Abstract — During the VITAL cruise 

 in the Bay of Biscay in summer 2002, 

 two devices for measuring the length 

 of swimming fish were tested: 1) a 

 mechanical crown that emitted a pair 

 of parallel laser beams and that was 

 mounted on the main camera and 

 2 1 an underwater auto-focus video 

 camera. The precision and accuracy 

 of these devices were compared and 

 the various sources of measurement 

 errors were estimated by repeatedly 

 measuring fixed and mobile objects 

 and live fish. It was found that fish 

 mobility is the main source of error 

 for these devices because they require 

 that the objects to be measured are 

 perpendicular to the field of vision. 

 The best performance was obtained 

 with the laser method where a video- 

 replay of laser spots (projected on 

 fish bodies) carrying real-time size 

 information was used. The auto-focus 

 system performed poorly because of a 

 delay in obtaining focus and because 

 of some technical problems. 



Precision and accuracy of fish length measurements 

 obtained with two visual underwater methods 



Marie-Joelle Rochet 



Ldboratoire MAERHA, IFREMER 

 Rue de nie d'Yeu, B.P 21105 

 44311 Nantes, Cedex 03, France 

 E-mail address mirochetifiifremerfr 



Jean-Francois Cadiou 



IFREMER (DNIS/SM/IM)-Centre Mediterranee 



B P 330 



83507 La Seyne, France 



Verena M. Trenkel 



Laboratoire MAERHA, IFREMER 

 Ruede llle d Yeu, BP 21105 

 44311 Nantes, Cedex 03, France 



Manuscript submitted 22 July 2003 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 20 April 2005 by the Scientific Editor. 



Fish. Bull. 104:1-9(20061. 



Visual sampling of marine systems by 

 SCUBA divers and underwater vehi- 

 cles is increasingly used to estimate 

 animal abundances, to observe natu- 

 ral behavior and response behavior 

 to fishing gear in situ, and to assess 

 community interactions (e.g., Bublitz, 

 1996; Auster et al., 1997; Davis et al., 

 1997; Uiblein et al., 2002; Trenkel et 

 al., 2004). Visual methods also allow 

 estimates of population-size struc- 

 tures without the bias caused by the 

 size selectivity of fishing gear. Visual 

 techniques have been used in the wild 

 for measuring the length of animals by 

 SCUBA divers (e.g., Yoshihara, 1997; 

 Pfister and Goulet, 1999; Harvey et 

 al., 2002a) or by submersibles (Love 

 et al., 2000; Yoklavich et al., 2000). 

 They have also been employed for esti- 

 mating the length frequency of the 

 catch of live tuna to be fattened after 

 capture (Harvey et al., 2003), and in 

 aquaculture to estimate the size range 

 offish (Petrell et al., 1997). Until now 

 these techniques were mainly used 

 in shallow waters or tanks. Because 

 of the optical characteristics of sea 

 water — its turbidity, the variations in 

 light intensity with depth and water 

 movements and fish movements, these 

 methods are subject to measurement 

 errors. Estimating the order of mag- 

 nitude of this measurement error has 

 been the focus of many studies (van 



Rooij and Videler, 1996; Yoshihara, 

 1997; Harvey et al., 2001, 2002a, 

 2002b, 2003). 



Efficient methods for measuring 

 fish length in situ can also be used 

 in deeper waters not accessible to 

 divers. Parallel laser projected from 

 a video camera onto the seafioor or 

 fish bodies permit accurate measure- 

 ments (Love et al., 2000; Yoklavich et 

 al., 2000). Albert et al. (2003) mea- 

 sured fish lengths on a video screen 

 and then transformed these mea- 

 surements into real length knowing 

 the distance of the camera from the 

 ground, its tilt angle, and the hori- 

 zontal opening angle of the camera. If 

 fish are not on or close to the bottom, 

 it is necessary to know their distance 

 off the bottom to apply this method. 

 Auster et al. (1997) and Norcross and 

 Mueter (1999) measured fish size on a 

 video screen when the fish appeared 

 between the skids of their ROV. The 

 screen measurement is then related 

 to the known distance of the skids. 

 This method relies on the fish and 

 skids being in the same horizontal 

 plane and on the fish being perpen- 

 dicular to the axis of the camera. 

 Krieger (1992) used a submersible to 

 estimate the size of rockfish. 



Two methods were tested during 

 the VITAL cruise in the Bay of Bis- 

 cay, in late August and early Septem- 



