Local environmental changes boost predation risk in forage fish: application to the sand lance in the eastern Seto Inland Sea

Changes in environmental conditions can modify the strength of prey-predator interactions in marine ecosystems by altering top-down and bottom-up drivers. This study examined potential top-down effects on the biomass of a forage fish, the sand lance Ammodytes japonicus, in oligotrophic waters of the eastern Seto Inland Sea, Japan. The biomass of sand lance has decreased over the last several decades, possibly due to reduced prey availability associated with declining nutrient levels. We analyzed 30 years of catch data for major omnivorous and piscivorous fish species in relation to temperature fluctuations. Additionally, laboratory experiments were conducted to test whether food manipulations simulating bottom-up trophic effects could amplify top-down impacts. The abundance of 14 predator fish species has shifted to high levels in sand lance habitats since 2016, coinciding with rising temperatures. Although nutritional status of sand lance did not significantly affect predation susceptibility, sand lance individuals with lower nutritional status spent more time searching for prey prior to estivation. These results suggest that warming-induced shifts in predator distribution within sand lance habitats may increase predation risk by creating more frequent predator encounters. This mechanism partly explains the sharp decline in sand lance biomass observed since 2017. Our findings highlight the importance of understanding how environmental changes influence predator-prey dynamics to better predict the sustainability of local populations and trophic structures in marine ecosystems.