  We present equivalent widths of the [OII] and Ha nebular emission lines for
77 brightest cluster galaxies (BCGs) selected from the 160 Square Degree
$ROSAT$ X-ray survey. We find no [OII] or Ha emission stronger than -15
angstroms or -5 angstroms, respectively, in any BCG. The corresponding emission
line luminosities lie below 6E40 erg/s, which is a factor of 30 below that of
NGC1275 in the Perseus cluster. A comparison to the detection frequency of
nebular emission in BCGs lying at redshifts above z = 0.35 drawn from the
Brightest Cluster Survey (Crawford et al. 1999) indicates that we should have
detected roughly one dozen emission-line galaxies, assuming the two surveys are
selecting similar clusters in the X-ray luminosity range 10E42 erg/s to 10E45
erg/s. The absence of luminous nebular emission (ie., Perseus-like systems) in
our sample is consistent with an increase in the number density of {\it strong}
cooling flow (cooling core) clusters between $\rm z=0.5$ and today. The decline
in their numbers at higher redshift could be due to cluster mergers and AGN
heating.
