In March 2023 and April 2025, large numbers of Calanus jashnovi were collected in the Kii Channel of the eastern Seto Inland Sea. To distinguish C. jashnovi copepodids from co-occurring closely related Calanus sinicus, which is the dominant large-sized copepod in the Seto Inland Sea, their prosome lengths from the copepodid stage CIV onward were measured. The length distributions of CIV and CV each showed clear bimodal patterns, of which the large and small modes were assumed to be C. jashnovi and C. sinicus, respectively. Adult C. jashnovi were far less abundant than CIV and CV. The appearance of C. jashnovi in the Seto Inland Sea is thought to result from ontogenetic seasonal vertical migration to the surface layer, combined with the occasional intrusion of warm Kuroshio surface water into the Kii Channel. The mean prosome length of C. jashnovi CV in April 2025 was much larger than in March 2023, which is considered to be due to the difference in food abundance and/or quality in the surface water where they grew up from CI to CV. In the Kii Channel, C. jashnovi numerically accounted for 70% and 44% of the Calanus population in March 2023 and April 2025, respectively, which would have contributed as a food source for planktivorous fish.
Ueda et al. (Fri,) studied this question.