Fishery Bulletin 104(1) 



232 mm 



232 mm 



Figure 1 



The four laser pointers mounted on a crown around 

 the main camera in front of the ROV Victor 6000. 

 The inner circle is the camera lens. 



Figure 2 



Laser spots (indicated by arrows) visible on and under a fish. 

 These laser spots documented on videotape provided size infor- 

 mation both in real time and during video replay. 



ber 2002.^ Victor 6000, a remotely operated vehicle 

 (ROV) equipped with several video cameras and re- 

 corders, was operated at depths ranging from 1100 to 

 1500 m. Fish size was measured both by using a pair 

 of parallel laser beams, and an auto-focus video camera 

 linked to software for estimating object size based on 

 the focal distance of the object in focus. In this study 

 three sources of measurement variability were investi- 

 gated: 1) systematic errors inherent to each method; 2) 

 variability due to observer differences; 3) variability due 

 to continuous fish movements and horizontal body orien- 

 tation. To estimate these error components separately, 

 rigid and articulated artificial objects ("artificial fish") 

 of known size were measured repeatedly by several 

 independent observers. Individuals belonging to several 

 deep-sea fish species were also repeatedly measured. 

 Mixed-effects models and heteroscedastic error models 

 were fitted to the resulting measurements to compare 

 the magnitude of errors due to different sources. 



Materials and methods 



Measurement devices 



Both measurement devices were installed close to each 

 other on the ROV Victor. The laser-beam crown was 

 mounted on the main camera, which was itself attached 



1 Trenkel, V. M., N. Bailly. O. Berthele, O. Brosseau, R. Causse, 

 F. de Corbiere, O. Dugornay, A. Ferrant, J. D. M. ordon, D. 

 Latrouite, D. Le Piver, B. Kergoat, P. Lorance, S. Mahevas, 

 B. Mesnil, J.-C. Poulard, M.-J. Rochet, D. Tracey, J.-P. Vach- 

 erot, G, Veron, and H. Zibrowius. 2002. First results of a 

 quantitative study of deep-sea fish on the continental slope of 

 the Bay of Biscav: visual observations and trawling. ICES 

 CM 20b2/L:18, 2002, 15 p. 



to the pan and tilt unit. The METRAU© (SONY, model 

 FCB-IX 47P) autofocus camera was mounted on the 

 same pan and tilt unit. 



Laser-beam pointers Four red laser pointers (10 mW, 

 635 nanometers [nm]) were mounted around the main 

 camera housing (Fig. 1). The distance between each 

 two opposite lasers was 232 mm. Red light is strongly 

 attenuated by water but because of the relatively high 

 power of the laser light-emitting diode (LED), a range 

 of up to 7 m is reachable in clear waters. 



To measure the fish and objects, the laser beams were 

 projected on the target (Fig. 2). The laser spots, visible 

 on the video, give size information both in real time and 

 during video replay. The principle is simple, but several 

 limitations exist. First, the measurement is correct only 

 for an object located in a plane perpendicular to the 

 laser axis. Second, the target should be large enough to 

 be reached by at least two laser beams; the more laser 

 impacts that are seen on the object to be measured, the 

 easier the measurement. 



For the measurement to be accurate, there must be 

 a strict parallelism between the laser beams. This is 

 complicated by the fact that the laser component itself 

 (the diode with its optic lens) does not necessarily have 

 a beam parallel to the axis of the component package. 

 Further, designing an accurate alignment mechanism 

 that is compatible with offshore and deep underwater 

 operating conditions is difficult. The residual error af- 

 ter alignment is about 0.15°, which entails an error of 

 10 mm for the distance between two opposite spots at 

 a distance of 4 meters (i.e., 4% of size). 



METRAU camera The METRAU system is based on 

 the autofocus video camera. The imaging device is an 

 original equipment manufacturer (OEM) camera module 

 similar to those used in off-the-shelf camcorders. The 



