THE CARP CONUNDRUM

Water Quality Issues and Possible SolutionsBy Rebecca Reeves, Stormwater Education and Outreach Manager

Common carp are ubiquitous freshwater fish that have become widespread in North America. Originally from Eurasia, carp were introduced by humans to control aquatic vegetation and for fishing. Unfortunately, their introduction has led to significant water quality issues due to their rapid reproduction, high appetites, and lack of natural predators in many areas.

These creatures are sometimes referred to as “ecological engineers” due to their ability to dramatically alter ecosystems. Their presence in lakes, rivers, and ponds leads to the loss of freshwater vegetation and habitat for native species, increased water turbidity (cloudiness), upheaval of nutrient-rich sediment, and the altering of local food webs.

Carp can establish themselves widely in water bodies due to their spawning abilities. Some species are able to spawn multiple times per season, and females lay thousands of eggs per season. Some carp species can eat up to 20% of their body weight daily (most fish only consume around 2%) allowing them to outcompete native species for food resources.

Through their feeding and swimming, carp also disturb the bottoms of rivers, lakes, and streams, muddying the clarity of water and cycling nutrients (namely phosphorus and nitrogen) that have settled into the sediment lining the bottom of these waters. These two nutrients are key drivers of declining water quality and in excess, can aid the growth of cyanobacteria. Sometimes referred to as blue-green algae, cyanobacteria is an ancient species that can produce toxins and lead to eutrophication, or the depletion of oxygen in lakes, rivers and ponds. Bodies of water with active cyanobacteria blooms are unsafe for swimming, fishing, and other recreation and are especially harmful to children and pets. These blooms typically appear like a green film or green spilled paint over the surface of the water.

This photo demonstrates the results of a 2007 carp enclosure experiment, where a rectangular area within Wisconsin’s Lake Winagra was protected from carp activity. The entire water body experienced a widespread cyanobacteria bloom, except in the lake area shown in brown. Carp had no access to that space.

Source: Carp Clean Lakes Alliance. Photo by Mike DeVries.

These fish are also very tolerant of eutrophic, or low-oxygen conditions, giving them an additional competitive advantage. Carp thrive in shallow muddy waters, making watersheds like the Roger Williams Park pond system an ideal habitat. The Stormwater Innovation Center, in collaboration with the Roger Williams Park Zoo and Providence Parks Department, has identified a large carp population in this watershed. As water quality in this watershed is already compromised, preliminary discussions have started to take place about how to mitigate the issue.

There are a variety of methods that other organizations and municipalities have employed to reduce carp’s impact on water quality. Due to carp’s ability to quickly populate areas, prevention is not feasible. Instead, population control methods such as baiting, trapping, netting, and electrofishing are used. These methods appear to be effective at reducing nutrient cycling and improving water quality. Some of these methods also utilize tags to track carp movement.

The Stormwater Innovation Center and our partners will continue to investigate the issue and consider solutions to help improve water quality in the Roger Williams Park watershed.

A special thank you to Dr. Art Gold, Visiting Professor at the Stormwater Innovation Center; Dr. Przemek Bajer, Research Assistant Professor at the University of Minnesota; and Heather Kinney, Coastal Restoration Scientist at The Nature Conservancy in Rhode Island, for their contributions and review of this article