In this letter we demonstrate, in an elementary manner, that given a partition of the single particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored into pairs of entangled modes, similar to a BCS state. We derive expressions for the entropy and for the particle number fluctuations of a subspace of a Fermi sea, at zero and finite temperatures, and relate these by a lower bound on the entropy. As an application we investigate analytically and numerically these quantities for electrons in the lowest Landau level of a quantum Hall sample. The study of quantum many particle states, when measurements are only applied to a given subsystem, are at the heart of many questions in physics. Examples where the entropy of such subsystems is interesting range from the quantum mechanical origins of black hole entropy, where the existence of an event horizon thermalizes the field density matrix inside the black hole [1, 2] to entanglement structure of spin systems [3]. In this work we address the relation of entanglement entropy of fermions with the fluctuations in the number of fermions.A general treatment of a BCS-like factorization of a Gaussian state on a bi-partite system was carried out in [4]. Here we concentrate on a particular case and show in an elementary way how a given subspace of the single particle Hilbert space, a Fermi sea, may be factorized into pairs of entangled modes in and out of the subspace, thereby writing the state as a BCS state. We then use this construction to calculate various properties of the fermions in one of the subsystems.While upper bounds on entropy were the subject of numerous investigations, especially since Bekenstein's bound [5], lower bounds on entropy are less known.We show that given a 'Fermi sea', the entropy of the ground state, restricted to a particular subspace A of the single particle space, relates to the particle number fluctuations in the 0305-4470/06/040085+07$30.00