Sampling of estuarine ark clams (cockles) in estuarine soft sediment habitats, to inform fisheries management

Cockles (ark clams) are infaunal molluscs that support important fisheries worldwide, but often occur across a complex range of strata. This creates challenges for designing independent survey programs to support fisheries management, and there is a need for additional published studies to aid development of such programs. Here, we improve the basal information available to inform the design of sampling programs for cockles, using Sydney Cockle ( Anadara trapezia ) as a case study. The combination of small quadrats and systematic searching of the quadrat area using the ‘finger dredge’ approach was highly effective. In a typical nested design (including quadrats, transects, subsites and sites), a considerable proportion of the overall variation in cockle abundance was detected at intermediate distances, with an upper limit of spatial dependence of <700 m. Approximately 77% of quadrats had all cockles visible upon initial observation (without ‘finger dredging’), meaning visible cockles provided a reasonable proxy for total (visible + buried) abundance. However, patterns were not consistent among subsites, suggesting site-specific factors may influence cockle burial. Presence of epiphytic algae (‘cockle weed’) provided a poor proxy for both presence and abundance, as algae was absent when cockles were present in 75% of observations. Analyses indicated that sampling effort should prioritise a greater number of shorter transects with fewer quadrats, allocated across a grid with <350 m centres. The patterns resolved are broadly useful for developing sampling programs for both cockles, and other infaunal bivalve molluscs.