This study conducted morphological and molecular phylogenetic analyses on three presumed natural interspecific hybrids of the genus Sarcocheilichthys, collected from the main stream of the Seomjin River and its tributary, Dongbokcheon Stream, as well as their parent species, S. nigripinnis morii and S. variegatus wakiyae, to identify whether they are natural hybrids and to determine their maternal and paternal species. The results of the molecular phylogenetic analysis clearly demonstrated that the three presumed natural hybrids are indeed natural hybrids of S. nigripinnis morii and S. variegatus wakiyae, with S. nigripinnis morii identified as the maternal species and S. variegatus wakiyae as the paternal species. The three natural hybrids exhibited intermediate or unique morphological traits that were not biased towards either parent species, and distinct morphological characteristics were observed in the dorsal fin and caudal fin. In the water bodies where the natural hybrids were collected, a variety of Acheilognathinae species and spawning hosts coexisted alongside the parent species S. nigripinnis morii and S. variegatus wakiyae. It is presumed that the overlapping spawning periods and locations of S. nigripinnis morii and S. variegatus wakiyae promoted the occurrence of hybrids. The unidirectional occurrence of natural interspecific hybridization is attributed to interspecific competition and population imbalance, and the sneaker behavior of S. variegatus wakiyae males was proposed as an important mechanism.

Current distribution of Korean Cottus pollux was investigated by conducting field surveys and environmental DNA analysis in September to November 2022 and March to May 2023. As a result of the field survey, the presence of C. pollux was confirmed in the uppermost stream of Hyeongsangang River, as well as in Deokdongcheon, Jeolgolcheon, and Hoamcheon Streams. The main habitat of C. pollux was the riffle area in the upper reaches of streams with clean water and boulder-cobble bottoms. As a result of environmental DNA analysis, the samples determined to be positive included all streams in which the presence of C. pollux was confirmed in the field survey. In addition, Namcheon and Singwangcheon Streams were determined to be positive, indicating potential as its habitats. Since C. pollux has a narrow distribution area and a small population size, continuous monitoring and conservation measures are required to immediately respond to damage caused by typhoons and river works, which are the main causes of habitat disturbance, in order to maintain a stable population.