What Scale Rings Reveal About Our Changing Lakes
In the cold, deep waters of Alpine lakes, a mystery unfolds beneath the surface. The whitefish, a silvery resident of these pristine waters, is changing—and scientists are listening to its story through a surprising source: its scales.
Beneath the mirror-like surface of Lake Starnberg in Germany, a team of scientists is unravelling an aquatic mystery. For over two decades, they have been collecting clues from an unlikely source—fish scales—to understand why the lake's coregonid fish, commonly known as whitefish, are changing. What they're discovering speaks volumes about the complex interplay between human activity, environmental change, and aquatic life 1 .
Coregonids, a group of freshwater fish including whitefish and ciscoes, were once the backbone of freshwater fisheries across Europe and North America. Just a century ago, these fish thrived in such abundance that they supported some of the largest freshwater fisheries ever recorded 2 . Today, they tell a different story—one of decline and adaptation that mirrors the health of our freshwater ecosystems.
Across the globe, fishermen and scientists alike have noticed troubling changes in coregonid populations. In the Great Lakes region of North America, as many as 11 species of coregonines once swam in abundance. Today, the situation is starkly different. At least two species are now extinct, and several others have disappeared from multiple lakes 2 .
The declines began with a perfect storm of threats. Overfishing depleted populations, invasive species disrupted food webs, and habitat loss removed critical spawning grounds 2 . More recently, another subtle but significant change has occurred—the fish are growing differently.
Research from Lake Starnberg reveals an intriguing pattern: while younger whitefish (ages 1-2) show stable or even slightly improved growth, mature fish (age 3) exhibit a significant decline in growth rates over the 22-year study period 1 . This isn't just a curiosity—it represents a fundamental shift in how these fish allocate energy in a changing environment.
Coregonid species once thrived in the Great Lakes
Species are now extinct
Years of research at Lake Starnberg
How do we know how a fish grew years ago? The answer lies in a technique similar to reading tree rings—but for fish scales.
Scientists carefully collect scales from the area just above the anal fin of captured whitefish, following established protocols 1 .
The scales are cleaned with dishwashing solution, with the epidermis gently removed using an interdental brush 1 .
Experienced staff independently determine each fish's age using a binocular microscope, only selecting fish whose age is agreed upon by multiple experts 1 .
Scales are mounted on slides and digitized using a high-resolution slide scanner at 3600 dpi 1 .
Using specialized software, researchers measure the distance from the scale center to each annual growth ring (annuli), calculating the incremental growth for each year of the fish's life 1 .
| Measurement | Description | Biological Significance |
|---|---|---|
| r1 | Distance from center to first annulus | Represents growth in Year 1 |
| r2 | Distance from center to second annulus | Represents growth in Year 2 |
| r3 | Distance from center to third annulus | Represents growth in Year 3 |
| rt | Distance from center to scale edge | Represents current year growth |
This meticulous process allows scientists to reconstruct the entire growth history of individual fish long after they've been caught—a powerful tool for understanding how environmental changes affect aquatic life over time.
What's causing these growth changes? The Lake Starnberg study points strongly to nutrient availability as a primary culprit. Over the study period, phosphorus concentrations in the lake decreased significantly, indicating reduced nutrient availability in the ecosystem 1 .
This connection between nutrients and fish growth might seem counterintuitive, but it's straightforward: less phosphorus means fewer algae, which means less food for the tiny organisms that whitefish eat. The result is a leaner menu throughout the food chain, ultimately affecting how well fish can grow 1 .
Meanwhile, another potential suspect—warming spring temperatures—showed no significant long-term trend during the study period, though researchers did find that temperature effects varied by fish age 1 .
| Factor | Trend Observed | Impact on Coregonid Growth |
|---|---|---|
| Phosphorus concentrations | Significant decrease | Primary driver of reduced growth in mature fish |
| Spring temperatures | No significant trend | Complex, age-specific effects |
| Reproductive investment | Not directly measured | Explains why mature fish show strongest declines |
The age-specific nature of the growth decline reveals a fascinating biological truth: life involves trade-offs. The research suggests that the observed decline in growth among mature individuals aligns with predictions from life-history theory, reflecting a potential allocation shift from somatic growth to reproductive investment after maturation 1 .
Simply put, once fish reach maturity, they must divide their limited energy between building their own bodies (somatic growth) and producing the next generation (reproduction). In an environment with reduced food availability, this trade-off becomes increasingly stark—you can't maximize both when resources are scarce.
What does it take to study these aquatic mysteries? Modern coregonid research relies on both traditional and cutting-edge tools.
Capturing fish for study - Standardized sampling across sizes and ages
Recording growth patterns - High-resolution analysis of growth history
Identifying populations - Conservation of diverse coregonid forms
Monitoring nutrient levels - Linking growth to environmental conditions
Measuring growth increments - Quantifying annual growth patterns
Digitizing scale samples - Creating precise images for measurement
Understanding these patterns isn't just academic—it's driving concrete conservation action. The Great Lakes Fishery Commission has launched a Coregonine Restoration Initiative, adopting a comprehensive framework that includes planning, restoration, and continuous evaluation 2 .
This initiative seeks to develop restoration plans that could include reintroducing extirpated species, restoring or connecting habitats, or using regulatory authorities to create refuges or otherwise limit fishing mortality 2 .
Meanwhile, at the University of Jyväskylä in Finland, researchers are working on multiple fronts—studying the effects of sulphate on coregonid reproduction, assessing the state of coregonid fish populations, and developing improved monitoring methods 3 . Their work includes collecting larval density data from research lakes for over 20 years, providing another critical long-term perspective on population health 3 .
A comprehensive framework for restoring coregonid populations in the Great Lakes region through planning, restoration, and evaluation 2 .
University of Jyväskylä researchers study sulphate effects, population assessment, and monitoring methods with over 20 years of data 3 .
The humble whitefish, once seen primarily as a commercial resource, has emerged as an important indicator of ecosystem health. Its scales tell a story of change, adaptation, and the intricate connections between human activity and aquatic life.
As restoration efforts expand across Europe and North America, the research continues. Monthly coregonine webinars hosted by the Great Lakes Fishery Commission bring together scientists from around the world, creating a collaborative community dedicated to understanding and conserving these remarkable fish 2 .
The next chapter of the coregonid story is still being written—in research stations along lake shores, in the careful analysis of fish scales, and in conservation plans that blend Western science with Indigenous Knowledge 2 . What remains clear is that the fate of these freshwater giants is inextricably linked to our own—and listening to their story is essential to protecting the aquatic ecosystems we all depend on.