  In many theories of quantum gravity quantum fluctuations of spacetime may
serve as an environment for decoherence. Here we study quantum-gravitational
decoherence of high energy astrophysical neutrinos in the presence of fermionic
dark sectors and for a realistic three neutrino scenario. We show how violation
of global symmetries expected to arise in quantum gravitational interactions
provides a possibility to pin down the number of dark matter fermions in the
universe. Furthermore, we predict the expected total neutrino flux and flavor
ratios at experiments depending on the flavor composition at the source.
