Introduction

The introduction of top carnivore fish greatly reduces the biodiversity of global and local freshwater ecosystems. Such fish reduce native fish populations via predation, competition, hybridization, habitat loss, and trophic alterations (Miller et al. 1989; Allendorf et al. 2001; Choi et al. 2021). Global species movements have accelerated the introduction and dispersal of new carnivore species (Scott et al. 2003; Kim et al. 2021). Established carnivore fish affect energy flow and trophic composition by filling vacant ecological niches and competing with native fish species, which makes conservation and restoration of native fish biodiversity difficult (Scott et al. 2003; Choi and Kim 2021).

Erythroculter erythropterus is a top carnivore fish that has been accidentally introduced (outside its native range) to watersheds of South Korea (Park and Oh 2004; Choi 2005; Lee et al. 2008). In 2011, it was officially reported in the middle and lower reaches of the Nakdong River (Yoon et al. 2012a). The population rapidly increased in the Nakdong River watershed given its high predation and fertility rates (NRWMC 2017). The species is large (up to 1.5 m long) and, in South Korea, is found mainly in major rivers and lakes with stagnant flow (Yoon et al. 2012a). The species adapts very well to different physical and chemical environments (Jang et al. 2012).

Eight weirs were constructed in the Nakdong River mainstream from 2009 to 2012 as part of the Four Major Rivers Restoration Project; these have changed the river ecosystem (Jang et al. 2012; Moon et al. 2019) by transforming the lotic system into a lentic one; which favors expansion of the E. erythropterus population. Also, the fish has no commercial value; fishers throw (dead) fish back into the water immediately; decomposition causes water pollution. The provincial government sought to eliminate the fish, but this reduced the diversity of native fish. Therefore, there is an urgent need to study the impact of E. erythropterus on other fish species and the ecology and distribution of the fish in the Nakdong River basin.

Previous research on fish communities found that E. erythropterus dominated the Gumi Weir and Imha Reservoir (relative abundance [RA] = 41%) (Moon et al. 2019). A 2016 study entitled “The Management of Migratory Freshwater Fish” found that the numbers of commercial fish species decreased as those of exotic species increased in Korean water bodies (Yoon et al. 2018a, b). The introduction of an exotic species into a watershed can change the trophic structure. The introduction of Korean native fish into the Nakdong River Watershed had less effect on the existing native fauna and trophic structure.

The E. erythropterus occupancy level limits the distribution of other fish in the river (Yoon et al. 2018b). One migration study indicated that the species repeatedly moved between the middle and downstream regions of the Nakdong River (Jang et al. 2012). However, the range of many individuals is now limited to around 10 km because of the weirs (Yoon et al. 2012a). Fish may migrate to predate, find new habitat, or spawn (Jeong and Han 2018; Kang 2011). It has been difficult to acquire data on population structures and movements. However, electronic tracking systems now provide quantitative data on migration and community structures. In particular, acoustic telemetry is affordable, simple, cross-compatible, and versatile (Crossin et al. 2017; Matley et al. 2022) and can identify movement times. It is mainly used to monitor the mobility and structure of sizeable exotic fish populations in large rivers and lakes. For example, Bajer et al. (2011) used acoustic telemetry to track invasive Cyprinus carpio to conspecific aggregations; this greatly aided removal of the fish using seine nets (52–94% removal rate). Acoustic telemetry has also been used to map the spawning locations of lake trout (Salvelinus namaycush) in Yellowstone Lake. These fish were successfully eradicated, and the largely devastated populations of native cutthroat trout (Oncorhynchus clarkia) recovered (Ruzycki et al. 2003). In Korea, the method has been used to determine fish migration patterns in reservoirs (Yoon et al. 2012b).

Information on E. erythropterus movements in the Nakdong River basin is sparse. Fish abundance is greatly affected by chemical disturbances; control of such events is key for ecosystem management. We analyzed the correlations between water quality and E. erythropterus abundance in the Nakdong River and provided important information on the distribution pattern. The seine netting at depth in winter may help maintain and restore the biological integrity of the river basin.

Materials and methods

Study area

The Nakdong River basin, which comprises 25% of South Korea’s total land area, begins in the Taebaek Mountains of Gangwon Province and runs through Gyeongsangbuk Province, Daegu Metropolitan City, Gyeongsangnam Province, Busan Metropolitan City, and Ulsan Metropolitan City. The Nakdong River basin is South Korea’s second-largest watershed. The basin area has an area of 23,817.3 km2, the length of the channels is 521.5 km, the circumference is 1,097.13 km, and the average slope is 32.26%.

Analysis of water quality parameters

Monthly surface water quality data from 2017 to 2021 were collected from the Ministry of the Environment Water Information Network (http://water.nier.go.kr). Water quality data were compiled from six mainstream sites (Andong, Chilgok, Waegwan, Jeokpo, Mulgeum, Hagutuk) and 20 tributaries (of the Naesung stream, Yeong river, Wi stream, Gam stream, Geumho river, Hoe stream, Hwang river, Nam river, and Miryang river) of the Nakdong River basin (Fig. 1, Table S1; supplementary file). The portable YSI Sonde 6600 V2 multiparameter analyzer was used on-site to measure pH, water temperature (WT), and electrical conductivity (EC). In addition, samples for measurement of total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD), total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), dissolved total nitrogen (DTN), nitrate (NO3–N), ammonia (NH4–N), dissolved total phosphorus (DTP), phosphate (PO4–P), and chlorophyll-a (CHL-a) were collected, preserved, and analyzed using protocols approved by the Korean Ministry of Environment (MOE 2000) from 2017 to 2021.

Fig. 1
figure 1

Map showing the study sites of Nakdong River basin

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Fish sampling methods

Fish sampling followed the survey and evaluation guidelines of the National Institute of Environmental Research (2017). Overnight, fish were sampled using fyke, gill, and trammel nets. Cast and kick nets were used to capture fish in runs, riffles, and pools. A boat was used to place fyke, gill, and trammel nets along the shoreline; cast and kick nets were placed in nearshore waters. The sampled river length was 200 m, and the sampling period was 60 min at each study location. All collected fish were released immediately after identification. Fish trophic and tolerance guild analyses were conducted as described in a previous regional study (An et al. 2004).

Migration of Erythroculter erythropterus

A programmable submersible ultrasonic receiver (SUR; frequency: 30–90 kHz) was used to determine the movements of E. erythropterus. The SUR detects (and logs to flash memory) fish tagged with Sonotronics ultrasonic transmitters. The SUR has a hydrophone, flash memory, and transponder; the user interrogates the unit from a distance to determine the presence/absence of data. The SUR was deployed at depths of 5 m (Gumi Weir) to 20 m (Chilgok Weir). Eight E. erythropterus were collected from the mainstream; miniature transmitters (Itty-Bitty [IBT] transmitters; frequency range: 70–77 kHz; range: 750 m) were attached to the dorsal fins (Fig. S1; supplementary file); and the fish were then released (Table S2; supplementary file). The SUR was placed between the Gumi and Chilgok weirs. When the sampled fish moved to shallow water (in the Gumi Weir region), the detection range was 203 m. In contrast, when the fish moved to deep water (in the Chilgok Weir region), the detection range was 391 m. The movement investigations proceeded as follows: 2020: first mainstream survey, October 5–8; first survey of tributaries, September 23–November 6, 2021: first surveys of the mainstream and tributaries, June 1–25; second survey, September 23–October 22.

Statistical analysis

Water quality parameters were log10-transformed to ensure data normality prior to regression analyses, which were performed using SigmaPlot (ver. 14) to determine causal relationships between water quality variables and the RAs of E. erythropterus. Principal component analysis (PCA) was used to examine the effects of water quality parameters on the RA of the fish (common name: Skygager). PCA was conducted with PAST software (Hammer et al. 2001).

Results and discussion

Comparative analysis of fish compositions and guilds

A detailed fish composition table for the mainstream and tributaries of the Nakdong River basin is given in the supplementary file (Tables S3 and S4). Tolerant guild analysis showed that tolerant species increased from upstream to downstream at mainstream sites and that sensitive species were absent from downstream sites (Fig. 2). Intermediate species dominated most tributary streams. Trophic guild analysis indicated that omnivores were dominant at mainstream sites M1 and M4, while carnivores were dominant at sites M2, M3, and M6. Omnivores dominated most tributary streams.

Fig. 2
figure 2

Tolerance and trophic guild analyses of the Nakdong River basin (M: mainstream and T: tributary)

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E. erythropterus RAs of the mainstream and tributaries

We found that the numbers of E. erythropterus at mainstream sites were higher than in tributaries (Table 1). The numbers of mainstream E. erythropterus ranged from 10 to 394, compared with 0–55 in tributaries. The RA of E. erythropterus was 0.9–25.35% in the mainstream, compared with 0–11.96% in tributaries. T tests indicated a significant difference between the mainstream and tributary E. erythropterus populations (Table 2).

Table 1 Maximum (max) and minimum (min) number of E. erythropterus individuals and mean RAs
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Table 2 Results of t tests comparing the mainstream and tributary E. erythropterus populations
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Movement of Erythroculter erythropterus

The movements of E. erythropterus were studied with the SUR placed at sites P1–P4 in the Nakdong River basin (Fig. 3). Fish number 1 (tag number 141) was released at point P1 on October 7, 2020, and was subsequently detected from 10 to December 25, 2020, at point P3, and again detected at point P1 from August 5 to 29, 2021 (Fig. 3, Table S2; supplementary file). Fish number 2 (tag number 144) was also released on the same day as fish number 1 at point P1, detected at point P1 on October 7, 2020, and quickly moved about 14 km for 5 days and detected at point P2 from October 12 to October 13, 2020. Fish number 2 stayed between P1 and P2 and was again detected at P1 on July 24, 2021, to August 19, 2021. Fish number 3 (tag number 143) was released on October 7, 2020, at P2 and was detected at P2 from October 7, 2020, to October 9, 2020, and was detected at P3 from October 26, 2020. It moved about 13 km over 10 days, stayed at P3 until June 20, 2021, and then detected at P1 from July 30, 2021, to October 29, 2021. Fish number 4 (tag number 142) was also released at P2 point on October 7, 2020, detected at P2 point from October 7 to October 9, 2020, and detected at P3 point from October 16, 2020, to February 16, 2021. It was found that it moved about 26 km during the spawning period and was found at P1 at the end of July to August 14, 2021. Fish number 5 (tag number 140) was released on October 7, 2020, at P3 and detected at P3 from October 30, 2020, to April 3, 2021, and it moved to P1 and detected from P1 between July 27 to August 14, 2021. Fish number 6 (tag number 139) was also released at the P3 point on the same date as fish number 5, but it was continuously detected at the P3 point from October 7, 2020, to January 28, 2021. It did not move anywhere. Fish number 7 (tag number 138) was also released at the P3 point on the same date as fish numbers 5 and 6, and it was also continuously detected at Point P3 from October 7, 2020, to March 22, 2021. In the case of Fish number 8 (tag number 137), It was released at P4 on October 8, 2020, but it was not detected at any point, and it is believed that it moved downstream for hibernation or to find a suitable habitat.

Fig. 3
figure 3

Movement of E. erythropterus in the Nakdong River basin

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Overall, tagged E. erythropterus moved from upstream (P1) to downstream (P3) sites (where the WT was low). During the spawning period (July–August), the fish moved to shallow water and were usually detected at P1. E. erythropterus moved continuously only within the region between the Chilgok and Gumi weirs due to the disconnection of habitat by constructing the weirs.

Relationship between E. erythropterus RA and water quality

Regression analysis was used to determine the interactions between water quality and the RA of E. erythropterus (Fig. 4). Water quality accurately predicted E. erythropterus populations. Regression analysis revealed that nutrient enrichment (TP: R2 = 0.57), organic pollution (BOD: R2 = 0.56, COD: R2 = 0.69, TOC: R2 = 0.70), suspended solids (SS: R2 = 0.68), and algal blooms (CHL-a: R2 = 0.57) significantly increased the proportions of E. erythropterus.

Fig. 4
figure 4

Effects of water quality parameters on the RA of E. erythropterus. COD: chemical oxygen demand, BOD: biological oxygen demand, TOC: total organic carbon, CHL-a: chlorophyll-a, SS: suspended solids, TP: total phosphorus). The blue dotted lines are 95% confidence intervals

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Insights from principal component analysis

The first two axes of the PCA explained 68.08% of the total variance (axis 1, 46.41%; axis 2, 21.67%) (Fig. 5). PCA was used to determine the effect of water quality variables on E. erythropterus abundance. PCA revealed two distinct groups of variables. The first group included highly significant water quality variables such as the TP, BOD, COD, TSS, TOC, and CHL-a, all of which are on the right of the graph and are thus highly correlated with the RA of E. erythropterus; these were characteristic of sites M2–M5. The second group included tributary sites. Analyses of PCA group 1 suggested that E. erythropterus numbers increased in mainstream regions with higher levels of TP, BOD, COD, TOC, TSS, and CHL-a.

Fig. 5
figure 5

Principal component analysis of water quality parameters in association with E. erythropterus abundance. M: mainstream, T: tributary, pH: hydrogen ions, DO: dissolved oxygen, TP: total phosphorus; TN: total nitrogen; DTP: dissolved total phosphorus, PO4–P: phosphate, DTN: dissolved total nitrogen, NO3–N: Nitrate, NH4–N: ammonium, BOD: biological oxygen demand; COD: chemical oxygen demand; TOC: total organic carbon, TSS: total suspended solids; EC: electrical conductivity; and CHL-a: chlorophyll-a

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Discussion

Biological invasions have wreaked havoc on native ecosystems (Elvira and Almodóvar 2001; Jang et al. 2006; Hur et al. 2018). The introduction of exotic freshwater fish poses a severe threat to the survival and genetic integrity of native fish worldwide (Elvira and Almodóvar 2001). Moyle et al. (1987) described the impact of introduced fish on native species as a “Frankenstein effect,” because the consequences of introductions are unpredictable.

In Korea, 28 fish species have been translocated to water systems that were not among their known original habitats, with most translocations occurring in the Nakdong River basin (Yoon et al. 2018a). E. erythropterus was translocated principally to the mid-to-lower reaches of the Nakdong River, where it may intensively predate native fish (Kim and An 2021).

The environmental conditions changed after the eight weirs were constructed in the Nakdong River; today, the river resembles a reservoir. The weirs slow water flow; the fish assemblage of the river currently consists principally of tolerant species, omnivores, carnivores, and large Cyprinidae and Bagridae species that prefer reservoir-like conditions (Yoon et al. 2017). E. erythropterus also prefers such lentic systems (Jeong and Han 2018). For example, in 2018, Jeong and Han (2018) reported that E. erythropterus was the dominant species (40.1%; 9,333 individuals) in the Imha Reservoir of the Nakdong River system. Likewise, E. erythropterus is the dominant species in the Asan and Chungju Lakes (89.8% and 54.5% of all biomass, respectively) (Heo et al. 2021).

Few Korean studies have explored the interactions between water quality parameters and E. erythropterus populations and found that water quality parameters predicted the number of E. erythropterus (Yoon et al. 2012a; Heo et al. 2021). A previous study used acoustic telemetry to obtain relatively continuous records of fish movements and indicated several advantages when studying migrating fish populations in large rivers (Yoon et al. 2012a). The Nakdong River is the longest river in South Korea, and the mainstream changed after the installation of large weirs. The used acoustic telemetry monitors the movements of E. erythropterus (family Cyprinidae) in the Nakdong River and found that tributary streams hosted fewer E. erythropterus than the mainstream due to a combination of ecological factors, including differences in habitat size, flow dynamics, resource availability, migration patterns, and competitive interactions. The mainstream of a river typically offers a larger and more complex habitat compared to its tributaries. Larger habitats provide more space for fish populations to thrive and access essential resources such as food and shelter (Jeong et al. 2018). In contrast, tributary streams are often smaller and may have simpler habitat structures, which can limit the carrying capacity for E. erythropterus populations. Mainstream river channels generally have more consistent and stable flow dynamics and a greater influx of nutrients, organic matter, and food resources than tributaries. The migratory behavior of E. erythropterus also plays a role and indicates that tributaries serve as secondary habitats or refuges rather than primary breeding or foraging grounds (Yoon et al. 2012b). The results found that a massive fish congregation occurred at the deepest depth (− 17 m) during winter. These fish could be removed using a seine net.

The predatory carp E. erythropterus feeds on benthic invertebrates and juvenile fish, but also preys near the surface (Jeong et al. 2018). The extensive horizontal range indicates that all prey items are of the same trophic level; there is no overlap with the trophic levels of other predatory species (Hur et al. 2018). Given the dramatic increase in the number and the spread of E. erythropterus in the Nakdong River system, the fish may disrupt the structures of existing fish communities, thereby inducing a cascade of negative trophic effects (Yoon et al. 2012a; Hur et al. 2018). Few studies on interspecific competition have sought to determine the impact of E. erythropterus on invaded (native) fish communities (Hur et al. 2018; Yoon et al. 2012b). Understanding the competitive interactions between E. erythropterus and native fish species provides essential information for decision-makers and conservationists. It may help to make informed choices about whether and how to manage the presence of this fish species in a given ecosystem. Studies on interspecific competition allow for prioritization by identifying which native fish species may be most vulnerable to competition from E. erythropterus and it ensures that resources are directed where they can have the greatest positive or negative impact on the ecosystem. Moreover, interspecific competition studies provide a baseline for ongoing monitoring and adaptive management of E. erythropterus. Such studies would aid ecosystem management and conservation and improve our understanding of the effects of introduced freshwater fish on native fauna.

Limitations

This study has several limitations including, the research focused exclusively on the Nakdong River Basin in South Korea, limiting the generalizability of results to other regions. Additionally, constraints related to data availability, including water quality and fish distribution data, may have impacted the comprehensiveness of our findings. The sample size of study sites and telemetry-tagged fish was restricted by logistical constraints, potentially affecting the robustness of the results. Finally, our study primarily explored the dynamics of the invasive species Erythroculter erythropterus, and while insightful, the implications may be specific to ecosystems with similar invasive species compositions.

Conclusion

Our study has shed light on the distribution and migration patterns of Erythroculter erythropterus and the profound influence of water chemistry within the Nakdong River mainstream and its tributaries. Beyond summarizing our findings, we aim to offer a deeper understanding of the empirical and conceptual aspects of our research, as well as the broader implications and advantages it presents. Our empirical investigation unveiled distinct ecological dynamics within the Nakdong River Basin. Notably, we observed a higher abundance of tolerant species in the mainstream compared to tributary sites, while tributary streams were dominated by intermediate and omnivore species. This finding highlights the sensitivity of fish communities to varying environmental conditions and habitat types. The distribution of E. erythropterus further underscored the species’ preference for the mainstream, where numbers were notably higher than in tributaries. Our research also uncovered the migratory behavior of E. erythropterus to shallow waters during spawning, a critical aspect of their life history. From a conceptual viewpoint, our study contributes to the growing body of knowledge on the ecological consequences of introduced species in freshwater ecosystems. By demonstrating the influence of nutrient enrichment, organic pollution, suspended solids, and algal blooms on the abundance of E. erythropterus, we provide key insights into the complex interactions between water chemistry and fish populations. The implications of our findings extend to ecosystem management and conservation efforts. Understanding the ecological drivers of E. erythropterus distribution and their preference for the mainstream can guide strategies for maintaining the ecological balance in the Nakdong River Basin. Moreover, our research highlights the importance of monitoring and addressing water quality parameters to safeguard native fauna. Our study’s use of telemetry technology represents a methodological advancement that enhances the precision of tracking fish movements and their responses to environmental variables. This innovative approach can serve as a model for future research in similar ecosystems.