Invaders at Delta Marsh

Restoring the Tradition - Invasive Carp Exclusion

Delta Marsh in Manitoba, is one of North America’s largest freshwater coastal marshes and is of great ecological and cultural significance. The presence of invasive common carp, among other factors, degraded the marsh and its ability to support migrating waterfowl.

About the Study

Lead Researcher: Dale A. Wrubleski, Ph.D (retired)
Common carp (Cyprinus carpio) is a freshwater invasive species in Canada. Dense populations of large carp degrade aquatic habitat by decreasing water clarity and damaging submersed aquatic vegetation through their destructive feeding and spawning behaviors. Submersed aquatic vegetation and the invertebrates associated with it are important food for fall-migrating waterfowl. In a massive project/undertaking with many collaborators and stakeholders, Ducks Unlimited Canada limited access of large common carp to Delta Marsh with the goal of restoring it to historical conditions (1970s) to support fall-migrating waterfowl. Delta Marsh is approximately 18,000 ha in size, making this carp exclusion project the largest of its kind in North America.

This important long-term study (launched 2012) was an opportunity to evaluate if carp exclusion structures could be used to restore a massive freshwater coastal marsh and improve conditions that support migrating waterfowl.

Project Phases and Timelines
Phase 1 (2009 – 2013)

Determine pre-exclusion baseline conditions
Exclusion structures constructed and installed in the winter of 2012/2013

Phase 2 (2013 – 2018)

In the 6 years following the construction of the structures research teams investigated: fish response, marsh nutrient budget, water clarity, submersed aquatic vegetation (SAV) response, and fall staging waterfowl response

About the work There was a need to evaluate if a balance could be reached excluding large carp without negatively impacting native fish populations. In the case of Delta Marsh, the answer was a resounding yes! The return of healthy aquatic habitat resulted in an increase in migrating waterfowl, and native fish populations were not negatively impacted.

Exclusion Structures and Results

Bar screens are deployed each spring to prevent migrating large carp from entering Delta Marsh from adjoining Lake Manitoba. The screens are removed mid-summer when the spawning season is over. The Province of Manitoba now manages the operation of the structures (since 2019).

Information from this study can be used by habitat managers around the world to help inform carp exclusion efforts. This study addressed a knowledge gap by evaluating optimal screen bar spacing size and timing of spring deployment. Results found that, in Manitoba, the optimal bar spacing in carp exclusion screens is 7 cm apart, and spring deployment of the screens should occur when channel water temperatures exceed 9 °C and continued warm air temperatures and southerly winds are forecast.

structure built to block the migration of common carp

Return of healthy aquatic habitat resulted in an increase in diving and dabbling ducks: Water clarity improved, largely driven by a reduction of inorganic suspended solids. Submersed aquatic vegetation doubled in percent cover through the 6 years of monitoring post-carp exclusion. Diving duck abundance increased with increasing submersed aquatic vegetation cover. There was a 339% increase in diving duck density and a nearly 400% increase in dabbling duck density between the pre- (i.e., 2000s) and post-exclusion periods.

Boat driving through delta marsh with a waterfowl flying overhead

Acknowledgements

This project was funded by Ducks Unlimited Canada, the U.S. Fish and Wildlife Service, Government of Manitoba Wildlife and Fisheries Branch, Wildlife Habitat Canada, as well as grants and funds from numerous individuals and foundations.

Delta Waterfowl and the U.S. Geological Survey Northern Prairie Wildlife Research Center, University of Manitoba, Department of Fisheries and Oceans provided in-kind support.

Invaders at Delta Marsh

About the Study

Exclusion Structures and Results

Acknowledgements

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