How cleaner water may pose risk to some Lake Erie fish species

How cleaner water may pose risk to some Lake Erie fish species

Study points to need for ecosystem-wide management approach

Achieving goals to lower nutrient runoff in Lake Erie, a move designed to prevent harmful algal blooms and reduce low-oxygen conditions called dead zones, could come at the unexpected cost of harming fishery health, a new study suggests.

Researchers combined a series of models to demonstrate the importance of monitoring multiple components of an ecosystem simultaneously, especially as climate change leads to higher temperatures and more precipitation. In the process, the team identified an unforeseen trade-off for fisheries as nutrient inputs are reduced to improve water quality - in particular, a lower harvest of the popular yellow perch species as dead zones were diminished.

"What we're finding is that as you clean up the lake by reducing nutrients, the ability to support important contemporary fisheries might also decline," said Stuart Ludsin, professor of evolution, ecology and organismal biology at The Ohio State University and a co-author of the study.

The modeling also suggests that projected warming will increase the extent of the dead zone related to an excess of nutrients - so there is no interest in easing up on restrictions, and the study provides estimates of nutrient reduction targets for the future. But the nuanced findings highlight the need for nutrient management decision-makers to give fisheries more consideration when setting targets, researchers say.

Don Scavia, professor emeritus at the University of Michigan School for Environment and Sustainability, was the lead author of the paper. The study was published recently in the journal Proceedings of the National Academy of Sciences.

Current federal water-quality guidelines aim to reduce nutrients - primarily phosphorous - in Lake Erie by 40% compared to 2008 levels through strategies such as improving wastewater treatment processes, restoring wetlands, and adopting agricultural and urban practices to reduce runoff. Excess nutrients can lead to hypoxia, or a loss of dissolved oxygen, causing aquatic life to die or leave the resulting dead zones.

Ludsin's lab led a 2023 study that set the stage for this paper, showing that historic fishery harvests for lake whitefish, yellow perch and walleye had peaked at differing levels of water quality in Lake Erie. The team labeled the scenario a "wicked" management problem with inevitable trade-offs.

In the new study, researchers linked predictive modeling of nutrient- and temperature-driven Lake Erie bottom hypoxia with commercial fishery harvest data over the past century and climate projections for 2030-2099.

The analysis suggested that nutrients and warmer air combine to drive up hypoxic conditions - an indication that climate change-related temperature increases need to be a factor in water quality management. And while nutrient loads have not been decreased to the goal of 40%, good progress has been made over the past 10 years, said Ludsin, also co-director of the Aquatic Ecology Laboratory at Ohio State.

And then there is the dead zone's effect on fish, which historically has not been a major factor in water quality management decision-making.

Results showed that more extensive dead zones in the 1960s and 1970s depressed harvests of lake whitefish and walleye at the same time yellow perch catches were at their highest. Lake whitefish appear to be the most in need of cleaner water. Walleye benefit from lower nutrient loads but have some food flexibility, and yellow perch do best when the nutrient load is high.

"When you clean up the lake, you might get rid of zones of low dissolved oxygen, but the loss in food production that comes from reduced nutrients seems to have a more negative impact on yellow perch," Ludsin said. "So whatever nutrient loading target we move to, there are going to be potential winners, such as lake whitefish, and there could be potential losers, especially for yellow perch and even walleye to some degree.

"The punchline of our research is that there didn't seem to be any single level of nutrient loading that is going to maximize the production of all three species."

The paper's authors push for adoption of ecosystem-based management that uses simple predictive models that can identify the trade-offs between water quality and fishery sectors and that gets managers from both sectors to interact more - they just don't have a history of working together.

"When you manage water quality, you should be considering more than just water quality. You need to consider impacts on fisheries, too," Ludsin said. "Likewise, when you manage for fisheries, it's not sufficient just to measure catch statistics. You need to be considering the state of the ecosystem and the broader environment."

Ludsin has received new long-term funding that will support his work to help agencies identify targets to control water quality while considering impacts on fisheries.

This work was supported by the U.S. Geological Survey, the U.S. National Science Foundation, the National Natural Science Foundation of China, the Ohio Department of Agriculture, The Andersons Charitable Foundation, the Northeast Ohio Regional Sewer District, and the Michigan Department of Environment, Great Lakes, and Energy.

Additional co-authors were Anna Michalak of Stanford University, Daniel Obenour of North Carolina State University, Mingyu Han and Yuntao Zhou of Shanghai Jiao Ton University, Laura Johnson of Heidelberg University, independent consultant Yu-Chen Wang and Gang Zhao of the Chinese Academy of Sciences.

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