What can fish tell us about ecosystem health?

by Brandon Peoples

Chances are if you’re reading this post, I don’t have to convince you that clean water is important. We’re responsible to ourselves to maintain enough clean water to support our society. Most obviously, we must maintain water quality for human consumption. In addition, agencies are required to manage water quality to protect imperiled species and fishery resources.

So exactly how do we do that?

When it was passed in the early 1970s, the Clean Water Act required states to establish numeric water quality standards—concentrations of specific pollutants that should not be exceeded. These standards helped to curb our big water quality problems—rivers in industrial cities stopped catching fire and regularly experiencing fish kills.

In 1969, a mass of oil and debris caught fire on the Cuyahoga River in Cleveland, OH. Source

But something was missing—numeric chemical standards aren't always necessarily best for protecting aquatic life.

Aquatic biota—from algae and bacteria to fishes and birds—respond to a combination of physical and chemical stressors. So, although individual pollutants may meet standards, elevated combinations of multiple pollutants can be harmful to aquatic life—especially fishes.

Enter bioassessment—assessing ecosystem health using biota. Bioassessment is based upon predictable relationships between water quality and biotic communities or populations. The two most common life forms used in bioassessment are macroinvertebrates (…bugs) and fish. Bioassessment indices usually incorporate multiple metrics, such as percentages of pollution-tolerant species and total numbers of species (species richness).

Pollution-sensitive organisms like this stonefly (left) and candy darter (right) can be informative about long-term ecosystem health. Photos by Brandon Peoples.

Bioassessment can be more effective than chemical monitoring at assessing ecosystem health for several reasons. Firstly, you may have heard that governmental agencies are facing a bit of a financial crisis. Chemical monitoring is expensive—states monitor dozens or even hundreds of chemical parameters, many of which must be processed in pricey in-house labs. Secondly, agencies can’t monitor chemistry in all water bodies at all times (well…not without a billion-dollar-a-year monitoring budget). It’s easy to miss events where water chemistry can reach harmful levels. Lastly, chemistry at sites must be sampled multiple times throughout the year to get a good handle on ecosystem health.

On the other hand, fish can do the monitoring for us. Overall, fish bioassessment can be cheaper than chemical monitoring. Fish communities only need to be sampled once each year, and they usually don’t require any lab processing. Although chemical monitoring can miss individual pollution events, these events will be evident in fish communities.

Backpack electrofishing is often used to conduct fish bioassessment. Photo by Brandon Peoples.

Although bioassessment is a useful tool for managers, agencies often meet considerable resistance when implementing biological standards. One argument against bioassessment is interpretation and implementation. For example, reducing nitrogen concentration by 0.5 mg/L in wastewater effluent is a fairly straightforward goal. On the other hand, what exactly must plant managers to increase species richness from 8 to 12?

In a perfect world, biologists like me would be out of a job—we wouldn’t need to monitor water quality. But until then, we’ll be hitting the rivers each summer to see just exactly what the fishes have to tell us.