A diver survey design to estimate absolute density, biomass, and spatial distribution of abalone

Abstract

Abalone surveys worldwide measure relative stock abundance. However, important advantages accrue if diver surveys measure absolute numbers or biomass per square metre. Principally, absolute biomass permits quota setting from a single survey using a decision table. Although relative abundance surveys have permanently fixed sampling protocols and locations, absolute abundance survey designs can be improved with technology over time. Furthermore, surveys can be directed to areas of principal management focus, and absolute survey population numbers by length with confidence intervals provide informative model input. We propose and test a transect survey design to estimate and map absolute density and biomass of abalone or other sedentary invertebrates. Divers count and measure all abalone within 1 m of a 100 m, boat-deployed leaded rope line. Semi-systematic transect locations provide spatially representative sampling inside bounded survey regions and geostatistical data for contour maps of abalone density and mean size. The effectiveness of the design for estimating change in population size under harvesting and for locating areas of fishable density was tested by a fish-down experiment, using surveys run before and after commercial harvest. The leaded-line survey design estimates of population change and spatial distribution showed agreement with the fish-down experimental harvest.