Demographic consequences of reserve placement under directional larval dispersal in linear coastal systems

Marine protected areas (MPAs) are widely implemented to protect or rebuild exploited populations, yet their population-level effectiveness remains variable. One unresolved question is how the spatial placement of no-take zones interacts with larval dispersal directionality, particularly in linear coastal systems where connectivity is often asymmetric. For sedentary species with planktonic larvae, such as European lobster, it remains unclear when reserve placement most strongly affects demographic recovery after exploitation. Here, we use a spatially explicit individual-based model parameterized for European lobster in a generic linear coastal system. We contrast Local, Broad dispersal, and Unidirectional larval dispersal scenarios across alternative MPA layouts with equal total protected area. We show that no-take zones increase abundance and the prevalence of large individuals relative to open areas. However, under Unidirectional dispersal, recovery depends on reserve placement: downstream no-take zones benefit from larval import and local retention, whereas upstream reserves primarily export larvae and show limited local recovery. These results identify dispersal directionality as a key determinant of when spatial configuration matters for MPA effectiveness.