264 



Abstract.— The objective of our study 

 was to model the performance of an air- 

 borne lidar survey system for northern 

 anchovy in terms of survey accuracy 

 and precision. 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/ml, about 

 half of the schools were detected by the 

 lidar during the day and about 64'7r 

 during the night. A greater proportion of 

 schools were detected during the night 

 because anchovy have a shallow ver- 

 tical distribution, whereas in the day, 

 schools may extend down to 155 m; 

 schools below about 40 m depth were 

 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 anchovy 

 (0.5 flshym') were detectable at night 

 throughout the upper 20 m of the water 

 column with a lidar With a substan- 

 tial 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 in our model. Such a 

 change would increase the ma.xinium 

 detection depth of the lidar system by 

 about 10 m but would have a negligi- 

 ble effect on the probability of detect- 

 ing schools during the day owing to the 

 skewed vertical distribution 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 based 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 acous- 

 tic and lidar surveys of school distribu- 

 tion is required. 



Modeling statistical performance of an 

 airborne lidar survey system for anchovy 



Nancy C. H. Lo 



John R. Hunter 



Southwest Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 P.O. Box 271, La Jolla, California 92038 

 E-mail address (for N.C.H Lo) Nancy Loanoaa gov 



James H. Churnside 



Ocean Remote Sensing Division 

 Environmental Technology Laboratory 

 325 Broadway. Boulder, Colorado 80303 



Manuscript accepted 18 November 1999. 

 Fish. Bull. 98:264-282 (2000). 



Airborne lidar sui-veys are an at- 

 tractive alternative to the methods 

 presently used in fishery-independent 

 surveys of epipelagic fishes (Hunter 

 and Churnside^. They would cost 

 much less per survey mile than 

 ship-based methods ( acoustic-trawl, 

 ichthyoplankton), and the survey 

 would extend to greater depths than 

 present aerial methods. A lidar, 

 (li[ght| d[etecting] and r[angingj) 

 system, in its most basic form, pro- 

 duces short pulses of laser light 

 that pass through the water sur- 

 face and reflect off fish and particles 

 in the water; a receiver measures 

 the returning reflected pulse; the 

 strength of the returning pulse sep- 

 arates fish targets from small par- 

 ticles, and the elapsed time from 

 start to return of pulses indicates 

 the range (depth below the sur- 

 face) of the target. The application 

 of lidar technology to fishery sur- 

 veys is still in its infancy. Fish 

 schools have been detected with a 

 variety of lidar systems (Churnside 

 and Hunter, 1996 1, but schools have 

 never been systematically studied 

 with lidar, nor has existing lidar 

 technology been adapted to fish- 

 survey needs; formal fish surveys 

 have never been conducted. 



A lidar survey system for fishery- 

 independent monitoring of epipelagic 

 fish stocks is being developed jointly 



by two laboratories of the National 

 Oceanic Atmospheric Administra- 

 tion (NOAA): Environmental Tech- 

 nical Laboratory, Boulder, CO; and 

 Southwest Fisheries Science Center, 

 La Jolla, CA. The approach is to 

 combine evaluations of prototype 

 instruments at sea with modeling of 

 survey performance to develop an 

 optimal lidar survey system. The 

 goal is to develop a system that will 

 deliver the greatest statistical preci- 

 sion for the lowest survey cost, while 

 minimizing potential biases. In our 

 study, we modeled a lidar survey 

 with the objective of evaluating how 

 instruments would affect survey 

 precision or accuracy. Two classes of 

 instrument design were considered, 

 those affecting fish schools in the 

 horizontal plane (swath width) and 

 those affecting the detection offish 

 schools in the vertical plane ( depth- 

 specific detection ). We also analyzed 

 a trade-off between swath width 

 and penetration depth, which is 

 analogous to changing from a visual- 

 based aerial sui-vey (wide swath, 

 shallow penetration ) to a lidar-based 



Hunter, J. R., and J. H. Churnside, eds 

 1995. Airborne fishery assessment tech- 

 nology — a NOAA workshop report. SWFSC 

 Admin. Rep. U-95-02. 33 p. Southwe.st 

 Fish. Sci. Ctr, NMFS, NOAA, P.O. Box 

 271,La Jolla. CA 92038. 



