2022 Volume 27 Issue 1 Pages 15-30
It has been pointed out that the decrease in nutrient concentration is a key factor in the decrease in sand lance catch in the Seto Inland Sea. In this study, aiming at having quantitative insight into the productive structure of sand lance, elucidating the relation with the nutrient environment, a Lagrangian numerical model of sand lance coupled with Eulerian lower-trophic ecosystem model was developed to simulate annual life cycle of the fishery resource. The model was validated with numerical experiment over the 3-year period from 2016 to 2018. As a result, the growth dynamics of sand lance from hatching through aestivation to spawning and hatching, as well as ecological features of the lower trophic ecosystem including plankton abundance and nutrient stocks, were reproduced fairly showing a good agreement with the observational data. The model turned out to provide a quantitative evaluation on the interaction between sand lance—the higher trophic organism— and plankton and nutrients—the lower trophic constituents—. Regarding the aestivation of sand lance, about 80% of all aestivating individuals settled down on the sandy seafloor in the eastern part of the Harima-Nada, such as Shikanose and Murotsunose, showing that the aestivation grounds were well reproduced. In terms of carbon flux, the transfer efficiency from primary production to secondary production during the peak of the 2016 fishing season around the Akashi Strait, the main fishing ground of sand lance, was as high as 45%. This indicates that zooplankton (copepods) exerted intensive feeding pressure on phytoplankton (diatoms) and led to the chronical shortage in abundance during this period.
