The lakes, rivers, and ponds you’ve been visiting for years are probably looking browner than they used to. And people fishing in these waters may be catching different types and sizes of fish than before.
Our study identified a link between these two developments. This means that trout, bass, perch, and whitefish may be less common in lakes without fish. But pike and walleye anglers may be in for a trophy-sized surprise.
Over the past few decades, many freshwater ecosystems in much of northeastern North America and northern Europe are darkening and, as a result, changing in other ways.
What is freshwater browning?
The unique phenomenon of water turning black, called “freshwater browning,” is caused by several factors. Reasons include climate change. Rising temperatures and increased runoff are increasing the amount and types of carbon compounds moving from soil and land into waters.
Similarly, as people took steps to reduce acid emissions from chimneys and other sources, the amount of acid falling as precipitation decreased and soil chemistry changed. These chemical changes also increase the flow of carbon into water bodies.
High carbon concentrations cause water to appear brown. This is because it is essentially dissolved plant matter that pollutes water, just like tea leaves.
Underwater visibility
Brown water makes it harder to see, making it harder for fish to find prey, escape from predators, and find suitable habitat to live in.
Our recent research combined a review of past research with some new analysis to examine how different types of fish behave in dark water. We worked with a large team of experts to compile results from previous studies that looked at the relationship between the darkness of a body of water and the growth rate of fish in that same body of water.
It turns out that fish often grow slower in brown water. Reductions in the growth rates of individual fish are likely to reduce the abundance of these fish, which may result in changes in the abundance and proportion of different types of fish in the lake.
However, freshwater browning does not affect all fish species equally.
Not surprisingly, it turns out that vision is very important when navigating brown waters. A study of fish communities in 303 Canadian lakes found that large-eyed fish species were more common in lakes with darker water.
Examining data on the populations of eight economically important fish species in 871 lakes in North America and Europe, they 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 populations were not affected by freshwater browning.
Browning was associated with increased populations of northern pike and walleye.
We think this is because walleyes, for example, have special retinas that help them see in the dark brown waters. Similarly, pike have a well-developed lateral line sensory system that allows them to sense vibrations, movement, and pressure changes in the water.
Changes for anglers
Those fishing brown lakes may consider appealing to the senses of fish that may be in the water. For example, when fishing in dark water, rather than using colorful or shiny lures to attract visual attention, consider using vibrating lures that can be detected by the fish’s lateral line system, or lures with scents that trigger an olfactory response.
By looking at the water and what’s happening in it, scientists and anglers alike can understand the changes we’re seeing and what they actually mean.
