Eyes on the size: accuracy of visual length estimates of white sharks, Carcharodon carcharias.

Cameron May,Charlie Huveneers,Sasha Whitmarsh,Lauren Meyer

doi:10.1098/rsos.190456

Abstract

Visual estimates have been used extensively to determine the length of large organisms that are logistically challenging to measure. However, there has been little effort to quantify the accuracy or validity of this technique despite inaccurate size estimates leading to incorrect population assessments and misinformed management strategies. Here, we compared visually estimated total length measurements of white sharks, Carcharodon carcharias, during cage-diving operations with measurements obtained from stereo-video cameras and assessed the accuracy of those estimates in relation to suspected biases (shark size, and observer experience and gender) using generalized linear mixed-models and linear regressions. Observer experience on board cage-diving vessels had the greatest effect on the accuracy of visual length estimates, with scientists being more accurate (mean accuracy ± standard error: 23.0 ± 16.5 cm) than crew (39.9 ± 33.8 cm) and passengers (49.4 ± 38.5 cm). Observer gender and shark size had no impact on the overall accuracy of visual length estimates, but passengers overestimated sharks less than 3 m and underestimated sharks greater than 3 m. Our findings show that experience measuring animals is the most substantial driver of accurate visual length estimates regardless of the amount of exposure to the species being measured. Scientists were most accurate, even though crew observe white sharks more frequently. Our results show that visual length estimates are not impacted by shark size and are a valid measurement tool for many aspects of C. carcharias research, provided they come from people who have previously been involved in measuring animals, i.e. scientists.

Highlights

Southern Shark Ecology Group, College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia
We compared visually estimated total length measurements of white sharks, Carcharodon carcharias, during cage-diving operations with measurements obtained from stereo-video cameras and assessed the accuracy of those estimates in relation to suspected biases using generalized linear mixed-models and linear regressions
Our results show that visual length estimates are not impacted by shark size and are a valid measurement tool for many aspects of C. carcharias research, provided they come from people who have previously been involved in measuring animals, i.e. scientists

Summary

Introduction

Southern Shark Ecology Group, College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide, South Australia 5001, Australia. A previous study has found that visual length estimates of whale sharks, Rhincodon typus, were underestimating the true size of the organism by several metres, which may have led to an underestimation of the number of mature R. typus in Ningaloo Reef, in Western Australia [14]. This underestimation may have distorted previous population models by wrongly inferring population growth due to underestimation of length and size-at-maturity. Unquantified errors in visual length estimates may have broad implications and studies need to assess the accuracy and skew of visual length estimates and identify factors that might influence this accuracy

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