Influence of river flow on the catch and biomass of Sakura shrimp in Suruga Bay, Japan.

Abstract

While coastal ecosystems are strongly influenced by riverine inputs that transport nutrients and stimulate productivity, they can degrade habitats through altered flow regimes. A notable example is Suruga Bay, the deepest bay in Japan, where the economically important Sakura shrimp (Lucensosergia lucens) fishery has substantially declined in recent decades; however, whether river discharge has contributed to this decline remains unclear. This study examines the effect of river discharge on the catch and biomass of Sakura shrimp in this bay. We integrated the statistical analyses of four decades of fishery data (1983-2020) with a three-dimensional ecosystem model (STOC-LT) configured for 2020 under altered river discharge scenarios from four major rivers. The ecosystem model successfully reproduced the observed seasonal patterns of Chl-a and water temperature in Suruga Bay, strengthening the basis for scenario analyses. The scenario analysis revealed fundamentally distinct seasonal pathways. In spring, the increased discharge enhanced the estimated catch through nutrient-driven spatial redistribution near the Fuji River plume, whereas the simulated biomass remained stable, indicating increased catchability rather than population growth. In autumn, both models converged on negative responses; a 20% increase in the river discharge reduced the simulated biomass from 5.22 × 10^-7 to 4.89 × 10^-7 mgC m^-3 and estimated catch from 43.5 to 33.1 ton/day, likely as the warmer river water constrains larval survival and reduces catchability. These findings demonstrate that the effects of river discharge on Sakura shrimp fisheries critically depend on the seasonal environmental context, highlighting the need for season-specific management of river discharge.