Investigation of shell banding in an arcid cockle alongside trace-element concentrations to evaluate potential suitability for age estimation

Arcid clams or cockles (Arcidae) are widespread sediment-associated bivalves that support commercial, recreational and cultural fisheries. Despite their importance, ageing in these species has received little attention, constraining stock assessment to length-based models and data limited approaches. In this context, sclerochronology (growth increments) and sclerochemistry (chemical proxies) offer valuable tools for population analysis and environmental reconstructions. Here, we investigated shell banding in Sydney Cockle (Anadara trapezia) alongside shell chemistry to assess the potential suitability for ageing. Specifically, we examined whether variation in Mg:Ca and Sr:Ca was associated with shell bands, as a means of validating seasonal growth increments. Banding patterns correlated closely with seasonal variation in shell chemistry, and application of innovative peak detection algorithms (spline quantile regression, and split moving window analysis) to chemical data improved the objectivity of increment identification, particularly for Mg:Ca and in larger, older shells. Overall, the results indicated that dark shell bands in Sydney Cockle are most likely annuli, and are likely to be appropriate for determining age composition and growth in the species. By enhancing objectivity and consistency, particularly for Mg:Ca, our integrated approach supports more robust age and growth assessments. The protocol developed for ageing and chemical analysis is relevant for ageing other arcid cockle species, but further validation work will improve confidence in ageing data using this approach.