A diver survey method to quantify the clustering of sedentary invertebrates by the scale of spatial autocorrelation

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.