Catfish
Large freshwater fish with firm, white, slightly fatty flesh and very few bones.
Catfish is a freshwater fish, mostly farmed in Central Europe and North America; its flesh is firm, moist, mild and nearly boneless, so it holds together when braised. In North America it is common in supermarkets, while in German-speaking countries it is found at fishmongers or larger fish counters, often only to order, and frozen by mail order.
Substitute:
Pike-perch (zander) or carp are the closest substitutes, pike-perch being leaner and more delicate, carp stronger in flavour and bonier; monkfish matches the firmness. Pangasius is also a catfish but softer and tends to fall apart in braises.