Fish Assemblage Responses to the Exclusion of Invasive Common Carp (Cyprinus carpio) from a Large Freshwater Coastal Wetland, Delta Marsh, Manitoba

Common carp (Cyprinus carpio) is an invasive fish species in North America and around the world. Through their feeding and spawning they contribute to the deterioration of wetland habitats. Exclusion structures are a frequently used management option for limiting their negative impacts. While habitat responses to common carp exclusion have been reported, little is known about impacts of exclusion structure operation on native fish assemblages. Here we describe a common carp exclusion project on a large freshwater coastal wetland, Delta Marsh, in south-central Manitoba, Canada. Most fish species, including common carp, overwinter in Lake Manitoba and migrate each spring to feed and spawn in the marsh. Exclusion structures with removeable screens were installed on connecting channels between the lake and marsh and were deemed the best management option to exclude common carp from the marsh. In this paper we contrast relative abundance and mean size of large-bodied fish species between pre- (three years) and post- (six years) exclusion periods, as well as inside and outside the common carp exclusion zone. In addition, we monitored fish migration into the marsh to evaluate initial management recommendations. Using a combination of delayed exclusion screen placement and 70 mm screen openings, we were able to reduce the number of large common carp present in the marsh with minimal impacts on the native fish assemblage. We also provide suggested changes to the timing of screen placement to increase common carp exclusion.