  We derive a purely fermionic no-sea effective theory, featuring
positive-energy states only for the Walecka model. In contrast to the so-called
mean-field theory approach with the no-sea approximation, where the Dirac sea
is simply omitted from the outset, we turn to the relativistic Hartree
approximation and explicitly construct a no-sea effective theory from the
underlying quantum field theory. Several results obtained within these two
approaches are confronted with each other. This sheds new light on the
reliability of the mean-field theory with the no-sea approximation as well as
the role of the Dirac sea. Restricting to 1+1 dimensions, we obtain new
analytical insights into nonuniform nuclear matter.
