Hiroshima University and others reveal how environmental changes altered sand lance behavior and increased predation risk

The sand lance (Ammodytes japonicus) is a small fish belonging to the family Ammodytidae. It is an important fishery resource in the eastern Seto Inland Sea, particularly in Osaka, Hyogo, and Kagawa Prefectures, and juvenile fish caught from March to April have long been widely enjoyed as an ingredient for simmered sand lances ("kugini"). Although catches in the eastern Seto Inland Sea had been on a declining trend, they remained above 10,000 tons per year until 2016. However, in 2017 the catch plunged to roughly one-tenth of the previous year's level, and has not recovered since, remaining at less than 3,000 tons—a persistently low level. The cause of this sudden and dramatic decline had not been clearly identified.

A research group led by Professor Takeshi Tomiyama from the Graduate School of Integrated Sciences for Life, Hiroshima University and Senior Researcher Michio Yoneda from the Fisheries Technology Institute, Japan Fisheries Research and Education Agency has clarified that a combination of environmental changes—rising water temperatures and food scarcity—sharply elevated the risk of sand lances being preyed upon in 2016, and that this was the primary factor behind the sharp decline in catches in 2017. Their findings were published in Marine Environmental Research.

The study involved long-term data analysis focusing on changes in the abundance of fish that prey on sand lances, as well as laboratory-rearing experiments to investigate the effects of rising water temperatures and food scarcity on sand lance behavior.

Sand lances are born in winter, actively feed to build up energy reserves from spring through early summer, and then burrow into the sand in summer to remain dormant until winter—a behavior known as "estivation." Their ability to accumulate sufficient energy reserves before entering estivation is therefore critical to their survival. The researchers analyzed catch data from January to July for 14 piscivorous fish species—including Japanese Spanish mackerel (Scomberomorus niphonius), Japanese amberjack (Seriola quinqueradiata), daggertooth pike conger (Muraenesox cinereus), Japanese seabass (Lateolabrax japonicus), and Japanese flounder (Paralichthys olivaceus)—to examine changes in predator distribution. The results showed a marked difference between conditions before 2015 and from 2016 onward, revealing that predator abundance had increased sharply from 2016.

The team also reared sand lances under both food-sufficient and food-scarce conditions and compared their behavior. The results showed that sand lances under food-scarce conditions delayed the onset of estivation. This effect was greater than that caused by rising water temperatures alone, indicating that under food-scarce conditions, sand lances need to remain active for longer periods in order to accumulate sufficient energy before estivation. This means increased opportunities for encounters with predators. In other words, a heightened "risk of predation."

Taken together, these findings demonstrate that sand lances in the eastern Seto Inland Sea were subjected to the combined effects of food scarcity, rising water temperatures, and an increase in predators. In particular, the sharp rise in predator abundance in 2016 is thought to have significantly reduced the number of sand lances entering estivation that year, which in turn led to a dramatic fall in spawning numbers the following winter.

Sand lance stocks and catches are declining nationwide. Because the causes may differ by sea area, each region needs to be studied separately. Research on stock fluctuations to date has focused primarily on bottom-up effects, factors such as water temperature and food availability that directly influence the growth and reproduction of organisms. This study, however, demonstrated that changes in predator abundance can also contribute to stock fluctuations, a top-down effect, opening the door to a new avenue of scientific investigation. Although top-down effects are difficult to verify directly in the field, the significance of this research lies in showing that scientific verification is possible through an integrated approach combining long-term catch data analysis and laboratory experiments. Going forward, it will be important to apply this research framework to other sea areas and other fish species to further elucidate the mechanisms of stock fluctuations occurring under rapid environmental change.

Journal Information
Publication: Marine Environmental Research
Title: Local environmental changes boost predation risk in forage fish: application to the sand lance in the eastern Seto Inland Sea
DOI: 10.1016/j.marenvres.2025.107827