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ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 20, 2009
ICES Journal of Marine Science: Journal du Conseil 2009 66(4):646-655; doi:10.1093/icesjms/fsp046
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Crown Copyright © 2009. Published by Oxford Journals on behalf of the International Council for the Exploration of the Sea. All rights reserved

Investigating and mitigating uncertainties in the assessment of Scottish Nephrops norvegicus populations using simulated underwater television data

N. Campbell, H. Dobby and N. Bailey

FRS Marine Laboratory, 375 Victoria Road, Aberdeen AB11 9DB, UK

Correspondence to N. Campbell: tel: +44 1224 295314; fax: +44 1224 295511; e-mail: n.campbell{at}marlab.ac.uk.

Campbell, N., Dobby, H., and Bailey, N. 2009. Investigating and mitigating uncertainties in the assessment of Scottish Nephrops norvegicus populations using simulated underwater television data. – ICES Journal of Marine Science, 66: 646–655.

Nephrops norvegicus is a commercially significant decapod crustacean that excavates and inhabits burrows in muddy sediments throughout the Northeast Atlantic and Mediterranean Sea. Because of its variable emergence behaviour, traditional surveys are considered a poor indicator of the status of its populations, and underwater television (TV) survey methods have been developed. The approach involves towing a TV camera over patches of muddy sediment and counting the number of Nephrops burrow complexes within a known area. Assuming a 1:1 rate of occupancy, the average population density can be estimated. This is raised to the known area of suitable sediment to give a measure of population size. Recently, underwater TV surveys have been used to provide absolute measures of abundance for a number of Nephrops stocks in the North Sea and Northeast Atlantic. It is therefore imperative to identify, quantify, and control for uncertainties in this process. Previous workers have suggested that visual recognition of Nephrops burrows and "edge effects", where single openings of burrow complexes at the sides of the viewed transect are counted, are important sources of uncertainty. This study shows edge effects to be responsible for an overestimation of population size of between 4 and 55%, depending on the width of the field of view and the mean size of the burrow complex. This overestimation is countered to some extent by variability in burrow entrance structure, which leads to Nephrops burrows going unrecognized. The scales of these errors are discussed in relation to observed sediment softness, sediment distribution, and burrow dimensions from sites around Scotland.

Keywords: edge effects, Nephrops norvegicus, simulation study, stock assessment, uncertainty, underwater television (UWTV)

Received 30 May 2008; accepted 29 November 2008; advance access publication 20 March 2009.


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N. Campbell, L. Allan, A. Weetman, and H. Dobby
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