Why do the police dub Jack Salmon Mr. Fish and his Huckleberry Hound in The Lovely Bones?

This title is first
applied to Jack Salmon by the police in Chapter Eleven, which charts the way that the police,
and in particular Len Fenerman, become increasingly concerned about the way in which Jack Salmon
becomes so fixated on the guilt of his neighbour, Mr. Harvey, in relation to his daughter's
disappearance. Note what the "final...