Fishery Bulletin 104(1) 



the known laser distance. The use of automated analy- 

 sis of video images may further reduce the observer 

 error source. 



The major difficulty in measuring fish length /;; 

 situ is caused by fish mobility, which causes them to 

 be in variable orientations and positions in relation to 

 the camera, and also to be flexed. We addressed both 

 variance components together by comparing measure- 

 ments of mobile objects ("artificial fish") and rigid 

 objects. With the laser beam method, the measurement 

 standard deviation of rigid objects was estimated to 

 be 20% of the standard deviation of mobile objects 

 (95% confidence limits: 6-75%). These components 

 may be of the same order of magnitude as those for 

 fish measurements, although fish measurements could 

 not be estimated in our study because the true size 

 of the fish was unknown. An attempt to disentangle 

 both components is provided by estimating the differ- 

 ence between the precision obtained for species with 

 contrasting behaviors. Bathypterois dubius individu- 

 als lie motionless on the bottom and seldom move, 

 but because they stand on their fins they are never 

 exactly perpendicular to the camera. By contrast, L. 

 eques swims close to the bottom and tends to escape 

 when the ROV is approaching too closely. This species 

 continuously moves its tail; therefore it is very diffi- 

 cult to obtain an image with the whole body properly 

 orientated and straight. The standard deviation of B. 

 dubius length measurements was estimated to be 66% 

 of that of L. eques. This difference is smaller than the 

 difference between rigid and mobile objects above; 

 therefore we conclude that the major part of variance 

 is due to the orientation of the fish in relation to the 

 camera. Similarly, the estimated CVs of 21 species 

 grouped by motion behavior differed only slightly. This 

 is consistent with previous studies which have shown 

 that relative errors of single-camera or stereo-video 

 measurements of silhouettes or frozen fish could reach 

 10% to 30%, depending on the distance to the camera, 

 when the angle to the camera was increased from 0° to 

 60°, whereas the measurement CVs increased fourfold 

 (Harvey and Shortis, 1996; Petrell et al., 1997; Harvey 

 et al., 2002b). By contrast, error due to tail flexion 

 and muscle contractions during swimming motions 

 was estimated at -5% in a comparison of "linear" to 

 "sinusoidal" length of dorsally photographed sharks 

 (Klimley and Brown, 1983) and at 0.5% for repeated 

 stereo-video measurements of swimming tunas (Har- 

 vey et al., 2003). 



In conclusion, the major source of measurement error 

 for live fish may be their orientation and position in 

 relation to the camera. For animals that are sessile or 

 lying immobile on the ocean floor, this would be much 

 reduced if the camera and laser beams were mounted 

 vertically instead of obliquely. Thus the laser-beam 

 method may be potentially useful for measuring ben- 

 thic animals. For mobile animals, however, stereo-video 

 methods (Harvey et al., 2001; Harvey et al., 2002a; van 

 Rooij and Videler, 1996) may be more promising, and 

 are continuously improving (Harvey et al., 2003). 



Acknowledgments 



We thank all observers for taking part in the experi- 

 ment, and the ROV pilots for their willingness and skill 

 at pursuing fish and in obtaining positions suitable for 

 being measured. An anonymous referee gave very helpful 

 comments on a previous version of this manuscript. 



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