Pla Tuu to Thai people is like sausage to German people; every one eats it and it is one of the most common dishes eaten at home. They taste similar to mackerel and look kind of like a small mackerel too. They are about 5 - 7 inches long and have a flat body. The back is bluish silver with silver belly.
These little fish you will find at an Asian grocery in the frozen food section. There are 3 or 4 in a flat package. If you are making Nam Prik Pla Tuu get the steamed version. The fresh would be used for other dishes.
There was a translation problem when the fish first was packaged for export, but the name stuck. "Steamed mackerel" is not actually steamed, but dipped quickly into boiling salted water to preserve the fish for transportation. The term 'nueng' literally means steamed. However, this 'Nueng' is shortened from 'thy nueng' which does not mean steamed but cooked from heat and preserved.
Thai cats also frequently eat rice mixed with leftover pla tuu head and bones.