The investigation focused on four large lakes and four smaller reservoirs, sampling twice annually to capture seasonal variations in fish assemblages. Researchers utilized cast nets, kick nets, and gill nets to collect a total of 9,105 individual fish representing 41 species from 11 families. Among these, the invasive alien species accounted for a substantial portion of the catch, with bluegill and largemouth bass together comprising 43.35% of all individuals collected in the lake sites. This high relative abundance indicates that these non-native fish have established dominant positions within the food webs of these ecosystems, often outcompeting or preying upon native species.

Statistical analysis of the collected data demonstrated a clear negative correlation between the density of invasive species and the diversity of native fish communities. Using Poisson-based analysis of variance models, the study found that sites with higher densities of largemouth bass and bluegill exhibited significantly lower Shannon diversity indices. The effect of bluegill density was particularly strong, with a highly significant impact on diversity, while largemouth bass density also showed a significant negative effect. Furthermore, the interaction between the two invasive species was significant, suggesting that their combined presence exacerbates the decline in native species richness and evenness more than either species alone.

Non-metric multidimensional scaling analyses further illustrated distinct community groupings based on waterbody type and the presence of invasive species. The ordination plots revealed that fish assemblages in lakes and reservoirs clustered separately, confirming size-related differences in community structure. Notably, Buckchosan Reservoir plotted distinctly from other sites after a complete removal project for largemouth bass was implemented in 2020. Following this intervention, no invasive alien species were detected in subsequent years, leading to a measurable increase in diversity and species richness. This case study provides empirical evidence that active management can reverse some of the ecological damage caused by invasive populations.

The distribution patterns of the two invasive species differed markedly between lakes and reservoirs. Bluegill were predominantly found in the larger lake sites, where they often acted as dominant or subdominant species. In contrast, largemouth bass were more prevalent in the reservoirs, although their abundance varied significantly across sites. In Gwanglyeong Reservoir, the relative abundance of invasive species reached 99.19%, indicating a severely degraded community where native fish were nearly displaced. This extreme imbalance highlights the vulnerability of smaller, enclosed water bodies to biological invasion, where limited habitat complexity and resources make native species more susceptible to disruption.

Historical context reveals that both species were intentionally introduced to Korea to improve fishery resources and address food shortages. Largemouth bass were introduced from North America in 1973, while bluegill arrived from Japan in 1969. Since then, both species have spread rapidly through rivers and reservoirs, adapting to local conditions and occupying higher trophic levels. Their generalist feeding habits allow them to exploit a wide range of prey, including aquatic invertebrates, small fish, and fish eggs. This predatory pressure, combined with competition for habitat and food, has led to a decline in the abundance of native species and a reduction in overall ecosystem health.

The study also examined the impact on Korean endemic species, which are particularly vulnerable to invasive competition. The average percentage of endemic species in Korea is estimated at 25.9%, but most study sites recorded lower proportions, with only Lake Chuncheon exceeding this baseline. The endangered species Pseudopungtungia nigra was captured once in 2019 but has not been recorded since, suggesting a potential local extinction due to invasive pressure. These findings emphasize that invasive alien species are not only reducing general biodiversity but also threatening unique and irreplaceable components of the local fauna, contributing to the broader global loss of freshwater biodiversity.

Management implications from the research suggest that strategies must be tailored to different habitat scales and waterbody types. The successful removal of largemouth bass from Buckchosan Reservoir demonstrates that targeted interventions can restore community balance, but such efforts require sustained monitoring to prevent re-invasion. The study recommends continued long-term monitoring to track population dynamics and assess the effectiveness of management actions. Additionally, expanding the number of sampling locations across different river basins is necessary to generalize these findings and develop robust policies for invasive species control in other regions of Korea.

As international trade and industrial development continue to facilitate the spread of alien species, the ecological and economic costs of inaction are becoming increasingly apparent. The global economic loss due to invasive species is estimated at approximately 530 trillion KRW annually, with freshwater ecosystems bearing a disproportionate share of the biodiversity loss. This five-year study provides a foundational dataset for understanding the long-term impacts of largemouth bass and bluegill in Korean waters, offering valuable insights for conservationists and policymakers. By identifying the specific mechanisms through which these species disrupt native communities, the research supports the development of more effective and targeted management strategies to preserve the integrity of freshwater ecosystems.

Subject of Research: Impacts of invasive alien fish species— Micropterus salmoides and Lepomis macrochirus (Actinopterygii, Centrarchiformes, Centrarchidae)—on freshwater fish communities in lakes and reservoirs of Korea: A five-year monitoring study

Article Title: Impacts of invasive alien fish species— Micropterus salmoides and Lepomis macrochirus ( Actinopterygii , Centrarchiformes , Centrarchidae )—on freshwater fish communities in lakes and reservoirs of Korea: A five-year monitoring study

Article References: Cho, Y. J., Kim, J.-W., Park, C.-W., Lee, S. I., & Kim, J.-G. (2025). Impacts of invasive alien fish species— Micropterus salmoides and Lepomis macrochirus (Actinopterygii, Centrarchiformes, Centrarchidae)—on freshwater fish communities in lakes and reservoirs of Korea: A five-year monitoring study. Acta Ichthyologica et Piscatoria, 55, 281-289. https://doi.org/10.3897/aiep.55.164833

Image Credits: AI Generated

DOI: 10.3897/aiep.55.164833

Keywords: Impacts, invasive, alien, fish, species, Micropterus, salmoides, Lepomis, macrochirus, Actinopterygii, Centrarchiformes, Centrarchidae