Not a Fish Story

When humans search for answers to complex problems, we often spend a good deal of time casting about in the dark. And our quests for answers commonly conclude with limited success or total failure. At least that is how it goes for me.

I suppose my quests are nerdier than average. In my research, I try to answer questions like: “Why do young loons seek to settle on lakes that resemble their natal one?” or “Why do female loons not yodel like males?” Still, the vexing puzzles I face in my work are not wholly different from the gnarly ones I confront in everyday life. “Why does the cupboard door under the sink never close properly?” “Why do plants keep dying in the southeastern corner of our yard?” Regardless of their origin, life is full of riddles that are challenging to address — and rewarding to solve.

This fall, I have been investigating factors that influence masses of adult loons and chicks. Most predictors of adult male mass* are easy to understand or, at least, lead to highly plausible and testable hypotheses. Males show a long-term decline in average body mass that began thirty years ago and continues today. This result suggests that human encroachment of some kind has made it harder for loons to maintain optimal body condition — and is getting worse. Recently we also found that adult males have lower mass on territories with a high density of docks. Again, we can understand how more docks — and more angling — might hinder adult male foraging and lead to lower mass.

Likewise, most predictors of chick mass are easy to explain. Short-term water clarity affects chick mass strongly. We have known for over a year that lower aquatic visibility hinders loon foraging. And I was not surprised to learn that a chick’s mass is correlated with that of its mother. It stands to reason that body size would be heritable in loons, as it is in other species. Finally, like their parents, chicks show a long-term downturn in mass over three decades that hints at environmental degradation. These three statistical relationships make perfect sense.

In fact, a brain twister did not emerge from this recent analysis until I looked at the relationship of dock density and chick mass. You see, chick mass is positively correlated with dock density. That’s right: the more docks there are on a lake’s shoreline, the fatter the local loon chicks become! The result is astonishing for two reasons. First, it runs counter to logic; second it is opposite to the male pattern. How could more docks — and thus more boaters and anglers — help chicks gain mass, when the same factor is harming their male parents?

I checked and double-checked the statistics. I scanned the data for abnormalities. Many days of fruitless pondering passed. At last I hit upon an idea that was hiding in plain sight: maybe the anomalous pattern had to do with food. Now, the data on fish abundance in our study lakes is of poor resolution and contains many holes. In fact, there are not enough data from any of our 205 Wisconsin study lakes to compare fish abundance with chick mass directly. Still, scientists have collected reams of data on abundance and size of fishes statewide. So we know a good deal about broad patterns.

A recent long-term analysis of fisheries shows that small fishes favored by loons — bluegill, yellow perch, crappies — have been declining in average size across the state for 70 years. The decline in panfish sizes — a consequence of overfishing, according to fisheries biologists — provides a plausible explanation for the long-term loss of mass in adults and chicks. Loons, it seems, must work harder each year to capture enough panfish to sustain themselves. This broad decrease in panfish size in Wisconsin lakes, therefore, might explain why masses of chicks and their male parents have declined in recent decades.

But how do we explain why loon chicks gain mass in busy lakes while males lose mass? Remember, when young, chicks can only swallow very small fish (as Linda Grenzer’s photo shows). Even at five weeks, chicks consume fish smaller than those that adults do. This appears to be the key point. Young chicks ingest small fishes — the very size-class of fishes that thrives when larger fishes are removed from the lake by anglers.**

So I have solved a puzzle related to loon feeding ecology. Well, more accurately, I have generated a highly plausible hypothesis that is consistent with the data but will require further testing. For the moment, I am quite satisfied with this outcome. Because I am a scientist, and that is how we roll.

*We have a large sample of males than females, so our analysis of male mass is more robust.

**While we might celebrate that young loon chicks appear to benefit from human overfishing, this news is not cheering. The first five weeks of a chick’s life, after all, is only one critical phase that it must pass through. This year’s hatchlings are 13 to 18 weeks old now. Their diet is now identical to their parents’. Like their parents, 2023 juveniles are finding fewer large bluegills, perch, and crappies. So they too must spend more time foraging than they did 30 years ago. Could the dearth of large panfish on the breeding grounds help us understand why loon survival from the late juvenile stage to adulthood has declined precipitously in the Upper Midwest?