Low downstream dispersal of young-of-year common carp from marshes into lakes in the Upper Mississippi River region and its implications for integrated pest management strategies

In lakes of the Upper Mississippi River Basin (UMRB), invasive common carp (Cyprinus carpio) employ a reproductive strategy in which adults migrate out of lakes to spawn in seasonally unstable marshes that have few egg and larval predators. The rates with which the juveniles outmigrate from marshes into adjacent lakes have critical management implications but have not been quantified using direct approaches. We used passive integrated transponder (PIT) tags to quantify outmigration of young-of-year (YOY) carp from marshes into lakes in two systems in UMRB. In both cases, marshes were located upstream of lakes and PIT antennas continuously monitored the movement of carp. In the first system, which we monitored for three years, 0.2% to 5.5% of PIT-tagged YOY carp outmigrated to the lake each year; highest outmigration occurred during mid-summer periods of low water level in the marsh. In the second system, which comprised much more extensive area of deeper marshes, 0.2% of YOY carp outmigrated to the lake. Ageing and mark-recapture analyses confirmed that relatively few YOY carp recruited into the lake populations from the marshes each year (between 5 and 50 per hectare). Our results show that YOY carp outmigration rates are low (< 6%) but can vary an order of magnitude among years. These findings are important for removal-based management strategies for carp in lakes of UMRB by showing what adult removal rates are needed to compensate for juvenile recruitment from marshy nurseries.