Feeding ecology and ecological risks of the invasive fish Coreoperca herzi revealed by gut content DNA and environmental DNA metabarcoding

By integrating gut content and environmental DNA metabarcoding, this study reveals that invasive Korean perch in Japan's Oyodo River system exhibit early-onset piscivory and opportunistic feeding on abundant aquatic insects and fish, posing significant ecological risks to native freshwater communities.

The Gist

Imagine a new, hungry tenant moving into a quiet apartment building (the Oyodo River in Japan). This tenant is the Korean Perch, a fish native to Korea that has recently established itself in Japan. The building's original residents (native fish and insects) are worried because this new tenant is known to be a voracious eater.

To figure out exactly what this new tenant is eating and how much trouble they might cause, scientists decided to play detective. Instead of just looking at what was left in the tenant's stomach (which is like trying to guess a meal by looking at a half-digested smoothie), they used two high-tech tools: DNA Metabarcoding and Environmental DNA (eDNA).

Here is the story of their investigation, broken down simply:

1. The High-Tech Detective Work

• The "Stomach Scan" (Gut Content DNA): The scientists caught 50 Korean Perches. Instead of just squinting at their stomach contents, they extracted the DNA. Think of this as finding a receipt in a trash can that lists every single item bought, even if the food is already digested. This allowed them to identify tiny bits of insects and fish that would have been invisible to the naked eye.
• The "Neighborhood Census" (eDNA): They also took water samples from the river. Fish and insects leave tiny traces of their DNA in the water, like footprints in the sand. By analyzing these footprints, the scientists could create a list of everyone living in the neighborhood and how many of them were there.

2. What Did They Find?

The investigation revealed some surprising facts about the Korean Perch's eating habits:

• They are "All-You-Can-Eat" Omnivores: The perch eats a mix of aquatic insects (like mayflies) and other fish.
• They are Visual Hunters: They seem to prefer insects that cling to rocks (like "epilithic clingers"). It's like they are looking for snacks that are easy to spot on a table, rather than digging through the dirt for hidden treats.
• The "Baby" Predators: This was the biggest surprise. Previous studies suggested that these fish only start eating other fish when they grow big. However, the DNA test showed that even the small, young perch (the size of a finger) were already eating other fish. They didn't wait to grow up to become predators; they started early.
• The "Feast or Famine" Rule: The perch doesn't seem to have a favorite specific fish to eat. Instead, they eat whatever is most abundant and easy to catch. If a certain type of fish is swimming in huge numbers, the perch eats them. It's like a diner who eats whatever is on the "Special of the Day" because it's the most available, rather than having a strict menu.

3. Why Does This Matter?

Think of the river ecosystem as a delicate house of cards.

• The Early Threat: Because the perch starts eating other fish when they are still small, they are putting pressure on the native fish populations much earlier than we thought. It's like a bully who starts stealing lunch money from kindergarteners, not just high schoolers.
• The Ripple Effect: By eating so many insects and small fish, the perch could be changing the entire structure of the river. It's not just about one species disappearing; it's about how the whole "neighborhood" functions.
• The Danger of Being "Too Good" at Hunting: The study found that the perch is very efficient. They can eat fish that are up to half their own size. Since they can grow quite large, they pose a long-term threat to medium-sized native fish, potentially stopping them from reproducing and causing their populations to crash.

4. The Takeaway

This study is like upgrading from a blurry security camera to a high-definition 4K camera with night vision. By using DNA technology, the scientists saw a much clearer picture of the danger the Korean Perch poses.

The Verdict: The Korean Perch is a highly adaptable, early-starting predator that is likely causing significant damage to the native fish and insect communities in Japan. Because they eat whatever is abundant and start hunting young, they are a serious threat that needs to be managed quickly before they completely disrupt the river's ecosystem.

In short: The new tenant in the river isn't just eating leftovers; they are eating the neighbors' kids, and they started doing it the day they moved in.

Technical Summary

1. Problem Statement

The introduction of non-native predators poses severe risks to freshwater biodiversity through predation and competition. The Korean perch (Coreoperca herzi), native to the Korean Peninsula, has recently established itself in the Oyodo River system in Kyushu, Japan. Previous studies relying on visual observation of stomach contents suggested that C. herzi primarily feeds on aquatic insects (specifically mayflies) and shifts to piscivory (fish-eating) only as it grows larger.

However, visual methods suffer from significant limitations:

• Digestion bias: Soft-bodied prey (like fish larvae) and partially digested tissues are often unidentifiable, leading to underestimation of dietary diversity.
• Ontogenetic gaps: Previous studies may have missed early-stage piscivory because small fish prey are rapidly digested and difficult to detect visually.
• Lack of context: Without data on prey availability in the environment, it is unclear if the predator is selecting specific prey or feeding opportunistically on abundant species.

The study aims to resolve these gaps by providing a high-resolution assessment of the Korean perch's diet and its ecological impact using molecular techniques.

2. Methodology

The researchers employed an integrated approach combining gut content DNA metabarcoding with environmental DNA (eDNA) metabarcoding to assess both diet and prey availability.

• Field Sampling:
  • Predators: 50 specimens of C. herzi were collected from the Okimizu River (a tributary of the Oyodo system) in September 2024 via night diving.
  • Environment: Water samples were collected in May 2024 from the same site to characterize the local prey community.
• Laboratory Analysis:
  • Gut Content: Stomach and intestinal contents were separated into solids and semi-liquids. DNA was extracted and amplified using three specific primer sets targeting:
    1. Fish: 12S rRNA (MiFish-U primers).
    2. Aquatic Insects: 16S rRNA (MtInsects-16S primers).
    3. Crustaceans: 16S rRNA (MiDeca primers).
  • eDNA: Quantitative fish eDNA analysis was performed using MiFish-U primers with spike-in standard DNA (known copy numbers) to estimate absolute DNA copy concentrations per liter of water, serving as a proxy for prey biomass/abundance.
• Bioinformatics & Statistics:
  • Sequencing was performed on an Illumina NovaSeq X Plus.
  • Data were processed using the PMiFish pipeline for denoising and species assignment.
  • Statistical Analysis:
    • Diet Composition: Calculated predation frequency (%F) for each prey taxon.
    • Ontogeny: Used Non-metric Multidimensional Scaling (NMDS) and PERMANOVA to test for diet shifts based on Standard Length (SL). A Generalized Linear Model (GLM) tested the relationship between body size and fish predation probability.
    • Selectivity: A GLM (Poisson distribution) correlated prey eDNA concentration (availability) with predation frequency (%F) to determine if the perch feeds opportunistically or selectively.

3. Key Contributions

• Methodological Integration: This is one of the first studies to simultaneously apply gut content metabarcoding and quantitative eDNA metabarcoding to an invasive predator, allowing for a direct comparison between what is eaten and what is available.
• Overcoming Visual Limitations: By utilizing DNA metabarcoding, the study detected prey taxa that were morphologically unrecognizable or fully digested in previous visual surveys, significantly revising the understanding of the species' diet.
• Quantitative Context: The use of spike-in standards in eDNA analysis allowed for the quantification of prey abundance, enabling a robust test of prey selectivity vs. opportunism.

4. Key Results

• Dietary Composition:
  • Aquatic Insects: Detected in 98% of specimens. The diet was dominated by epilithic clinger taxa (mayflies like Heptageniidae and Baetidae), suggesting visual hunting on stone surfaces. Notably, black fly larvae (Simuliidae), common in the environment, were rarely found in guts, indicating selective avoidance or habitat mismatch.
  • Fish: Detected in 66.7% of specimens. The most frequent prey were Pseudogobio esocinus (48.9%) and Opsariichthys platypus (40.0%).
  • Crustaceans: Rarely detected (17.8%), likely due to low environmental abundance rather than avoidance.
• Ontogenetic Dietary Shifts:
  • Contradiction of Previous Studies: Unlike visual studies suggesting piscivory begins only in larger individuals, this study found no significant relationship between body size and fish predation.
  • Early Piscivory: Fish predation was detected in 70.0% of small individuals (58–99 mm SL) and 64.0% of large individuals (100–165 mm SL). This indicates that C. herzi begins eating fish at a very early life stage, and the ontogenetic shift is not pronounced.
• Prey Selectivity:
  • A significant positive relationship was found between the eDNA concentration of prey fish (those small enough to be eaten) and their predation frequency.
  • This suggests the Korean perch is an opportunistic feeder, primarily targeting abundant and size-accessible prey rather than exhibiting strong species-specific preference.
• Regurgitation Evidence: Partially digested fish (P. esocinus and O. platypus) were found regurgitated in transport containers, confirming that even small perch (approx. 35–65 mm prey) are consumed.

5. Significance and Implications

• Ecological Risk Assessment: The findings indicate that C. herzi poses a higher and more immediate threat to native fish populations than previously thought. Because they prey on fish from a very early size class (age-0), they can suppress recruitment and cause population declines across all life stages of native species, not just adults.
• Management Strategy: The lack of a distinct ontogenetic shift means that control measures cannot rely on size-selective harvesting to protect juvenile native fish; the predator is a threat throughout its growth.
• Behavioral Insights: The preference for epilithic insects and abundant fish suggests the perch is a visual predator that exploits high-density prey patches. The absence of black fly larvae in the diet despite their environmental abundance highlights specific foraging constraints or preferences.
• Future Research: The study underscores the necessity of combining molecular tools with traditional methods to accurately assess invasive species impacts, particularly when prey are soft-bodied or rapidly digested.

In conclusion, the Korean perch in Japan is a highly efficient, opportunistic generalist predator that exerts intense pressure on both benthic insects and native fish populations from the earliest stages of its life, posing a significant risk to the biodiversity of the Oyodo River system.