The Good, The Bad, and The Ugly: Understanding Fisheries and Eutrophication

Harmful algal blooms, dead zones, and fish kills are the results of a process called eutrophication—which begins with the increased load of nutrients to estuaries and coastal waters. The Mississippi River carries millions of tons of nutrient-rich sediment into the Gulf each year. (Credit: NOAA's National Ocean Service via Flickr Public Domain)

While eutrophication is often portrayed as a negative in limnological circles, fisheries managers may have a different view of high productivity. Though high levels are linked to poor water quality, productive water bodies tend to increase fish yields thanks to increased food production for fish.

Effects of Eutrophication

Of course, too much of a good thing can be bad, and eutrophication is no different. A 2023 study published in Ecology and Society explains that while eutrophic conditions lead to increased food generation for fish, it also deteriorates water quality—making it a complex problem for resource managers.

Eutrophication simply refers to the process of fresh or saltwater becoming more productive due to excessive nutrients accumulating. The severity of eutrophication can vary throughout the year, depending on runoff, pollution, and other variables. The higher production means more aquatic vegetation for fish to feed on, however, severe eutrophication can deteriorate overall water quality.

Denser vegetation on the surface can trap heat, making water temperatures throughout the water column higher, causing colder temperature fish to become sick or die. Sediment and vegetation also become suspended in water, impacting turbidity levels, water clarity, and fish behavior. As algae falls to the bottom of a water body and decays, dissolved oxygen is consumed, often leading to hypoxia, which suffocates aquatic life.

As a result, eutrophication is much more nuanced than simply being considered “bad” in the world of environmental monitoring and management.

Balancing the Positives and Negatives of Eutrophication

From a fisheries perspective, high production can be beneficial, resulting in improved fishery yields. However, from the human perspective, poor water quality can result in lower-quality drinking water, beach closures, and other disruptions.

Still, the study cites several examples where eutrophication has been halted altogether, resulting in lower fishery yields due to decreased productivity. As a result, resource managers of multi-use aquatic systems must ensure the success of the fishery while still protecting other critical ecosystem services.

In order to meet this balance, allowing for intermittent eutrophication in multi-use systems may be the best approach to meeting the needs of all ecosystem services. While this may seem like an ideal solution, the study stresses that, in reality, it is much more difficult to manage.

In addition to meeting human ecosystem needs, many water bodies support various fisheries, all of which have varying habitat needs. The study looks at Lake Erie’s fisheries, specifically whitefish, walleye, and yellow perch.

Results of the assessment showed that each species was maximized at different levels of ecosystem productivity. In short, this means that ecosystem-based management is essential to meeting the demands of the different species and managing resource use and protection.

Conclusion

In the end, the study concludes that each system is different, and resource managers have to consider both fisheries and ecosystem services in management plans. Ultimately, managers are faced with the choice of prioritizing fisheries or water quality–a question that the study concludes does not have a single simple solution. Without a thorough, well-balanced ecosystem-based management plan, fishery yields could decline, and ecosystem services may be compromised.