Carp Management Effectiveness Assessment

Project Overview and Objectives

Researchers and lake managers often consider common carp to be a driver of poor ecological conditions in Minnesota lakes, citing their negative impact on water clarity and aquatic plants. As a result, managers throughout the state have implemented carp management programs to improve lake conditions. However, in practice, these programs have produced variable results, suggesting success depends on multiple factors related to management, the carp population itself, and underlying lake and watershed characteristics. While individual program evaluations exist, no statewide assessment has been conducted to inform where and when carp management can be an effective lake restoration strategy.

To further understand the factors that impact the effectiveness of common carp management, MAISRC researchers studied the effects of management on carp biomass, water quality, and aquatic plant communities in a diverse array of lake types and ecosystems across the state of Minnesota.

The goal of this project was to: 

1. Provide insight into the underlying factors that influence management success and
2. Allow stakeholders and lake managers to set data-driven guidance and expectations for future management efforts. 

This project and resulting research products (dataset, data process and analysis code and workflow, management-focused report, and a peer-reviewed scientific publication) form a foundation for understanding when carp management is likely to be impactful as a lake restoration strategy, as well as when it is unlikely to result in meaningful improvements.

Project Findings and Lake Management Interpretations

This project’s results focus on changes in water quality and aquatic plant communities because, for lake managers, successful lake restoration is defined by improved water and/or aquatic plant quality, following carp biomass reductions. Overall, this assessment’s results demonstrated that common carp management can produce marginally positive water quality outcomes and variable aquatic plant responses. 

• On average, lakes displayed a Total Phosphorus (TP) reduction of 22 μ/L following carp removal. Shallow lakes typically exhibited larger reductions compared to deep lakes.
• Seasonal differences were also common. Carp had a stronger effect on water quality in the early season, while internal nutrient cycling may have a greater impact on late season (July – September) water quality.
• On average, plant diversity increased, but carp removal did not result in consistent improvements to overall coverage. 

These findings suggest the benefits of carp management are most meaningful in shallow lake systems already near state water quality standards. Additionally, the most successful projects were often accompanied by other restoration strategies—such as watershed nutrient reduction and internal load management—showcasing that carp management is not a stand-alone solution for lake restoration. Rather, the findings support integrated management approaches, which rely on strategic, data-informed prioritization of multiple management strategies to achieve lasting improvements. 

Background

Common carp are known to disturb lake sediments and dislodge aquatic plants, which can alter a lake’s ecological makeup and adversely affect water quality. Common carp management can be an effective tool to improve water quality in some lakes, but the benefits are variable (Figure 1) and have been most often evaluated in shallow, unstratified lakes for which results may not be generalizable (Bajer and Sorensen 2015). Long-term improvements to water and floristic quality are often dependent on more than just carp response, so an understanding of carp management outcomes across a wider range of lake types and ecosystems could be used to more reliably inform when and where carp management should or should not be prioritized, to measurably improve water and floristic quality.

Evaluating the effects of carp management in impaired lakes is complex and can be confounded by other, concurrent management efforts (Figure 2). Reducing biomass is often the focus of implementing carp management as well as evaluating its effectiveness, whereby success is defined as reducing the population below a specific threshold (e.g., < 100 kg/ha; Bajer et al. 2009). However, successfully reaching a biomass threshold does not necessarily lead to positive water and floristic quality responses, which are also dependent on context-specific interactions between lake processes, common carp, additional sources of nutrients, watershed characteristics, and other management efforts.

In partnership with the Minnehaha Creek Watershed District, and in coordination with the Minnesota Department of Natural Resources and the Minnesota Pollution Control Agency, the UMN research team evaluated statewide data from common carp management projects to:

1. Quantify diverse impacts of management efforts on common carp populations and
2. Quantify diverse lake responses to carp management, especially regarding water quality and macrophyte community composition.

Data Collection and Analysis

Evaluating common carp biomass responses: The project team compiled and evaluated carp management efforts across the state. Thanks to contributions from carp management contractors, watershed districts, lake associations, and other stakeholders, the team compiled carp removal data from 90 lakes, 86 of which also had carp biomass data. Based on analysis of pre- and post-data, reduction in carp biomass could be related to covariates such as lake area, littoral area (or proportion of littoral area), and shoreline development factor, which were also collected.

Evaluating water and floristic quality responses: The project team also evaluated the effects of carp management on water quality (e.g., total phosphorus, Secchi depth, and chlorophyll-a concentrations) and aquatic plant communities. Analyses of the aquatic plant community data included species responses, floristic quality responses, and community responses. Covariates included:

• lake depth
• nutrient concentrations
• pre-management water clarity
• lake morphological characteristics
• relevant management history (e.g., alum treatments)
• and watershed characteristics (e.g., size, watershed to lake area ratio, and land cover)

The team compiled available water quality data from 63 lakes with carp removal or biomass data (Minnesota Pollution Control Agency, LAGOS Research Platform) and aquatic plant community point-intercept survey data from 40 lakes with carp removal or biomass data (Minnesota Aquatic Invasive Species Research Center “PI Charter App”).

References 

Huser, B. J., Bajer, P. G., Chizinski, C. J., & Sorensen, P. W. (2016). Effects of common carp (Cyprinus carpio) on sediment mixing depth and mobile phosphorus mass in the active sediment layer of a shallow lake. Hydrobiologia, 763(1), 23–33. https://doi.org/10.1007/s10750-015-2356-4

Bajer, P.G., Sullivan, G. & Sorensen, P.W. Effects of a rapidly increasing population of common carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake. Hydrobiologia 632, 235–245 (2009). https://doi.org/10.1007/s10750-009-9844-3