  We searched for an optical counterpart to the first gravitational wave source
discovered by LIGO (GW150914), using a combination of the Pan-STARRS1
wide-field telescope and the PESSTO spectroscopic follow-up programme. As the
final LIGO sky maps changed during analysis, the total probability of the
source being spatially coincident with our fields was finally only 4.2 per
cent. Therefore we discuss our results primarily as a demonstration of the
survey capability of Pan-STARRS and spectroscopic capability of PESSTO. We
mapped out 442 square degrees of the northern sky region of the initial map. We
discovered 56 astrophysical transients over a period of 41 days from the
discovery of the source. Of these, 19 were spectroscopically classified and a
further 13 have host galaxy redshifts. All transients appear to be fairly
normal supernovae and AGN variability and none is obviously linked with
GW150914. We illustrate the sensitivity of our survey by defining parameterised
lightcurves with timescales of 4, 20 and 40 days and use the sensitivity of the
Pan-STARRS1 images to set limits on the luminosities of possible sources. The
Pan-STARRS1 images reach limiting magnitudes of i = 19.2, 20.0 and 20.8
respectively for the three timescales. For long timescale parameterised
lightcurves (with FWHM=~40d) we set upper limits of M_i <= -17.2 -0.9/+1.4 if
the distance to GW150914 is D = 400 +/- 200Mpc. The number of type Ia SN we
find in the survey is similar to that expected from the cosmic SN rate,
indicating a reasonably complete efficiency in recovering supernova like
transients out to D = 400 +/- 200 Mpc.
