For a long time I've been puzzling over what these are. The are labelled as 芒鱼 (máng yú). Google does turn up a few similar looking fish, but the vast majority look nothing like these.
Today I worked out what they are!
They are about 30 cm - 51 cm / 12" - 20" long, dark skinned on the back and lighter on the belly and shiny bodied. I was fairly sure that they were a seawater species, but whether they were just a local or more widely distributed one, had no idea. One suggestion that they are caught in coastal areas around Guangdong and Hong Kong proved false.
I decided we really needed a better look at the mysterious 芒鱼 (máng yú), particularly its unusual head. Today, I looked a bit more closely. And got some stats. The fellow below is 48 cm / 19 inches from tip of nose to end of tail. It weighs 1.2 kg or 2.7 lbs before evisceration.
They are a river fish, but imported from Vietnam's Mekong Delta where the fish enjoy the brackish water, hence them being included in the seafood sections of the supermarkets. Yes it's good old pangasius / basa fish as discussed above.
Apparently, the 芒鱼 term is a Cantonese translation of the scientific name for the entire genus, although Cantonese uses the rare character 𩷶 which is not available in Mandarin.
He's now sitting in my fridge waiting to become fish and chips for tomorrow's lunch!