I don't know what I had for lunch.
In the supermarket they were labelled 黄尾鱼 which means yellow tail fish. That is only slightly more helpful than just 'fish', but with their being literally dozens of fish varieties called yellowtail, I'm still don't know what they are.
But I bought some anyway. When the woman weighed and priced my selection, I noticed that the barcode sticker she printed read "金丝鱼“ or "gold thread fish". Again this name appears to be given to many different species, none of which look like these.
Whatever they are they made a nice lunch. I gutted them, salted them then deep fried them whole. In less than a minute they were good to go.
A bit of lemon juice and I had a very satisfying lunch with some home made bread and butter on the side. I ate six of the critters. They are only 16cm / 6.3 inches long.