Spatial segregation of early life stages: fish eggs and larvae occupy distinct marine environments in the southern coastal waters of Korea

Fish eggs and larvae are key planktonic life stages influencing recruitment success and marine ecosystem functioning, yet their fine-scale habitat use remains insufficiently resolved. This study examines stage-specific spatio-temporal distributions of ichthyoplankton in the southern coastal waters of Korea, with a focus on differences between coastal and inner-bay environments. Ichthyoplankton were collected from April to September 2023 in southern coastal waters and a semi-enclosed embayment (Yeoja Bay). Species composition was identified using morphological analysis, DNA barcoding, and metabarcoding. A total of 126 species were detected, with higher richness in the coastal waters (121 species) than in the inner bay (57 species). Egg species richness increased from April and declined in early summer, while larval richness increased from May and remained relatively stable thereafter. Clear stage- and species-specific distribution patterns were observed. Jack mackerel (Trachurus japonicus) occurred exclusively in the coastal waters. Whereas silver croaker (Pennahia argentata), three-lined tongue sole (Cynoglossus abbreviatus), and fivespot flounder (Pseudorhombus pentophthalmus) were present in both regions, exhibiting spatial shifts between egg and larval stages. For example, P. argentata eggs were concentrated in the inner bay in spring, while larvae occurred sequentially in the inner bay and coastal waters during summer. Temperature and salinity analyses showed that eggs and larvae of C. abbreviatus were associated with relatively low-temperature and high-salinity conditions, whereas P. pentophthalmus larvae occurred in warmer waters. These results demonstrate stage-specific habitat partitioning of ichthyoplankton at fine spatial and temporal scales, highlighting the importance of resolving early life stage–environment relationships in coastal ecosystems.