Modeling statistical performance of an airborne lidar survey system for anchovy

The objective of our study was to model the performance of an airborne lida r survey system for northern anchovy in terms of survey accuracy and precis ion. Our analyses indicated that swath width would have little or no effect on the probability that at least one fish school would be encountered. In typical coastal waters off California (attenuation coefficient=0.1/m), abou t half of the schools were detected by the lidar during the day and about 6 4% during the night. A greater proportion of schools were detected during t he night because anchovy have a shallow vertical distribution, whereas in t he day, schools may extend down to 155 m; schools below about 40 m depth we re not detectable to the laser. Although schools tended to be more diffuse during the night than during the day, even the very diffuse schools of anch ovy (0.5 fish/m(3)) were detectable at night throughout the upper 20 m of t he water column with a lidar. With a substantial increase in instrument and survey costs, it would be possible to increase the equivalent laser-pulsed power by a factor of 10 over that of the "off-the-shelf system," as used i n our model. Such a change would increase the maximum detection depth of th e lidar system by about 10 m but would have a negligible effect on the prob ability of detecting schools during the day owing to the skewed vertical di stribution of anchovy schools. More effective approaches for improving the accuracy and precision of potential lidar surveys for fisheries would be to improve school detection algorithms and to develop a lidar survey model ba sed on line transect theory to obtain an unbiased estimate of abundance. To produce an accurate reconstruction of the average vertical distribution of schools for a particular season and region, a synthesis of acoustic and li dar surveys of school distribution is required.