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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on April 23, 2009
ICES Journal of Marine Science: Journal du Conseil 2009 66(5):907-915; doi:10.1093/icesjms/fsp108
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© 2009 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

How to collect verifiable length data on tuna from photographs: an approach for sample vessels

Shui-Kai Chang1, Ta-Te Lin2, Ghung-Hui Lin1, Hsiang-Yun Chang2 and Ching-Lu Hsieh3

1 College of Marine Sciences, National Sun Yat-Sen University, 70 Lien-hai Road, Kaohsiung 804, Taiwan
2 Department of Bio-Industrial Mechatronics Engineering, National Taiwan University, 1 (Section 4) Roosevelt Road, Taipei 106, Taiwan
3 Department of Biomechatronics Engineering, National Pingtung University of Science and Technology, 1 Shuefu Road, Pingtung 912, Taiwan

Correspondence to S-K. Chang: tel: +886 7 5252000 ext. 5303; fax: +886 7 5250050; e-mail: skchang{at}faculty.nsysu.edu.tw.

Chang, S-K., Lin, T-T., Lin, G-H., Chang, H-Y., and Hsieh, C-L. 2009. How to collect verifiable length data on tuna from photographs: an approach for sample vessels. – ICES Journal of Marine Science, 66: 907–915.

Length frequencies are essential data for fish stock assessments, particularly for longer-lived species. They are usually provided by commercial vessels, or by port sampling, observers, or sample-vessel programmes, but each of these has limitations. Collection by sample vessels might be the most balanced way if the data quality is verifiable. We introduce a photograph-based length-measurement approach for sample vessels to photograph fish images with a calibration board, using a regular digital camera to obtain length estimates that can be verified after the images are transformed to reduce errors of perspective distortions. We analyse this approach under ideal conditions, develop a set of objective criteria for choosing acceptable photographs from observers, and compare estimated lengths for bigeye tuna (Thunnus obesus) based on this approach with lengths measured by observers. The criteria can serve as guidelines for photographing: if images are captured following these guidelines, the approach shows the potential for obtaining cheaply a large quantity of length estimates that deviate around 3% (up and down) on average from the actual measurements taken by observers.

Keywords: fish length, photograph-based length measurement, sample vessel

Received 10 October 2008; accepted 18 March 2009; advance access publication 23 April 2009.


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