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

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posted on 2019-01-01, 00:00 authored by C May, L Meyer, Sasha WhitmarshSasha Whitmarsh, C Huveneers
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 .

History

Journal

Royal Society Open Science

Volume

6

Article number

ARTN 190456

Pagination

1 - 11

Location

England

ISSN

2054-5703

eISSN

2054-5703

Language

English

Publication classification

C1.1 Refereed article in a scholarly journal

Issue

5

Publisher

ROYAL SOC