The lakes, streams, and ponds you have visited for years now appear increasingly brown, and anglers are observing changes in the species and size of fish they catch.
Our research has established a connection between these trends, indicating that trout, bass, perch, and whitefish may become less abundant in unstocked lakes, while pike and walleye anglers could experience a notable surge in trophy-sized catches.
Over the past several decades, freshwater ecosystems across much of northeastern North America and northern Europe have become darker and undergone additional transformations.
What is freshwater browning?
Freshwater browning, the darkening of water, results from several factors, notably climate change, which brings higher temperatures and greater runoff, thereby increasing the volume and variety of carbon compounds leaching from soils into water bodies.
Likewise, reduced acidic emissions from smokestacks and related sources have led to less acid in precipitation, altering soil chemistry and further enhancing carbon fluxes into aquatic systems.
Elevated carbon concentrations give water a brown hue by dissolving plant material that stains the water similarly to tea leaves.
Underwater visibility
Reduced visibility in browner waters hampers fish ability to locate prey, avoid predators, and identify suitable habitats.
Our recent study integrated a literature review with new analyses to assess how various fish species respond to darker water, compiling findings from earlier research that examined the link between water darkness and fish growth rates.
We discovered that in browner waters, fish tend to grow more slowly, and this reduced growth at the individual level can diminish population sizes, potentially altering the abundance and composition of different fish species within a lake.
However, freshwater browning impacts fish species unequally.
Consistent with expectations, vision proved crucial for navigating browner waters; analysis of fish communities across 303 Canadian lakes revealed that species with larger eyes were more prevalent in darker lakes.
Examining population data from eight economically important fish species across 871 lakes in North America and Europe, we observed that browning correlated with reduced populations of lake trout, lake whitefish, yellow perch, largemouth bass, and smallmouth bass, while brook trout numbers remained unaffected.
Conversely, browning was linked to increased populations of northern pike and walleye.
We attribute this to physiological adaptations: walleye possess a specialized retina that enhances vision in low‑visibility, brownish water, while pike have a robust lateral‑line system that detects vibrations, movements, and pressure changes.
A change for anglers
Anglers fishing in browner lakes can better target likely species by appealing to their non‑visual senses; instead of relying on bright or reflective lures, they might use vibrating lures that stimulate the lateral line or scented lures that activate the olfactory system.
By studying the water’s condition and its inhabitants, both researchers and anglers can grasp the observed changes and their practical implications.
