Darkening waters are slowing fish growth and shifting species balance, favoring fish that rely less on vision. Ecosystems and fishing patterns are changing as a result.
Freshwater ecosystems across northern regions are gradually darkening, a shift linked to changing climate and land chemistry that is subtly reshaping aquatic life. As visibility declines, fish growth and population dynamics are altered, favoring species with sensory or visual adaptations suited to murkier conditions.
The lakes, streams, and ponds you’ve visited for years are likely looking more brown than they used to. And people who are fishing those waters are likely catching different species and sizes of fish than in the past.
Our research has identified a link between those two developments, which means that trout, bass, perch, and whitefish may become less common in unstocked lakes. But pike and walleye anglers may be in for a trophy-sized surprise.
In the past several decades, across much of northeastern North America and northern Europe, many freshwater ecosystems are getting darker, and they are changing in other ways as a result.
Causes of Freshwater Browning
The specific phenomenon of darkening water, called “freshwater browning,” is driven by a few factors. Among the reasons are climate change, as higher temperatures and increased runoff are combining to increase the amount and types of carbon compounds that move from soil and land into bodies of water.
Similarly, as people have taken steps to reduce acidic emissions coming from smokestacks and other sources, less acid has fallen as precipitation, changing the chemistry of soils. Those chemical changes are also increasing the flow of carbon to bodies of water.
Higher levels of carbon make water look brown because it’s basically dissolved plant matter that stains the water like tea leaves would.
How Darker Water Affects Fish
It’s harder to see in browner waters, which makes it harder for fish to locate prey, escape from predators, and find suitable habitat to live in.
Our recent study combined a review of past research with some new analyses to examine how different kinds of fish do in darker water. Working with a large team of experts, we tallied findings from previous studies that looked at the relationship between the darkness of a body of water and fish growth rates in that same body of water.
We found that in browner waters, fish often grow more slowly. The decreased growth rate in individual fish appears to reduce the population sizes of these fish, which may, in turn, change the quantities and proportions of different kinds of fish in a lake.
Winners and Losers Among Fish Species
But freshwater browning doesn’t affect all species of fish equally.
Unsurprisingly, we found that vision appeared to be quite important for navigating browner waters. When we studied fish communities in 303 Canadian lakes, we found that in lakes with darker water, fish species with larger eyes were more common.
When we looked at data on populations of eight economically important fish in 871 lakes across North America and Europe, we found that browning was associated with smaller populations of several species, including lake trout, lake whitefish, yellow perch, largemouth bass, and smallmouth bass. Brook trout abundance was not affected by freshwater browning.
Why Some Fish Thrive in Murky Water
Browning was associated with larger populations of northern pike and walleye.
We believe that’s because walleye, for example, have a specialized retina that helps them see in browner waters with poorer visibility. Similarly, pike have a well-developed lateral-line sensory system that allows them to sense vibration, movement, and pressure changes in the water.
People fishing in browner lakes may consider appealing to the senses of the fish that are likely to be in the water. For example, rather than using colorful or shiny lures to attract their visual attention, when fishing in darker water, consider using vibrating lures that a fish’s lateral line system can detect or scented lures that trigger an olfactory response.
By examining what’s happening to the water and in it, both scientists and people who enjoy fishing can understand the changes we’re seeing and what they mean in practical terms.
Adapted from an article originally published in The Conversation.
This contribution was made possible by a working group grant, led by A. M. R., from the Groupe de Recherche Interuniversitaire en Limnologie/the Interuniversity Research Group in Limnology (GRIL; https://doi.org/10.69777/341034), which is funded by the Fonds de recherche du Québec Nature et Technologie (FRQNT).
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