A diver survey method to quantify the clustering of sedentary invertebrates by the scale of spatial autocorrelation
Richard McGarvey A C , John E. Feenstra A , Stephen Mayfield A and Erin V. Sautter BA SARDI Aquatic Sciences, South Australian Research and Development Institute, 2 Hamra Avenue, West Beach, SA 5024, Australia.
B 32 Cremorne Street, Fullarton, SA 5063, Australia.
C Corresponding author. Email: richard.mcgarvey@sa.gov.au
Marine and Freshwater Research 61(2) 153-162 https://doi.org/10.1071/MF08289
Submitted: 13 October 2008 Accepted: 4 July 2009 Published: 25 February 2010
Abstract
Sedentary benthic invertebrates exhibit clustering at a range of spatial scales. Animal clustering reduces the precision of diver surveys and can accelerate overexploitation in dive fisheries. Dive harvesters target the densest aggregations of males and females that produce the highest rates of egg fertilisation during mass spawning events. By quantifying these effects of harvesting on fertilisation success, measuring animal clustering can inform stock management for reproductive sustainability. We present a method to measure the spatial extent of density aggregations down to 1 m, extending a previously described leaded-line survey design. Applying this method to abalone, research divers counted individuals in successive 1 × 2 m2 quadrats lying along adjoining pairs of 1 × 100 m2 transects. Clusters were observed as neighbouring quadrats of high animal density. Spatial autocorrelations at inter-quadrat distances of 1 to 100 m were calculated for four surveys, with eight pairs of transects swum in each survey. For all four surveys, inside two survey regions, spatial autocorrelation declined to non-significant levels at a distance of ~20 m. Quantified by the distance within which density counts are correlated, this quadrat-within-transect method provides a diver survey measure of the scale of spatial aggregation for sedentary invertebrates such as abalone, sea cucumbers and urchins.
Additional keywords: aggregations, benthic invertebrates, fertilisation success, quadrat, transect.
Acknowledgements
We thank the South Australian SARDI abalone research dive team, Brian Foureur, Kate Rodda, Brian Davies, Peter Preece, Coby Matthews, Thor Saunders, Brian Davies and James Brooks, who collected all greenlip abalone counts analysed in the Coal Ground and West Bottom leaded-line surveys. We also thank Dr J. R. Naylor and a second referee for valuable comments. Assistance with figures was provided by Janet Matthews. For financial support, we gratefully acknowledge the South Australian abalone industry, SARDI Aquatic Sciences, PIRSA Fisheries and the Australian Fisheries Research and Development Corporation (Project No. 2001/076).
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