Formatted Title:

Improved Aquatic System Outcomes from Carp Management

Background/Objectives:

A common carp (carp) management program was implemented in 2019 concurrent with a remedial investigation (RI) for polychlorinated biphenyls (PCBs) in Area 6 of Operable Unit 5 of the Kalamazoo River Superfund Site. Area 6 comprises 9.8 river miles and an impounded lake of approximately 1,700 acres bounded by dams. Carp are known bioturbators and their invasive presence have likely reduced water quality, bed stability, biodiversity, aquatic vegetation coverage, and the recreational value of the lake. Baseline studies confirmed high carp abundance (biomass over 180 to 250 kilograms per hectare [kg/ha]) dominated by older age classes (>20 years). A 2018 pilot study confirmed box netting, a method that uses bait to elicit large feeding aggregations, was a practical, effective approach for carp removal. As a result, an adaptive carp management program using box nets was initiated in 2019 with the potential to continue for up to five years. The goal of the adaptive carp management program was to reduce the carp population to an ecological management threshold of approximately 100 kg/ha or less. Reducing the carp biomass to levels near this threshold would allow for system improvements in water quality, enhanced sediment bed stability, reduced nutrient re-suspension, increased aquatic vegetation, and improved native fish community. The results of recent studies, discussed below, showed significant changes to the system.

Approach/Activities:

Baseline and follow-on studies (fish community surveys, aquatic vegetation, water quality, and surface water PCBs) were conducted in Area 6 and a control area upstream of Area 6 (i.e., Area 5) to measure the ecological and physical effects of carp removal. Baseline data were collected in 2019 and follow-on studies were conducted in 2022. Fish community and aquatic vegetation surveys were conducted in accordance with Michigan state agency protocols for method acceptability and general comparison to other regional data. Water quality data were collected at six surface water stations positioned across approximately 15 river miles of Areas 5 and 6. Each station used one of two instrument packages of water quality, optical, and current velocity instrumentation and are identified as: OPTICS (OPTically-based In-Situ Characterization System) and HyST (Hydrodynamics and Sediment Transport system). This instrumentation collected near-continuous data every 15 minutes between April and November 2019 and between April and August 2022. Freely-dissolved PCBs were collected with passive samplers in the study and control area. At the time of the follow-on studies, over 138,500 carp had been removed from Area 6 (a reduction from 442 kg/ha to 155 kg/ha). An additional 11,861 carp were removed in 2022 and 4,875 removed in 2023 for a total program carp removal through 2023 of 156,631, resulting in the Area 6 carp population being below the 100 kg/ha biomass threshold.

Results/Lessons Learned:

Significant changes have occurred in Area 6 that appear to be related to the removal of carp in Area 6. Fish abundance, age class distribution, and integrity of the fish community have improved since the 2019 fish community assessment. Area 6 aquatic vegetation species occurrence, surface area coverage, and density metrics have increased since the 2019 baseline aquatic vegetation survey. Peak diel amplitudes observed in turbidity at Area 6 water quality monitoring stations in 2022 were similar to 2019 monitoring results while upstream stations showed higher peak amplitudes in 2022 compared to 2019, suggesting more mobile particulate matter in the river. This diel pattern in near-bottom water turbidity is consistent with nocturnal carp feeding behavior and associated sediment bioturbation. Given the difference between Area 6 and the control area, it appears reasonable to assume that carp removal resulted in the change in turbidity signals. The measured fish community, aquatic vegetation, water quality, and freely-dissolved PCB concentration improvements between 2019 and 2022 provides a significant basis to understand how carp removal has changed the lake considering those 3 years represented more than 90% of the total carp removed over the entire program. Surface water and fish tissue PCB data collected as part of the long-term monitoring program provide additional lines of evidence with respect to the sequestering of PCBs via enhanced natural sedimentation in the biologically active layer. An evaluation of lines of evidence for recovery will be performed as part of the ongoing RI/FS and remedy selection/implementation process.

Matt Basler

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Improved Aquatic System Outcomes from Carp Management