Adult-juvenile interactions in intertidal shellfish populations

Adult-juvenile interactions are integral for regulating population dynamics in intertidal shellfish beds. Disturbances that reduce shellfish abundance are becoming more frequent and can disrupt these crucial relationships, affecting population dynamics. We manipulated the density gradient of adult cockles Austrovenus stutchburyi along 23 intertidal sites across 4 estuaries to explore how reduced adult density affects juvenile retention, shell growth, and condition. Within each site, we reduced adult (>15 mm shell length) cockle densities from 9 m 2 ‘removal’ plots and established ‘non-removal’ plots with ambient densities. Juvenile cockles (8-12 mm shell length) were stained with a calcein marker, reseeded into the centre of each plot (0.5 m 2 ), and then sampled after 7 mo. We hypothesised that relationships between adult cockle density and juvenile retention, shell growth, and condition would be non-linear and influenced by hydrodynamic conditions (i.e. wind-wave exposure) and sediment characteristics such as mud content, microphytobenthic biomass, and shell hash. Our statistical models revealed that adult cockle density and wind-wave exposure were important in driving changes in juvenile retention. Adult density was an important predictor of juvenile shell growth but switched from positive to negative effects in high and low mud, respectively. Juvenile condition was predominantly predicted by sediment chlorophyll a content (a proxy for microphytobenthic biomass). Our study highlights that adult-juvenile interactions are positioned within a network of biophysical interactions, making simple cause and effect relationships difficult to detect when seeking generality. Whilst complex, these interactions are critical to inform disturbance-recovery dynamics and restoration of cockle beds.