  The Daya Bay experiment has observed correlations between reactor core fuel
evolution and changes in the reactor antineutrino flux and energy spectrum.
Four antineutrino detectors in two experimental halls were used to identify 2.2
million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles
for each of six 2.9 GW$_{\textrm{th}}$ reactor cores at the Daya Bay and Ling
Ao nuclear power plants. Using detector data spanning effective $^{239}$Pu
fission fractions, $F_{239}$, from 0.25 to 0.35, Daya Bay measures an average
IBD yield, $\bar{\sigma}_f$, of $(5.90 \pm 0.13) \times 10^{-43}$
cm$^2$/fission and a fuel-dependent variation in the IBD yield,
$d\sigma_f/dF_{239}$, of $(-1.86 \pm 0.18) \times 10^{-43}$ cm$^2$/fission.
This observation rejects the hypothesis of a constant antineutrino flux as a
function of the $^{239}$Pu fission fraction at 10 standard deviations. The
variation in IBD yield was found to be energy-dependent, rejecting the
hypothesis of a constant antineutrino energy spectrum at 5.1 standard
deviations. While measurements of the evolution in the IBD spectrum show
general agreement with predictions from recent reactor models, the measured
evolution in total IBD yield disagrees with recent predictions at 3.1$\sigma$.
This discrepancy indicates that an overall deficit in measured flux with
respect to predictions does not result from equal fractional deficits from the
primary fission isotopes $^{235}$U, $^{239}$Pu, $^{238}$U, and $^{241}$Pu.
Based on measured IBD yield variations, yields of $(6.17 \pm 0.17)$ and $(4.27
\pm 0.26) \times 10^{-43}$ cm$^2$/fission have been determined for the two
dominant fission parent isotopes $^{235}$U and $^{239}$Pu. A 7.8% discrepancy
between the observed and predicted $^{235}$U yield suggests that this isotope
may be the primary contributor to the reactor antineutrino anomaly.
