  In order to estimate the contribution of low surface brightness (LSB)
galaxies to the local (z<0.1) galaxy number density, we performed an optical
search for LSB candidates in a 15.5 deg^2 part of the region covered by the 65
deg^2 blind Arecibo HI Strip Survey (AHISS). Object detection and galaxy
profile fitting were done with analytical algorithms. The detection efficiency
and the selection effects were evaluated using large samples of artificial
galaxies. Our final catalogue is diameter-limited and contains 306 galaxies
with diameters >18" at the limiting surface brightness of mu_{B,lim}=25.2+-0.31
mag/arcsec^2. Of these 306 galaxies, 148 were not catalogued previously. Our
results indicate that low surface brightness galaxies contribute at least to 30
% to the local galaxy number density. Without additional distance information,
choosing the limiting diameter and the surface brightness at which the diameter
is measured is crucial. Depending on these choices, diameter-limited optical
catalogues are either biased against LSB galaxies, or contaminated with
cosmologically dimmed high surface brightness galaxies, which affects the
implied surface brightness distribution. The comparison to the AHISS showed
that although optical surveys detect more galaxies per deg^2 than HI surveys,
their drawback is the need for spectroscopic follow up observations to derive
distances. Blind HI surveys have no diameter limits, but tend to miss gas-poor
galaxies and all galaxies which lie outside their redshift limits. HI and
optical surveys thus provide complementary information and sample different
parts of the LSB galaxy population.
