In the Name of Allah, the Inspirer of Truth. Assalamu Alaikum Warahmatullahi Wabarakatuh
There is no slaughtering method for fish unlike other animals. An exception to the rule i.e. a dispensation bestowed on us by Allah: "Two carions are made Halaal for us; Fish and locust." (Mishkaat vol.2 pg.361).
King prawns would fall into the same ruling as prawns.
Please see the following link(s):
The Jurisprudence of Halaal and Haraam Animals: http://www.gmwa.org.uk/foodguide2/index.php?page=viewquestion&id=112
