Scientists studied what pikeperch (a type of predatory fish) ate in a Czech reservoir by comparing two time periods: the 1960s-70s and the 2010s-20s. By examining the stomachs of 875 fish, they discovered that pikeperch are eating less overall, but eating more cyprinids (a common fish type) and less perch. Interestingly, they also found pikeperch eating each other more often in recent years, which suggests there may be fewer other food options available. This research helps us understand how fish communities change over time and what happens when ecosystems shift.

The Quick Take

  • What they studied: What pikeperch fish eat and how their food choices changed between the 1960s-70s and the 2010s-20s in a large water reservoir
  • Who participated: 875 pikeperch fish caught from Lipno Reservoir in the Czech Republic during two different time periods spanning about 50 years
  • Key finding: Pikeperch significantly changed what they eat over 50 years—they’re eating fewer total prey items but eating more cyprinids (common fish) and less perch. They’re also eating each other more often, which wasn’t seen in the earlier period
  • What it means for you: This research shows that fish communities in lakes and reservoirs can change dramatically over decades. If you fish or care about water ecosystems, understanding these changes helps predict what fish will be available in the future and how healthy the ecosystem is

The Research Details

Scientists collected pikeperch from Lipno Reservoir during two separate time periods: 1966-1970 and 2010-2022. They carefully examined the stomach contents of 875 fish total to see what each fish had eaten. By comparing what fish ate in the 1960s-70s to what they ate in the 2010s-20s, researchers could track how the diet changed over approximately 50 years.

For each fish, they recorded what type of prey was in its stomach, how many prey items it had eaten, and the size of both the predator (pikeperch) and prey. They calculated something called a ‘predator-prey length ratio’ which shows the relationship between how big the pikeperch was compared to what it was eating. They also used a special calculation called an ’electivity index’ to determine which prey types pikeperch preferred to eat versus what was just available to eat.

This approach is like looking at a restaurant’s trash over 50 years to see how customers’ food preferences changed—it gives direct evidence of what was actually consumed rather than just guessing.

Stomach content analysis is one of the most reliable ways to understand what fish actually eat in nature. By comparing two time periods, researchers can see how ecosystems change over decades. Pikeperch are important predators that control other fish populations, so understanding their diet helps us know if the entire ecosystem is healthy or stressed.

This study examined a large sample size (875 fish), which makes the findings more reliable. The researchers compared two distinct time periods separated by about 40 years, which allows them to see real long-term changes. However, the study doesn’t explain why these changes happened—it only documents that they did occur. The findings are specific to one reservoir, so results might be different in other lakes or reservoirs.

What the Results Show

The most striking finding was that pikeperch dramatically changed their diet over 50 years. In the 2010s-20s, pikeperch ate significantly fewer total prey items compared to the 1960s-70s, suggesting that overall food availability in the reservoir decreased. However, the types of fish they ate shifted: cyprinids (a common family of fish) made up a much larger percentage of their diet in recent years, while perch and ruffe (two other fish types) became less common in their diet.

One surprising discovery was cannibalism—pikeperch eating other pikeperch. This behavior was observed in the 2010s-20s data but wasn’t seen in the 1960s-70s samples. Cannibalism typically indicates that a predator is struggling to find its preferred food sources and is turning to eating its own species out of desperation or necessity.

The predator-prey length ratio (how big the pikeperch was compared to its food) decreased as the fish grew larger, meaning bigger pikeperch were eating relatively smaller prey. Additionally, the ratios in this reservoir were lower than in other studied reservoirs, suggesting pikeperch in this particular location might grow more slowly than in other places.

While perch and ruffe were still eaten by pikeperch, the electivity index (a measure of preference) showed that pikeperch only actively preferred to eat other pikeperch, not the other available fish types. This suggests that pikeperch may be eating perch and ruffe simply because they’re available, not because they prefer them. The shift toward eating more cyprinids could reflect changes in what fish species are available in the reservoir or changes in where different fish live within the water.

This study adds important long-term data to our understanding of fish ecosystems. Previous research has shown that predator-prey relationships can shift when environmental conditions change, but this 50-year comparison is particularly valuable because it spans such a long time period. The appearance of cannibalism in recent years aligns with other research showing that predators eat their own species when food becomes scarce. The lower predator-prey ratios in this reservoir compared to others suggest that local conditions—possibly water quality, temperature, or food availability—may be different here.

This study only examined one reservoir in the Czech Republic, so the findings may not apply to pikeperch in other locations. The researchers didn’t investigate why the diet changed—they only documented that it did. They couldn’t determine if changes were due to fewer prey fish available, changes in where fish live in the reservoir, or other environmental factors. The study also doesn’t explain what caused the apparent reduction in prey availability or the emergence of cannibalism. Additionally, fishing practices or how the reservoir is managed may have changed over 50 years, which could affect fish populations but wasn’t analyzed in this study.

The Bottom Line

This research suggests that fish ecosystems can change significantly over decades. If you’re involved in managing fisheries or water resources, monitoring what predatory fish eat can be a useful indicator of ecosystem health. For recreational fishers, understanding that pikeperch diet has shifted may help predict where to find them and what bait might work best. However, these findings are specific to one reservoir, so local conditions matter significantly. (Confidence: Moderate—based on solid data from one location, but limited to that specific ecosystem)

Fishery managers and conservation professionals should pay attention to these findings because they show how fish communities can shift over time. Recreational fishers interested in pikeperch fishing may find this useful for understanding fish behavior. Environmental scientists studying how lakes and reservoirs change over time will find this valuable. However, if you don’t fish or work with water ecosystems, this research is more of general scientific interest than directly applicable to daily life.

These changes occurred over approximately 50 years, so they represent very long-term shifts in the ecosystem. If similar changes are happening in other reservoirs, they would likely take decades to become noticeable. This isn’t something you’d see change in weeks or months, but rather something that becomes apparent when comparing data across many years.

Want to Apply This Research?

  • If using a fishing or ecosystem monitoring app, track the types of fish caught in your local reservoir or lake over time. Record the species, size, and date. Over months and years, you can see if the fish community is changing similar to what researchers found in this study.
  • Use the app to log what fish species are most abundant in your area and whether you notice changes in predator fish behavior or diet over seasons. Share observations with local fishing communities or environmental groups to contribute to citizen science efforts that track ecosystem changes.
  • Set up long-term tracking in your app to compare fish populations and predator behavior year-over-year. Create alerts to compare data from the same season across different years to identify trends. This personal data collection mirrors what scientists did in this study and can help you understand your local ecosystem’s health.

This research describes changes in fish diet in one specific reservoir and should not be used to make medical or health decisions. The study is observational and does not prove cause-and-effect relationships between environmental changes and diet shifts. If you rely on fishing for food or income, consult with local fishery management authorities about the health and safety of fish in your area. This information is for educational purposes and should not replace professional advice from fishery biologists or environmental managers regarding your specific location.