Validation of otolith daily increment formation and early growth analysis of Japanese flounder (<em>Paralichthys olivaceus</em>)

Abstract

The Japanese flounder is a vital economic species among East Asian released and cultured fish. Here, we sought to validate daily increment formation and analyze the early morphological development of sagittal otoliths and lapillus in Japanese flounder larvae and juveniles over 50 days. Our analysis focused on the otolith shapes, the number of daily rings present, and their length and width measurements. The results revealed that sagittal otoliths and lapillus formed before the fertilized eggs hatched. During the larval and juvenile stages, sagittae evolved gradually, transforming from nearly round to oval structures, eventually adopting arrow- or pear-shaped forms. Similarly, lapillus transformed from nearly round to mussel-shaped structures. The first daily ring became evident on the 5th-day post-hatching, demonstrating a daily periodicity during the larval phase. We observed a direct relationship between incubation days (D) and the number of daily otolith increments (n), expressed as D = n + 5. This deposition trend exhibited a daily periodicity for both sagittae and lapillus. We also noted exponential correlations between the length (Ls, μm) and width (Ws, μm) of the sagittae and the total length (TL, mm). Contrastingly, the lapillus measurements demonstrated linear correlations. Furthermore, a linear relationship emerged between the sagittae and lapillus lengths: Ll= 0.3028Ls+ 28.534 (R2= 0.9366). Additionally, we found a similar linear relationship between the sagittae and lapillus widths: Wl = 0.3791Ws+ 21.508 (R2= 0.9426). The formation pattern of daily increments in lapillus was notably more distinct than that in sagittae, making it highly suitable for age identification and group recognition. This study not only contributes to the field of fish ecology but also provides a foundational understanding for future research. It can assist in reconstructing early life histories and inferences about the spawning periods and grounds of stocked Japanese flounder.