Drivers of growth in a keystone fished species along the European Atlantic coast: The common cockle Cerastoderma edule

A better understanding of growth drivers in shellfish populations including the common cockle Cerastoderma edule is essential, as their future is challenged by unsustainable fishing practices and climate change. In this study the spatial and temporal variabilities in common cockle growth across latitudes were assessed and compared with historical data. Six locations were examined at bimonthly intervals over 19 months; three Irish, two Welsh and one French, spanning the latitudes 54°N to 44°N. The results demonstrated that local abiotic and biotic factors have a larger impact on cockle growth than latitude. Cockles at similar latitudes grew at different rates and sizes, possibly due to factors such as density, fishing activity and interspecific competition. Cockles (0–3 years) impacted by low salinity and parasites (trematodes), exhibited reduced growth in later years. At the warmest, southernmost site growth was lowest in cockles >2 years. Previously, cockles at that site have been shown to spawn almost year-round, possibly diverting energy to gonad development rather than growth. The results opposed previously literature which demonstrated significantly greater growth at lower latitudes. These findings affirm that cockle growth and size is variable due to local abiotic (reduced salinity) and biotic (potentially trematode infection) drivers. Additionally, the synergistic relationship between these factors, i.e. warmer temperatures driving prolonged spawning, and the potential association between lower salinities and trematode prevalence, is concerning due to predicted climate related increases in temperature, precipitation and trematode prevalence/transmission, which may result in northern cockles reaching smaller maximum sizes.