Big Fish, Little Fish: Decades’ Worth of Basic Measurements Tell a Complex Tale in One Wisconsin Lake

A new study is highlighting the power of long-term research when it comes to helping scientists understand ecological processes. 

In one Wisconsin lake, researchers at the UW-Madison’s Center for Limnology were able to both interpret the complexities of predator-prey dynamics between two species of fish and document impacts of an invasive zooplankton by using only the most basic measurements fisheries scientists take when they sample fish. 

The key was that they had four decades-worth of that data. 

Ben Martin.

“Whether it’s birds, mammals, amphibians or fish, all ecologists have to measure some sort of body size of the organisms we sample,” explains Ben Martin, a recent Ph.D recipient at the Center for Limnology and now a postdoctoral researcher at the University of Notre Dame. 

For fish, those measurements are of length and weight and they are good indicators of the overall health of an individual fish. But, Martin and his colleagues found, under the right circumstances, they can tell a much bigger tale. Their results were published last Friday in the journal Ecospheres

Trout Lake, in northern Wisconsin, has been one of the National Science Foundation’s “long-term ecological research” sites since 1981. Over those decades, researchers there have documented fluctuating changes in the population of the lake’s native top predator, the lake trout, and its preferred species of prey, a fish called cisco. They did this using labor intensive research techniques like driving back and forth across the lake taking hydroacoustic surveys by boat, or catching, tagging and recapturing fish in nets. 

When Martin went back over all that data, he found something surprising – from lake trout and cisco, at least, the only data he needed to see these big population shifts was the length and weight measurements from a handful of fish.

When researchers were pulling skinny lake trout out of the lake, it meant that their population numbers were so high that they were having trouble finding enough cisco to go around. But when scientists were catching fatter trout, it was a sure sign that cisco were far more abundant and lake trout had plenty to eat.  

Only a few lake trout and a few dozen cisco – like the young one pictures here – could indicate the state of Trout Lake’s fishery. Photo: Alice Ogden-Nussbaum

“One of the more surprising things,” Martin continues, “was the minimum total number of fish that we would need each year to be an indicator.” In some years, he found that length and weight measurements of as few as three lake trout or only 50 cisco were enough to accurately infer the health of the entire population.

But the power of length/weight measurements didn’t stop there. The data also captured the story of how cisco populations were impacted by the invasion of the spiny water flea, a non-native zooplankton that arrived at Trout Lake a little over a decade ago. The spiny water flea is a voracious predator of native zooplankton in lakes and can decimate their populations, which means those native zooplankton are scarce for the young fish that evolved to eat them. 

But, Martin says, his study found that in Trout Lake, at least, the spiny water flea story wasn’t all bad. While the invader does result in fewer native zooplankton and skinny, struggling young cisco, once those fish reach a certain length, the tables are turned and they are big enough to feed on the spiny water flea. As a result, these larger cisco have higher length/weight ratios – a sign that they are well-fed.   

The results suggest that invasive species impacts aren’t uniform even for a single native fish, Martin says. That’s important to acknowledge because in some lakes, “spinies will probably never do well because cisco will eat them too quickly. But in Trout Lake they invaded right after a period where we saw a low amount of cisco in the lake because lake trout [populations] were really high,” he says. The current state of a lake’s fishery might become an important factor in predicting invasive species impacts. 

Martin cautions that the power of these length/weight measurements to reveal what’s going on beneath the surface may work best when species are “strongly coupled” in the food web. The lake trout in Trout Lake eat almost exclusively cisco, which means the two species’ fortunes are intertwined. There are other examples of strongly coupled predator-prey dynamics – for example, Canadian lynx and snowshoe hares – but, usually, an ecosystem’s food web is a “mesh of weak interactions” where a predator eats a variety of other organisms and its prey species support a lot of other predators, he says. In those cases, simply analyzing length and weight data may not be enough to reveal population dynamics. 

Still, Martin says, the power of simple, long-term measurements to reveal important ecological processes shouldn’t be overlooked. “It’s really, really hard to do population estimates,” he says. “We were just going back to these very simple data and asking how body size changes over time and what drivers [caused those changes].”

It’s something, he says, that can get overlooked in studies utilizing complicated models and statistics but, for the fish in Trout Lake, at least, these simple, modest measurements allowed his team to read ‘a very interpretable story” in all those decades of data.


The paper is available here: