There's been some discussion on backing with sturgeon and wolf fish recently. I guess only certain types of fish can be used? What should I look for? I believe some types, like eel, can be used as a structural element as an alternative to rawhide, but others are used more as decoration. What types work well, what should I look for/avoid. I really want to know about species I might find in a fish mongers, for instance, can I use things like monkfish, Pollock or hake?