Delta Marsh, located about 25 km north of Portage la Prairie, is a Canadian Heritage marsh and an internationally recognized wetland of great importance. Every year thousands of migrating birds nest in its waters. The countless insects that inhabit the marsh provide a great source of protein for the hatchlings of Canada geese and canvasback ducks that make the yearly trek, as well as for the many species of indigenous fish that over-winter in the deeper waters of Lake Manitoba.
But the marsh is also home to one not-so-indigenous species of fish.
The common carp (Cyprinus carpio) is a close relative of goldfish and koi. It is native to Asia and Eastern Europe where it is grown for both food and sport. An omnivore, this fish will feed on aquatic vegetation or animal material scavenged from the lake or riverbed in which it finds itself. It survives in a variety of temperature ranges and can thrive in waters that have been polluted by human development. They are prized by anglers in Europe for their size (growing up to 1.6 metres long and weighing up to 37.3 kg) and for the challenge of reeling them in. In some places, such as Poland, carp are even revered — or unlucky — enough to find themselves alongside the turkey as part of a traditional Christmas dinner. All of these factors go toward making carp the primary stock fish for aquaculture in the world. Today the amount of carp processed by China annually is greater than all other fish, produced by all other countries, combined.
When native fish stocks started declining in 19th-century North America due to over-harvesting and pollution, the U.S. government started looking for something a little hardier to replace them; something that would be good for sport tourism as well as food. The first official importing of carp was by the U.S. Fish Commission from Germany in 1872. These fish were stocked in ponds as well as rivers and lakes where they used the waterways to make their way throughout the continent. Allowing carp free reign of North American rivers and lakes was seen as a good idea at the time.
Hindsight, however, has shown that it wasn’t so good for Delta Marsh.
The river environment from which the carp originates is fundamentally different from the environment it has invaded at Delta Marsh. As a bottom-feeder, the carp has little effect on its native riverbed where rocks stabilize the sediment, keeping it in place while the carp feeds. But there are no such rocks on the bottom of Delta Marsh. There the fish’s aggressive eating and mating habits dredge up the silt brought into the marsh from the floodway, turning the water cloudy, which blocks the sunlight needed by plants living along the bottom of the marsh. The loss of the plants, which help anchor the soil, leads to more erosion, feeding the cycle.
Studies conducted by the University of Manitoba’s Delta Marsh Field Station between 2001 and 2004 have shown that the presence of carp increases the levels of phytoplankton and suspended sediment in test pools. Phytoplankton absorbs much of the oxygen in the water during the dark hours, killing anything in the marsh pools, including fish and much of the insect larva. The carp, due to their ability to survive in low-oxygen environments, are relatively unaffected by this lack of oxygen. With the larva gone, waterfowl have no protein source to feed hungry hatchlings. This destruction of natural habitats has led to the declaration of carp as an invasive species in the many parts of the world where it has been introduced. In southern Australia it is even illegal to release a captured carp back into the wild.
This fish, that was supposed to be sport for the anglers and food for the common fisherman, has instead damaged the marsh and its native inhabitants in ways that may be irreparable. Estimates now indicate that carp account for as much as half of the large fish biomass in the marsh, and their numbers continue to climb.
On the Mississippi river, a species of carp, known as Asian carp, have even started to pose a danger to pleasure boaters. The fish, which can grow to 9 kg, have taken to jumping out of the water when startled, to the point that there have been reports of people being knocked out cold by giant fish, flying through the air.
Adding to the frustration over carp in North America, the people who were originally supposed to benefit from the introduction of carp have turned their backs on them as well; while those found in the rivers of Asia and Europe are considered quite tasty, the carp that filter-feed on the mud in the marsh are described as having a distinctly muddy taste.
And who besides carp, wants to eat something that tastes like the bottom of a marsh?
Efforts are underway to combat the invasion. The University of Manitoba Delta Marsh Field Station along with Ducks Unlimited Canada and other partners have begun a research and monitoring project, blocking carp from the marsh when they make the yearly migration back from the lake in the spring. There is still a lot of work to do; screens blocking passage into the marsh must be tested for size to make sure they don’t keep out native species and surveys of fish populations must be conducted to make certain the screens are working, but the hopes are that the larger carp will be unable to return to the lake to mate and that the species will die off, returning the marsh environment to its normal, pre-invasion state.