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Aquatic Biology

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AB - Vol. 11, No. 2 - Feature article
Seabed image of rock habitat, illustrating variability in abundance and spatial distribution of benthic taxa.
Photo: Roger Coggan, Cefas.

Barry J, Coggan R


The visual fast count method: critical examination and development for underwater video sampling


Underwater video techniques provide a non-destructive method of sampling seabed habitats and are becoming increasingly important in assessment and monitoring studies. However, quantitative analysis of the video record can be very time consuming. The Visual Fast Count (VFC) is a sub-sampling technique that significantly reduces analysis time, but gives biased estimates of taxon abundance. We demonstrate this algebraically; bias increases if species are rare or if individuals are spatially clustered. Consequently, we develop and test variants of the VFC estimator which significantly reduce this bias. We also recommend randomising the order in which the sub-sample 'segments' of the video are examined. Minimising the bias of the VFC estimator is an important development in ensuring the technique is fit-for-purpose.


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