In places like Bangladesh where prawns are almost a staple food, it is said that they are halaal on the basis of Urf.

Answer

Urf (Customary Practice) can never make halaal what the Shariah has made haraam. The Urf here in South Africa is to consume halaalized carrion chickens and halaalized carrion meat. Obviously the Urf does not make it halaal. There are many haraam deeds which have become customary. However, these acts remain haraam regardless of how prevalent they become. Since the hurmat (prohibition/being haraam) of ghairsamak (that which is not fish) is Mansoos for the Ahnaaf, the Urf cannot halaalize it. In Bangladesh and elsewhere they consume shrimps/prawns by self-deception. They do not say that it is halaal because of urf. They say that prawns are fish, hence halaal. For eating prawns, they select to become temporary morons, hence they argue abortively that prawns are fish.