Invasive alien fish species, such as the largemouth bass, Micropterus salmoides (Lacepède, 1802), and bluegill, Lepomis macrochirus Rafinesque, 1819, have rapidly spread across freshwater ecosystems in Korea, raising serious ecological threats. This study investigated the impact of these invasive alien species on fish communities in large lakes and small reservoirs over a 5-year monitoring period (2019–2023). Eight sites, including four lakes and four reservoirs, were surveyed twice a year. Fish were collected using cast nets, kick nets, and gillnets (for lakes). Species diversity indices (Shannon’s Hʹ, Pielou’s Jʹ, and Margalef richness) and dominance indices were calculated. Poisson-based analysis of variance models was applied to examine the relationship between the densities of invasive alien species and the diversity of fish communities. Sites with higher densities of largemouth bass and bluegill had significantly lower diversity indices. Non-metric multidimensional scaling analyses indicated distinct community groupings according to the type of waterbody and the presence of invasive alien species. These results emphasize the need for continued monitoring and management strategies tailored to different habitat scales.

invasive alien species, Lepomis macrochirus, Micropterus salmoides, aquatic ecosystems, Long-term monitoring, closed waterbodies

The growth of international trade and industrial development has resulted in an increase in the influx of alien species (Bellard et al. 2013; ME 2006). According to the Global Invasive Species Program, there are four types of introduction pathways of alien species: intentional, accidental, contained, and post-introduction spread via vectors. In Korea, the majority of alien species have been intentionally introduced for purposes such as agriculture, ornamental use, and the pet trade, although some cases have involved unintentional or contained introductions (Bang et al. 2004). Among alien species, fish are often deliberately introduced for fisheries and biological control. Nevertheless, unintentional introductions have also occurred due to the escape of farmed individuals, released pets, or mixing during the release of other fish species. In Korea, both intentional and unintentional introductions appear to be the major pathways for the invasion of alien fish species. The introduction of such alien species can result in both ecological and economic damage; in fact, it has been reported that the global economic loss due to the spread of alien species amounts to approximately 530 trillion KRW annually (IPBES 2023). Moreover, invasive alien species are among the most significant direct drivers of global biodiversity loss. In particular, biological diversity has been significantly altered in freshwater ecosystems by invasive alien species over the past 50–100 years, with their impact on biodiversity intensifying continuously (IPBES 2023). As per the most recent national survey, 2653 alien species have been identified in Korea, showing a steady increase from 894 species recorded in 2009 (NIE 2021). Among them, alien fish accounted for 879 species (NIE 2023), comprising 33.13% of all recorded alien species.

In Korea, the most prominent invasive alien fish species are the largemouth bass, Micropterus salmoides (Lacepède, 1802), and bluegill, Lepomis macrochirus Rafinesque, 1819. The largemouth bass is a species native to the southeastern United States. It was introduced to Korea from North America in 1973 to improve fishery resources, being first released into Cheorwon and Lake Paldang, after which it spread through rivers and reservoirs throughout the country. The bluegill, also native to the southeastern United States and belonging to the same family, was introduced to Korea from Japan in 1969 for the same purpose, after which it spread to rivers and lakes nationwide.

Aquatic ecosystems are relatively enclosed systems that are particularly vulnerable to disruption caused by the introduction of alien species (Strayer 2010). As invasive alien species, fish occupy the higher trophic levels in ecosystems and induce more profound changes than other taxonomic groups (Orians 1995; Strayer 2010). Moreover, alien fish may compete with native fish species and alter the food web, potentially affecting water quality and nutrient cycling (Gallardo et al. 2015). In particular, the introduction of novel predators exerts the greatest impact on native benthic invertebrates (Gallardo et al. 2015).

The largemouth bass reduces fish species diversity in invaded ecosystems and modifies food web structure (Carpenter and Kitchell 1993; Takaharu et al. 2015). Due to its generalist feeding habits and behavior of guarding spawning grounds, it often becomes an apex predator in aquatic ecosystems (Hill and Cichra 2005; Almeida et al. 2012), exerting significant negative effects on reservoirs and rivers (Ko et al. 2008). Bluegills are omnivorous and feed on both plant and animal material; however, they often exhibit a preference for animal prey, depending on the environment. Their broad diet and rapid population growth pose a threat to invertebrate and fish communities (Song et al. 2012). Consequently, both the largemouth bass and bluegill have been designated as ecosystem-disturbing species under the Natural Environment Conservation Act since 1998 (Act on the Conservation and Use of Biological Diversity, Article 21-2, Risk Assessment) and are subject to management in Korea (NIE 2021; ME 2024).

Freshwater ecosystems are typically classified into lotic systems, such as rivers, and lentic systems, such as lakes, reservoirs, and wetlands. Unlike rivers, lakes and reservoirs exhibit stagnant water with minimal flow and generally support a single stable community structure (Dobson and Frid 2009). Over time, fish species inhabiting lentic environments develop species-specific ecological and physiological strategies that allow them to adapt to these conditions and maintain their populations (Kim et al. 2005). Nevertheless, in such single-community lentic systems, the introduction of alien species can cause a decline in biodiversity (Vitousek et al. 1996), and studies have also reported a significant reduction in fish species diversity in lakes and reservoirs invaded by alien species (Maezono and Miyashita 2003; Maezono et al. 2005). The impact of alien species may vary depending on the size of the reservoir. In small lakes with relatively low ecological complexity, there is limited available space, resources are scarce, and native fish communities are more vulnerable to disruption. In contrast, large lakes with greater ecological complexity and more available habitats may buffer the spread of alien species to some extent.

Therefore, this study focused on assessing the relative abundance and community structure of fish communities in lakes colonized by invasive alien species, namely bluegill and largemouth bass, based on 5 years of monitoring data from large and small lakes. The outcomes of this research may support management strategies that aim to maintain balanced fish communities by regulating populations of these species.

Fish monitoring was conducted twice annually (in summer and autumn) from 2019 to 2023 at eight sites where largemouth bass and bluegill were present. These sites included four lakes—Lake Chuncheon, Lake Chungju, Lake Daecheong, and Lake Jangseong—and four reservoirs—Susim, Gangjeong, Buckchosan, and Gwanglyeong (Fig. 1).

Figure 1. 

Map showing the eight study sites located in South Korea. A. Lake Chuncheon, B. Lake Chungju, C. Lake Daecheong, D. Lake Jangseong, E. Gangjeong Reservoir, F. Buckchosan Reservoir, G. Susim Reservoir, H. Gwanglyeong Reservoir.

Cast nets (mesh size: 5 × 5 mm) and kick nets (mesh size: 4 × 4 mm) were used to evaluate the fish fauna at each site. Furthermore, in the four large lakes, gill nets (4- and 12-panel types, each 50 m in length) were set for more than 12 h and then retrieved. Cast nets were thrown at least 10 times per site, and kick netting was conducted for 40 min. All captured fish were identified and counted on-site. Native fish were released back into the habitat after identification, excluding the invasive alien species. The relative abundance of each species was calculated to compare fish assemblages among the sites. Community structure was analyzed using indices of species diversity, evenness, and richness.

Species diversity was calculated using the Shannon–Wiener index (Pielou 1969):

$H^{\text{'}}=\sum _{i=1}^s\left[\frac{n_i}{N}\ln \frac{n_i}{N}\right]$

where S is the total number of species, n_i is the number of individuals of species i among the collected individuals, and N is the total number of individuals collected.

Evenness was estimated with Pielou’s evenness index (Pielou 1975):

$J^{\text{'}}=\frac{H^{\text{'}}}{\ln \left(S\right)}$

which expresses the ratio of observed diversity to the maximum possible diversity.

Species richness was evaluated using Margalef’s richness index (RI) (Margalef 1958):

$\mathrm{RI}=\frac{(S-1)}{\ln \left(N\right)}$

where N is the total number of individuals in the sample.

The effect of the relative abundance of largemouth bass and bluegill on fish species diversity was evaluated by analysis of variance (ANOVA) using R (ver. 4.4.2) based on the Poisson distribution. ANOVA was conducted using the aov function with the specified Poisson family, and the Type II sum of squares was applied using the car package (ver. 3; Fox and Weisberg 2019). For post hoc analysis, the emmeans package (ver. 1.10.7; Russell et al. 2021) was used for comparing pairwise mean differences, and Tukey’s HSD was applied for identifying statistically significant group differences. The relationship between sampling sites and fish assemblages was further analyzed using non-metric multidimensional scaling (nMDS) with the metaMDS function from the vegan package (ver. 2.6-10; Oksanen et al. 2025) in R (ver. 4.4.2). Dissimilarity among sites was evaluated using the Bray–Curtis distance. The ordination results were interpreted based on the stress value, where values < 0.2 indicate acceptable ordination and values < 0.05 indicate excellent fit (Clarke and Warwick 1994).

Over the 5-year monitoring period, 9105 individual fish representing 41 species from 11 families were collected from the four lake sites, with the invasive alien species bluegill and largemouth bass accounting for 3947 individuals (a relative abundance of 43.35%; Table 1). Among the lakes, Lake Chuncheon exhibited the highest relative abundance (72.98%; 1383 individuals) and the greatest species richness (27 species). Bluegill and largemouth bass were consistently dominant in Lake Chuncheon, resulting in elevated dominance values and a corresponding decline in evenness. In Lake Chungju, bluegill remained a persistent subdominant species, although cyprinids, such as Coreoperca herzi Herzenstein, 1896, were also abundant, resulting in a slightly different community structure.

In Lake Daecheong, the abundance of invasive alien species declined in 2023 compared with earlier years; however, their relative abundance still reached 40.78%. As 18 species were present, both the diversity index and species richness were the highest among the four lakes. Lake Jangseong also exhibited a lower proportion of invasive alien species in 2023; however, the small number of species collected implied that the richness and diversity indices were only minimally affected.

Bluegill was predominant in the four reservoirs (Table 2). In Gangjeong, only bluegill was recorded, although the overall abundance was comparatively low. In Buckchosan, largemouth bass was dominant until 2020; however, after a complete removal project, no invasive alien species were detected, resulting in increases in diversity and richness. In Susim, only largemouth bass was observed, although 1780 individuals representing 20 species were captured; hence, the dominance values were low. In contrast, both largemouth bass and bluegill were detected in Gwanglyeong, and these two species were also found for several years in this reservoir, indicating a marked community imbalance. Across the lakes, C. herzi and bluegill were typically dominant or subdominant, whereas in the reservoirs, C. herzi, bluegill, largemouth bass, and the goby, Rhinogobius brunneus (Temminck et Schlegel, 1845), were frequently dominant or subdominant (Table 3).

Korean endemic species were collected in Chuncheon (8 species, mean endemic rate 29.6%), Chungju (2 species, 10.5%), Daecheong (2 species, 8.7%), Jangseong (5 species, 23.8%), Gangjeong (2 species, 10.0%), and Susim (3 species, 15.0%), but were not observed in Buckchosan or Gwanglyeong (Tables 2, 3). In addition, the endangered species, Pseudopungtungia nigra Mori, 1935, was captured once in Chuncheon in 2019 but has not been recorded thereafter.

nMDS analysis of fish communities and largemouth bass/bluegill densities revealed that all sites contained invasive alien species (Tables 1, 2; Fig. 2). Among the lakes, Lake Chuncheon had the highest proportion of invasive alien species (72.98%), whereas among the reservoirs, Gwanglyeong showed 99.19% (Table 3). Bluegill was dominant in the lakes, whereas largemouth bass was dominant in the reservoirs (Fig. 2).

Figure 2. 

Density of Micropterus salmoides (A) and Lepomis macrochirus (B) at the study sites in South Korea during the monitoring period.

A Type II Poisson-based ANOVA revealed that the density of both largemouth bass and bluegill significantly affected species diversity (Table 4). Largemouth bass density exerted a significant effect on diversity (F_{1, 38} = 8.58, P = 0.0057), and bluegill density exerted a very strong effect (F_{1, 38} = 21.02, P < 0.0001), and the interaction between them was also significant (F_{1, 38} = 14.87, P < 0.001). Neither lake size nor its interactions with largemouth bass or bluegill density significantly affected diversity (P > 0.05), and the largemouth bass × lake size interaction was similarly nonsignificant (P > 0.05).

The nMDS ordination further revealed distinct clustering of communities according to waterbody size (lakes vs. reservoirs; Fig. 3), confirming size-related differences in fish assemblages. Buckchosan plotted separately from the other reservoirs, indicating community shifts after the complete removal of largemouth bass in 2020.

Figure 3. 

Ordination plots of fish species from four lakes and four reservoirs, in South Korea, based on non-metric multidimensional scaling (nMDS; stress value = 0.18).

Largemouth bass and bluegill have been introduced to improve fishery resources in several countries. Similarly, they were introduced to Korea to address food shortages and improve fish stocks. Since their introduction, both species have adapted to and spread across most rivers and reservoirs, causing ecological disturbances. Largemouth bass are now widely distributed throughout the country, including Jeju Island, and are especially abundant in the main river channels, with higher densities typically observed in low-gradient rivers (Kim and An 2021). Largemouth bass prefer lentic habitats with slow or stagnant water flow (Lee et al. 2009). Similarly, bluegill prefer slow-flowing environments and are now widely distributed across rivers and reservoirs in Korea, with particularly high occupancy rates in weedy dam lakes and reservoirs (Son and Song 2006). This indicates differences in microhabitat between the two species.

In the presently reported study, bluegills were largely absent from reservoirs, except for one individual in Buckchosan and a few individuals in Gwanglyeong. However, their abundance was high in larger lakes, exhibiting a distinct distribution pattern compared with that of largemouth bass, which were present at all sampling sites. Lake Chuncheon had the highest number of species and the highest proportion of invasive alien species (72.98%). In contrast, the other lake sites (Chungju, Daecheong, and Jangseong) exhibited lower relative abundances of invasive alien species (29.64%, 33.56%, and 40.58%, respectively). In reservoirs, except for Gwanglyeong (99.19%), the relative abundance of invasive alien species was lower, ranging from 9.38% to 16.24%, indicating a generally lower load of invasive alien species than that in lakes.

These differences can be attributed to several factors. First, the sampling methods differed between lakes and reservoirs. Cast nets and kick nets used in lakes have limitations, especially in deeper waters; hence, gill nets were also deployed, which were particularly effective in collecting species such as largemouth bass and bluegill that were active at depths of 2–3 m. Second, lakes may have been prioritized earlier in fishery improvement projects, resulting in earlier and more extensive introduction of invasive alien species, which are now reaching ecological saturation. In contrast, most reservoirs have been established for agricultural purposes and may have experienced a delayed introduction of invasive species. For instance, in Buckchosan, largemouth bass was dominant until 2020 (relative abundance 63.08%–75.66%); however, after a complete removal project, no invasive alien species have been observed since 2021, resulting in an overall decrease in their 5-year cumulative abundance. In Gwanglyeong, only a few individuals of native species, such as carp Cyprinus carpio, crucian carp, Carassius Carassius (Linnaeus, 1758), topmouth gudgeon, Pseudorasbora parva (Temminck et Schlegel, 1846), and northern snakehead, Channa argus (Cantor, 1842), were recorded, whereas most captured fish were invasive alien species, indicating a severely degraded fish community.

The analysis of largemouth bass and bluegill densities in lakes and reservoirs revealed that both species were significantly negatively correlated with fish species diversity. These results are consistent with those of Lee et al. (2020), who reported that the abundances of largemouth bass and bluegill significantly and negatively correlated with the total abundance of native fish. This suggests that increased densities of invasive alien species correspond to decreased species diversity. Because diversity reflects both species richness and evenness, the dominance of invasive alien species implies a decline in ecosystem health and stability. Largemouth bass feed on aquatic insects, fish, and shrimp, and those inhabiting lakes primarily prey on fish (Ko et al. 2008; Park et al. 2019). Bluegills are omnivorous and feed on both plant material and animal prey; however, in introduced habitats, they tend to prefer animal prey and feed on aquatic invertebrates, small fish, and fish eggs (Mittelbach 1984; Azuma 1992). At invaded sites, both species probably caused declines in the abundance of native species through direct predation and competition for food and habitat, leading to reduced species richness and abundance.

It has been almost 50 years since the introduction of largemouth bass and bluegill into the freshwater ecosystems of Korea, and numerous studies continue to report their negative impacts on fish communities, native species, and endemic fauna. According to Kim (1997), the average percentage of endemic species in Korea is 25.9%. Compared with this value, all study sites exhibited lower proportions, except for Chuncheon (29.6%). Therefore, it is important to develop ongoing strategies and conduct consistent monitoring to protect and restore river ecosystems. The presently reported study was conducted in lakes and reservoirs within the Hangang, Geumgang, and Yeongsangang River basins; therefore, some variability may exist in other watersheds, such as the Nakdonggang and Seomjingang river systems. Moreover, because of the relatively small number of study sites, it is necessary to expand the number of sampling locations to ensure more robust conclusions. Nonetheless, this study provides valuable long-term monitoring data on invasive alien species in lakes and reservoirs over 5 years and may serve as a foundational resource for the future management of invasive alien species and the development of policies.

This work was supported by a grant from the National Institute of Environment Research (NIE) of the Republic of Korea (NIE-A-2025-09).