Partial exclusion of spawning Cyprinus carpio to improve coastal marsh habitat may come at the cost of increased carp population growth

Common Carp (Cyprinus carpio) feeding and spawning behaviours, when abundant, negatively impact shallow water habitats essential for marsh ecosystem functioning. A common coastal marsh restoration practice is to create barriers to carp movement into a marsh from connected lakes. These barriers often do not exclude all carp from a marsh, and the lower adult density and improved habitat may increase overall carp recruitment success. A population model was created for Delta Marsh on Lake Manitoba, Canada. Varying the proportion of the total carp population spawning in the marsh indicates that, with partial carp exclusion, the improvement in spawning conditions increases carp population growth rate and decreases generation time. Increasing fishing mortality can help mitigate this effect, indicating that a multi-tiered management approach is best. It is important that fish exclusion programs be implemented with awareness of this unanticipated result.