I had them at a restaurant years back. As I recall they were simply fried in a flour and seasoning mixture. Tasted a bit like Smelt (Small fish). Not bad, but IMO not worth the extra $$. Smelt is not usually too hard to find around here, especially in season.
This reminds me that I REALLY need to go out on a smelt dipping trip one day. I know my grandpa used to do it every year. I remember cleaning them suckers when dad brought them home.