Development of rapid sampling procedures for an exploited bivalve in the swash zone on exposed ocean beaches

Abstract

We assessed a time-based technique for estimating the relative abundance and size composition of populations of the beach clam, Donax deltoides, in the swash zone of exposed ocean beaches by comparing it with a standard quadrat-based method. The time-based method consisted digging small plots of sand by hand and scooping sediment and clams into a mesh bag attached to a rigid frame. We tested three digging times (30, 60 and 120s) and two mesh sizes (12 and 19mm). Compared to a standard box-quadrat, the time-based diggings were more effective and efficient in terms of numbers of clams collected per time taken to do a sample. The timed digging technique was also much simpler and less problematic to use in the swash zone, which is important when industry are involved in sampling. Although a greater total number of clams were collected in the 120s diggings, when the CPUE data were standardized to number per 30s, a greater proportion of clams were collected in the shortest time frame tested. This suggests most clams were captured in the first 30s of digging, with fewer caught per unit of time thereafter. A major benefit of using the shortest digging time is that a greater number of replicate samples and patches of clams on a beach can be sampled per given unit of time, potentially improving overall precision without large increases in costs. An optimal sampling design would involve sampling more patches on a beach than replicates within a patch. Given a sampling window of 3h either side of low tide, we suggest that future sampling should incorporate 6 replicate 30-s diggings at each of 8 patches on a beach. We further recommend that a 12mm mesh bag be used as it retained a greater proportion of small clams (<20mm). This study highlights the importance of doing pilot studies to develop appropriate sampling gears and for determining optimal, cost-effective sampling strategies for large-scale surveys.