Environmental DNA (eDNA) has emerged as a promising tool for aquatic biodiversity monitoring, yet its collection in lentic ecosystems remains technically constrained by filtration capacity and field logistics. In this study, we applied a novel eDNA concentration system, QuickConcTM, to evaluate freshwater mussel diversity in lakes, and compared its performance with the conventional GF/F filtration method. Water samples were collected from four reservoirs at surface, mid, bottom, and waterside layers, and processed using both filtration techniques. Metabarcoding of mitochondrial 16S rDNA revealed that QuickConcTM captured a higher average number of amplicon sequence variants (ASVs) and exhibited greater species richness and diversity indices (Shannon and Simpson), although the differences were not statistically significant. QuickConcTM samples showed a greater capacity to detect rare taxa and to recover higher ASV richness in certain cases, suggesting its potential to enhance biodiversity resolution. Species composition remained consistent across methods, with Cristaria plicata and Sinanodonta lauta being dominant in both cases. However, slight spatial variations in species assemblages were observed between center and waterside sampling points, highlighting the influence of habitat heterogeneity on eDNA distribution. Overall, our results demonstrate that the QuickConcTM system offers a practical and efficient alternative to traditional filtration methods for eDNA-based freshwater mussel monitoring, particularly in environments with high suspended solids. The findings underline the need for adaptive sampling strategies that consider both methodological and ecological factors when designing eDNA surveys in lentic ecosystems.

Freshwater bivalves contribute to key ecological functions in lake ecosystems, yet their cryptic and benthic lifestyles often hinder detection through conventional surveys. In this study, we applied environmental DNA (eDNA) metabarcoding to assess the diversity and distribution of unionid bivalves in six lakes across Republic of Korea. Water samples were collected from three sampling strategies-Center Surface, Center Mix, and Waterside Surface-and processed using 16S rDNA-targeted primers followed by high-throughput sequencing. A total of four unionid species (Cristaria plicata, Sinanodonta lauta, Unio (Nodularia) douglasiae, and Anodonta woodiana) were detected across 18 sampling points. Notably, eDNA successfully identified unionid presence in all lakes, even where conventional surveys failed to observe individuals. Among the sampling strategies, Center Mix exhibited the highest values for Shannon and Simpson indices as well as ASV richness. Waterside Surface samples generally showed lower diversity and detection frequency. A Venn diagram of ASV occurrences revealed three ASVs shared across all sampling strategies and one unique ASV found only in Center Mix. These results indicate that sampling location significantly affects detection sensitivity and diversity representation in eDNA-based bivalve monitoring. Combined application of Center Mix and Center Surface strategies may enhance both detection efficiency and species diversity coverage in lentic environments.

Environmental DNA (eDNA) analysis has emerged as a powerful tool for biodiversity monitoring due to its efficiency, standardization potential, and cost-effectiveness. We evaluated the applicability of eDNAbased zooplankton monitoring in Korean lakes by comparing three DNA methods-eDNA, iDNA, and eiDNA-with traditional microscopy. Sampling was conducted in six lakes with varying conditions. eDNA was obtained from lake water, iDNA from unpreserved zooplankton incubated in water, and eiDNA from zooplankton incubated in ethanol. DNA metabarcoding detected more taxa than microscopy, but dominant taxa overlapped, mainly Daphnia. While DNA methods showed higher richness, Simpson and Shannon indices were higher in microscopy, reflecting differences in quantification methods. These discrepancies reflect methodological differences in how taxa are quantified and suggest that DNA-based approaches may overrepresent certain groups in richness estimates. In addition, false negatives were observed for several common rotifer species (e.g., Keratella, Polyarthra), likely due to incomplete reference databases and high intraspecific genetic diversity. Conversely, some taxa detected only by DNA-particularly small-bodied or rare crustaceans-may represent false positives relative to microscopy. These findings emphasize the importance of improving reference libraries and interpreting DNA results with caution, while also supporting the utility of DNA-based methods as complementary tools in zooplankton monitoring and national biodiversity assessments.

Phytoplankton play a vital role as primary producers in freshwater ecosystems, contributing to the nutrient cycle, energy flow, and ecological stability. To accurately assess phytoplankton diversity and community composition, this study compared traditional microscopy and environmental DNA (eDNA) metabarcoding in six small lakes located in the Han, Geum, and Nakdong River basins in Korea. eDNA analysis identified 268 species from 161 genera, approximately 2.4 times higher than microscopy, which detected 113 species from 68 genera. The eDNA data were dominated by picocyanobacteria such as Synechococcus and Cyanobium, while microscopy primarily revealed larger taxa, including Stephanodiscus and Scenedesmus. Nonmetric multidimensional scaling (NMDS) based on Bray-Curtis similarity showed clear separation between the two methods, with average similarity values of 0.0326 (1st survey) and 0.0221 (2nd survey) at the species level. Only 6.8% of the 429 total species were commonly detected by both methods, while overlap at the genus level was 18.8%. Spatial heterogeneity in phytoplankton communities based on eDNA was also evident depending on the sampling location, with the centre of the surface showing the highest species richness and overlap, suggesting its suitability for biodiversity monitoring. These findings demonstrate the high resolution and sensitivity of eDNA metabarcoding in capturing phytoplankton diversity and highlight its complementary role in existing biomonitoring programmes. Further improvements in the quantitative reliability of eDNA-based assessments will require efforts such as copy number normalisation, methodological standardisation, and refinement of reference databases.

This study analyzed the structural characteristics and influencing factors of fish assemblages according to lake size (small, medium, large) based on fish survey data from 90 lakes designated under the National Aquatic Ecosystem Health Monitoring Program in Korea. From surveys conducted between 2022 and 2024, a total of 107 fish species belonging to 32 families were recorded. The dominant species was the bluegill (Lepomis macrochirus, relative abundance [RA]: 21.2%), followed by Hemiculter eigenmanni as the subdominant species (RA: 13.6%). Statistical analysis based on lake size revealed significant differences across eight ecological indicators including species richness, abundance, diversity, evenness, richness, dominance, number of endemic species, and number of exotic species according to lake size. These indicators tended to increase with lake size. While this pattern may be initially attributed to positive factors associated with larger lakes, such as greater habitat heterogeneity and food resource diversity, it is considered that the primary factor influencing these results is the difference in the number of sampling sites per lake. Since sampling sites were designated based on the national water quality monitoring network, they are considered representative of each lake’s environmental conditions. Bray-Curtis similarity and SIMPER analyses identified patterns in assemblage similarity and the key contributing species for each size group. This study provides empirical evidence that demonstrates the influence of lake size on fish assemblage composition and structure and highlights the necessity of incorporating lake size and typology into fish-based assessments of lentic ecosystem health.

This study analyzed and presented zooplankton species occurrence, diversity distribution, and community composition in ninety lakes across South Korea using samples collected through the “Survey of Lake Aquatic Ecosystem Status and Health Assessment.” When comparing our results with the National Species Checklist, we identified factors within each of the three taxa that warrant improvement due to their influence on diversity assessments. To bridge the gap between the ongoing lake ecosystem surveys and the continually updated National Species Checklist-and to enhance the accuracy of diversity evaluations-we conclude that (1) greater taxonomic rigor must be reflected in the national checklist, and (2) the limitations of morphology-based identification (α-taxonomy) must be addressed. Because the National Species Checklist does not distinguish among species, subspecies, and morphospecies, it can give rise to taxonomic oversplitting and taxonomic inflation, leading to ambiguous diversity-index results. Moreover, the low resolution of morphological identification for zooplankton (at the genus, family, or class level) can introduce errors when comparing communities across habitats or detecting non-native introductions. Although alternatives such as environmental DNA and functional diversity exist, they require further refinement before being adopted in policy; therefore, they should be implemented alongside and in comparison with current aquatic ecosystem health assessment methods.

This study investigated seasonal variations in phytoplankton community composition and physicochemical water quality across 90 lakes in South Korea surveyed 2022 to 2024, and examined their relationships with environmental factors. Over the course of four seasons, a total of 952 phytoplankton taxa were identified, with Bacillariophyta (diatoms) and Chlorophyta (green algae) accounting for 64.2% of the total species richness. Diatom species such as Aulacoseira granulata, Fragilaria crotonensis, and Cyclotella meneghiniana were frequently observed regardless of season. In terms of cell density, Cyanophyta were dominant, comprising 85.1% on average, with particularly high summer densities driven by blooms of Microcystis aeruginosa and Aphanizomenon sp. In contrast, diatoms accounted for the highest relative abundance in winter (54.1%). Canonical correspondence analysis (CCA) revealed that Cyanophyta and Chlorophyta were positively correlated with water temperature, TP, COD, and Chl-a, whereas Bacillariophyta and Cryptophyta showed negative correlations with these variables. These results indicate that phytoplankton communities in Korean lakes are highly responsive to variations in temperature and nutrient concentrations, and that summer rainfall-driven nutrient inflow plays a critical role in triggering cyanobacterial blooms. This study provides a scientific basis for understanding seasonal ecosystem dynamics in Korean lakes and offers foundational data for eutrophication management and the development of biological water quality assessment indices.

Lakes and reservoirs represent key freshwater ecosystems that host diverse aquatic organisms and perform essential functions such as water cycling, nutrient retention, and ecosystem service provision. Due to their semi-closed hydrological structure and limited inflow-outflow dynamics, lakes exhibit complex biological communities shaped by regional climate, topography, and land use. However, these ecosystems are increasingly exposed to multifactorial stressors-including climate change, urbanization, nutrient enrichment, and invasive species-that are causing significant structural and functional shifts in aquatic biodiversity. This review provides an integrative overview of (1) the structural and ecological characteristics of lake ecosystems and the primary drivers of biodiversity alteration, (2) the ecological functions and bioindicator potential of key organism groups including phytoplankton, zooplankton, aquatic macrophytes, benthic macroinvertebrates, and fish, and (3) the emerging role of environmental DNA (eDNA) metabarcoding in lake biodiversity monitoring. We highlight both the technical principles and challenges of eDNA analysis and discuss its potential to complement traditional survey methods for the development of integrated ecosystem health assessments. Positioned at the forefront of the 2025 special issue of Ecology and Environment, this article establishes a conceptual and methodological foundation for a national-scale freshwater biodiversity monitoring framework in Korean lakes.

This study analyzed the epilithic diatom community and ecological health of freshwater streams using environmental DNA (eDNA)-based metabarcoding technology. eDNA metabarcoding is a method that analyzes biological communities by performing PCR amplification followed by next-generation sequencing (NGS), offering higher sensitivity and faster results compared to traditional microscopic analyses. The study compared the eDNA metabarcoding results of ribulose bisphosphate carboxylase large chain gene (rbcL) targeting epilithic diatoms according to Taq polymerases (SuperFi II, GainBlue, EzPCR, and AccuPower). SuperFi II and GainBlue yielded the highest number of reads and zOTUs, with GainBlue showing particularly uniform read distribution, allowing for more accurate analysis for community diversity of epilithic diatoms. On the other hand, EzPCR and AccuPower exhibited lower number of reads and zOTUs, making them less suitable for the community diversity. In terms of community similarity analysis, SuperFi II and GainBlue produced highly similar results, while EzPCR and AccuPower showed significant differences. This study demonstrates that PCR Taq polymerases significantly influence community diversity and similarity analyses of epilithic diatoms, with GainBlue providing the most stable and accurate results. Our findings serve as a valuable foundation for improving the accuracy of eDNA-based metabarcoding analyses of diatoms.

Fish require movement for feeding, spawning, growth, and evading predators, making swimming ability a critical factor in their survival. Swimming speed in fish are generally classified into sustained, prolonged, and burst speed. This study assessed the swimming capabilities of native benthic freshwater fish and provided data to inform the design and construction of fishways. Three fish species were examined: Pseudobagrus fulvidraco (family Bagridae), Tridentiger brevispinis and Rhinogobius brunneus (family Gobiidae). Measurements of total length, standard length, body weight, and body height were collected for each species, and swimming ability was evaluated using a swimming tunnel apparatus. Swimming ability was determined by measuring burst speed to calculate the critical swimming speed (Ucrit) and the relative swimming speed (TL s-1). The mean critical swimming speeds were 0.73±0.15 m s-1 for P. fulvidraco, 0.75±0.1 m s-1 for T. brevispinis, and 1.34±0.12 m s-1 for R. brunneus. Additionally, a significant correlation was found between swimming ability and total length, indicating that swimming capabilities may vary based on body type and behavior. These findings contribute essential insights into the swimming abilities of native freshwater fish, offering valuable data for improving fishway design. Further research is suggested to evaluate the swimming capabilities of a broader range of freshwater fish species to enhance fishway efficiency.

Captive breeding and reintroduction are crucial strategies for conserving endangered species populations. However, fish raised in predator-free environments, show a lack of recognition of predationrelated stimuli such as chemical and visual signals. It is critical to recognize chemical signals from injured conspecifics, also known as alarm signals, and the order or shape of predators to indicate the spread of predation risk in the habitat. We conducted a laboratory experiment to determine and adjust the optimal exposure period to induce appropriate anti-predator behavior response to different types of stimuli (Chemical, Visual and Chemical+Visual) for the endangered species Microphysogobio rapidus. Our results demonstrate that predator avoidance behavior varies depending on the types of stimuli and the duration of predation risk exposure. First, the results showed captive-breed M. rapidus show lack of response against conspecific alarm signal (Chemical cue) before the predation risk exposure period and tend to increase response over predation risk exposure time. Second, response to predator (visual cue) tend to peak at 48 hours cumulative exposure, but show dramatic decrease after 72 hours cumulative exposure. Finally, response to the mixed cue (Chemical+visual) tend to peak prior to the predation risk exposure period and show reduced response during subsequent exposure periods. This experiment confirms the lack of responsiveness to conspecific alarm signals in captive-bred M. rapidus and the need for an optimal nature behavior enhancement program prior to release of endangered species. Furthermore, responsiveness to predator visual signal peak at 48 hours cumulative exposure, suggest an optimal predation risk exposure period of up to 48 hours. Key words: predator cognition, captive breeding, chemical signal, visual signal, endangered

This study analyzed the monitoring results of fishways at 16 weirs constructed on four large Rivers to provide data helpful for the operation and management of fishways. The average utilization rate of the fishways at the weirs was confirmed to be 64.9%. When comparing the dominant species in the mainstream and fishway monitoring results, differences were observed in 9 weirs (56.3%). This indicated that the species prevalent in the mainstream were not necessarily the ones most frequently using the fishways. The average number of individuals using the fishways per day was 336. When classifying the fish species using the fishway by life type, 92.3% were primary freshwater fish, and migratory species accounted for only 5.6%. Analysis based on the season of fishway usage revealed that an average or higher number of fish species used the fishways from May to October, with the highest number of individual users occurring from June to August. Between May and July, 80% of the fish species using the fishways were in their spawning period, while during other season, less than 40% were species that move during the spawning period. The fishways that showed a significant alignment between the spawning period and the fishway passage period were Rhinogobius brunneus, Leiocassis nitidus, Squalidus chankaensis tsuchigae, Pseudogobio esocinus, Acheilognathus rhombeus, and Pungtungia herzi, in that order. When comparing the fishway monitoring results of the Gangjeong-Goryeong Weir and the Dalseong Weir with the upper part water level of the weir, both the number of fish species and individuals using the fishway showed positive correlations with the upper part water level of the weir. This suggests that a higher water level of the weir increases the inflow discharge within the fishway, leading to increased use by fish (number of individuals in Gangjeong-Goryeong Weir, P<0.001; number of species in Dalseong Weir, P<0.05). This study summarized and analyzed the results of fishway monitoring at 16 weirs built on four large Rivers, considering fishway efficiency, operation and management, monitoring period, and regulation of water level in the upper part of the weir. It is thought that this will help understand the status of fish use in fishways on large River and aid the construction, operation, and management of fishways in the future.

Small ponds, which exhibit unstable succession pattern of plankton community, are less well studied than large lakes. Recently, the importance of small ponds for local biodiversity conservation has highlighted the necessity of understanding the dynamics of biological community. In the present study, we collected zooplankton from three small reservoirs with monthly basis and analyzed their seasonal dynamics. To understand the complicated zooplankton community dynamics of small reservoirs, we categorized zooplankton species into four groups (LALF Group, Low Abundance Low Frequency; LAHF Group, Low Abundance High Frequency; HALF Group, High Abundance Low Frequency; HAHF Group, High Abundance High Frequency) based on their occurrence pattern (abundance and frequency). We compared the seasonal pattern of each group, and estimated community diversity based on temporal beta diversity contribution of each group. The result revealed that there is a relationship between groups with the same abundance but different occurrence frequencies, and copepod nauplii are common important component for both abundance and frequency. On the other hand, species included with LALF Group throughout the study period are key in terms of monthly succession and diversity. LALF Group includes Anuraeopsis fissa, Hexarthra mira and Lecane luna. However, groups containing species that only occur at certain times of the year and dominate the waterbody, HALF Group, hindered to temporal diversity. The results of this study suggest that the species-specific occurrence pattern is one key trait of species determining its contribution to total annual biodiversity of given community.

This study carried out from March 2021 to October 2021 in the upper part (St. 1) and middle part (St. 2) section of Yongsu stream, a branch of the Geum river, using PIT telemetry to understand the movement patterns and habitat characteristics of Odontobutis interrupta, a Korean endemic species. O. interrupta collection was used kick net (5×5 mm) and fish trap (5×5 mm). After collecting fish, PIT tag insertion was performed immediately in the site. Reader (HPR Plus Reader, biomark, USA) and portable Antenna (BP Plus Portable Antenna, biomark, USA) were used for detection of fish to monitoring the tagged O. interrupta. As a result of PIT telemetry applied to 70 individuals, mean movement distance was 36.5 (SE, ±6.6) m. There was a significant difference between total length and movement distance (P≤0.05). O. interrupta was mainly identified in average water depth, 36.2±1.9 cm, average water velocity, 0.03±0.07 m s-1 and average distance from watershed, 4.4±0.3 m. Extent of rock used for habitat was varied from 32 to 4,000 cm2. There was no statistical difference between the area of the first selected rock and the area of the after selected rock (P>0.05). but there was significant difference between total length and the area of the rock except for detection before 24 hours (P<0.01). Therefore, to restore the habitat, it is considered necessary to create various substrate structures by providing various habitat environments (water depth, flow rate, stone, etc.) for each individual size.

Estuary is important in terms of biodiversity because it has the characteristics of transition waters, created by the mixing of fresh- and seawater. The estuarine water circulation provides a variety of habitats with different environments by inducing gradients in the chemical and physical environment, such as water quality and river bed structure, which are ultimately the main factors influencing biological community composition. If the water circulation is interrupted, the loss of brackish areas and the interception of migration of biological communities will lead to changes in the spatial distribution of biodiversity. In this study, among the sites covered by the Estuary Aquatic Ecosystem Health Assessment, we selected study sites where changes in biodiversity can be assessed by spatial gradient from the upper reaches of the river to the lower estuarine area. The α-, γ- and β-diversity of diatom, benthic macroinvertebrates, and fish communities were calculated, and they were divided into open and closed estuary data and compared to determine the trends in biodiversity variation due to estuarine circulation. As results, all communities showed higher γ-diversity at open estuary sites. The benthic macroinvertebrate community showed a clear difference between open and closed estuaries in β-diversity, consequently the estuarine transects were considered as a factor that decreases spatial heterogeneity of their diversity among sites. The biodiversity trends analyzed in this study will be used to identify estuaries with low γ- and β-diversity by community, providing a useful resource for further mornitoring and management to maintain estuarine health.

Total of 325 estuaries in Korea were surveyed to analyze the effect of presence of sluice gate on the estuary environment and fish community from 2016 to 2018. Fish community in closed and open estuaries showed differences generally, and the relative abundance (RA) of primary freshwater species in the closed and migratory species in the open estuaries were high. The result of classifying species by habitat characteristics in closed and open estuaries showed similar tendencies at the estuaries of south sea and west sea. The relative abundances of primary freshwater species in the closed estuaries at the estuaries of south sea and west sea were the highest, but estuarine and migratory species were high in both closed and open estuaries at the estuaries of east sea. Primary freshwater species showed higher abundances in the closed estuaries with reduced salinity due to blocking of seawater since they are not resistant to salt. However, primary freshwater species in open estuaries at east sea was higher than that of the closed estuaries, which is considered to be the result of reflecting the characteristics (tide, sand bar, etc.) of the east sea. Korea Estuary Fish Assessment Index (KEFAI) was showed to be higher at open estuaries than closed in all sea areas (T-test, P<0.001), the highest KEFAI was observed in closed estuaries at south sea, and open estuaries in east sea. Fish community of closed and open estuaries in each sea areas showed statistically significant differences (PERMANOVA, East, Pseudo-F=3.0198, P=0.002; South, Pseudo-F=22.00, P=0.001; West, Pseudo-F=14.067, P=0.001). Fish assemblage similarity by sea areas showed a significant differences on fish community in closed and open estuaries at east sea, south sea, and west sea (SIMPER, Group dissimilarity, 85.85%, 88.36%, and 88.05%). This study provided information on the characteristics and distribution of fish community according to the types of estuaries. The results of this study can be used as a reference for establishing appropriate management plans according to the sea areas and type in the management and restoration of estuaries for future.

Fish assemblage of total 325 of Korean peninsula estuaries were surveyed to analyze the characteristics of community structure and diversity by sea areas for three years from 2016 to 2018. The scale (stream width) of Korean estuaries were various (14~3,356 m), and 68.9% of all estuaries showed salinity of less than 2 psu. Total 149 species classified into 52 families of fish were identified, and the dominant and sub-dominant species were Tribolodon hakonensis (relative abundance, RA, 12.5%) and Mugil cephalus (RA, 9.5%), respectively. The estuary of the Korean Peninsula had different physical and chemical habitat environments depending on the sea area, and accordingly, fish community structure also showed statistically significant differences (PERMANOVA, Pseudo-F=26.69, P=0.001). In addition, the NMDS (nonmetric multidimensional scaling) results showed the patterns that indicating fish community difference by sea areas, even though low community similarity within sea area (SIMPER, 21.79~26.39%). The estuaries of east sea areas were distinguished from the others in the aspects of which, the higher importance of migratory fishes and endangered species, and that of brackish species were characterized at south sea estuaries. However, the estuaries of west sea showed higher importance of species that have a relation with freshwater (primary freshwater species, exotic species), which is the result that associating with the lower salinity of west sea estuaries because of the high ratio of closed estuaries (78.2%). The SIMPER analysis, scoring the contribution rates of species to community similarity, also showed results corresponding to the tendency of different fish community structures according to each sea area. So far, In Korea, most studies on fish communities in estuaries have been conducted in a single estuary unit, which made it difficult to understand the characteristics of estuaries at the national level, which are prerequisite for policy establishment. In present study, we are providing fish community structure characteristics of Korean estuaries in a national scale, including diversity index, habitat salinity ranges of major species, distribution of migratory species. We are expecting that our results could be utilized as baseline information for establishing management policies or further study of Korean estuaries.

The conversion of all carbon preforms to dense SiC by liquid infiltration can become a low-cost and reliable method to form SiC-Si composites of complex shape and high density. Reactive sintered silicon carbide (RBSC) is prepared by covering Si powder on top of 0.5-5.0 wt% Y2O3-added carbon preforms at 1,450 and 1,500°C for 2 hours; samples are analyzed to determine densification. Reactive sintering from the Y2O3-free carbon preform causes Si to be pushed to one side and cracking defects occur. However, when prepared from the Y2O3-added carbon preform, an SiC-Si composite in which Si is homogeneously distributed in the SiC matrix without cracking can be produced. Using the Si + C = SiC reaction, 3C and 6H of SiC, crystalline Si, and Y2O3 phases are detected by XRD analysis without the appearance of graphite. As the content of Y2O3 in the carbon preform increases, the prepared RBSC accelerates the SiC conversion reaction, increasing the density and decreasing the pores, resulting in densification. The dense RBSC obtained by reaction sintering at 1,500 oC for 2 hours from a carbon preform with 2.0 wt% Y2O3 added has 0.20% apparent porosity and 96.9% relative density.

The conversion of carbon preforms to dense SiC by liquid infiltration is a prospectively low-cost and reliable method of forming SiC-Si composites with complex shapes and high densities. Si powder was coated on top of a 2.0wt .% Y2O3-added carbon preform, and reaction bonded silicon carbide (RBSC) was prepared by infiltrating molten Si at 1,450oC for 1-8 h. Reactive sintering of the Y2O3-free carbon preform caused Si to be pushed to one side, thereby forming cracking defects. However, when prepared from the Y2O3-added carbon preform, a SiC-Si composite in which Si is homogeneously distributed in the SiC matrix without cracking can be produced. Using the Si + C → SiC reaction at 1,450oC, 3C and 6H SiC phases, crystalline Si, and Y2O3 were generated based on XRD analysis, without the appearance of graphite. The RBSC prepared from the Y2O3-added carbon preform was densified by increasing the density and decreasing the porosity as the holding time increased at 1,450oC. Dense RBSC, which was reaction sintered at 1,450oC for 4 h from the 2.0wt.% Y2O3-added carbon preform, had an apparent porosity of 0.11% and a relative density of 96.8%.

The accurate estimation of fish assemblages is highly dependent on the sampling gear used for sampling. We used data from 15 sampling sites along the Nakdong River, which is a large river in South Korea, to identify differences in assemblages and sizes of freshwater fishes collected with either cast nets or gill nets, the two most commonly used sampling gear in South Korea. The two gears differed in the fish assemblages they captured, with more species caught by gill nets. Further, due to its tighter mesh size, the cast net caught significantly smaller fishes than the gill nets (independent t-test, p<0.05). We found the cast net to be appropriate for species that inhabit shallow (less than 2 m) and open water, but inappropriate for deep water, habitats with plant beds, and nocturnal species. Thus, cast net sampling is not efficient in a large river environment, and a combination of sampling methods is more suitable for understanding fish assemblages in such habitats. In general, appropriate selection of fishing methods to specific habitats is necessary to improve data quality and minimize the misrepresentation of environmental conditions.