Recruit–recruit interactions, density-dependent processes and population persistence in the eastern oyster Crassostrea virginica

Abstract

The supply and successful settlement of larvae is vital for the development and persis- tence of populations of many marine species. Strongly influenced by hydrodynamic processes, the rate of supply affects the interaction between recruits. Population structure is determined by density- independent mortality when recruitment is limited, and by density-dependent mortality when satu- rated; however, the extent to which these processes determine population structure remains unclear. The relationship between recruitment and density-dependent processes in population persistence was examined in South Carolina for the eastern oyster Crassostrea virginica. Over a range of recruit densities, the resultant change in population structure was compared at locations characterised by high or low water velocities. A regionally consistent relationship between recruit density and popu- lation size indicated both density-independent and density-dependent mortality effects. Population growth was strongly positive at low and intermediate recruitment, where recruitment rates exceeded mortality. Population growth was strongly negative at extremely low and high recruitment, where density-independent mortality exceeds survival at very low recruitment rates, and density- dependent mortality reduces population size at high recruitment rates. Flow velocity affected recruit- ment rates, with high flow rates enhancing short-term recruitment; however, in the long-term, popu- lations in high and low flow regions exhibited comparable recruit densities, suggesting resource limitation. Therefore, population structure of C. virginica is likely determined by carrying capacity and fluctuating rates of density-dependent mortality and not by rate of supply.