Rochet et al.: Precision and accuracy of fish length measurements obtained with two visual underwater methods 



Measurement variance for observers and fish movements 



The standard deviation of the observer random effect, 

 aio), amounted to approximately 20% of the residual 

 standard deviation. The standard deviation of the 

 random effect for mobile objects, On,obih'^fmobih^^ was 

 40'7c higher than that for rigid objects, indicating that 

 the measurement variability was lower for fixed than for 

 mobile objects, as expected. The slope and intercept of 



the fixed effects did not significantly differ from 1 and 

 0, respectively. Thus the laser method is unbiased and 

 performs equally well for all sizes in the range tested 

 (Table 3). 



Accuracy and precision of measurements for artificial objects 



Size measurements carried out using the laser beams 

 in real time or on registered videos were found to be 



