-kosher fish residue on the equipment, and from the common practice of leaving all of the fish in buckets overnight, leaking brine on each other.
But trusting the fish counter is, itself, problematic. This in from ABC News this past week:
Investigation Uncovers Rampant Fish Fraud
A new investigation provides fresh evidence that restaurants and markets continue to dupe seafood lovers into paying top dollar for low-grade fish.
As part of a special “Fishy Business” series, the Boston Globe spent five months buying fish from dozens of establishments throughout Massachusetts and sending the samples off to a lab in Canada. DNA tests found 48 percent of the fish had been mislabeled as a more expensive type of fish.
Fish samples were gathered from 134 restaurants, grocery stores and seafood markets, and the results were staggering. Every one of 23 white tuna samples tested turned out to be something other than tuna. In most cases the fish labeled tuna was escolar, which the Globe said was “nicknamed the Ex-Lax of fish by some in the industry for the digestion problems it can cause.”
Milk remains unique in its acceptability without kosher supervision in North America, because there is no incentive for milk producers and distributors to cheat - it would be ridiculously counterproductive for them to raise pigs and milk them. But many other products, including products people take for granted, are often adulterated or outright switched with inexpensive substitutes. Hence the need for hashgachah.