Annual variation in condition factor of sand lance Ammodytes japonicus during the estivation period in Harima-Nada, eastern Seto Inland Sea, Japan

Abstract The sand lance Ammodytes japonicus has been one of the most important fisheries resources in the eastern Seto Inland Sea, but long-term variations in annual catch have shown a decreasing trend. This species estivates for about 5 -months and does not eat during the period in Harima-Nada, eastern Seto Inland Sea. Therefore, a sufficient condition factor is important to survive the estivation period and start maturity thereafter. In this study, we investigated the condition factor of age-0 fish, which account for most of the catch, during the estivation period (July–November) together with water temperature in Harima-Nada from 2010 to 2017. The mean condition factor of the age-0 fish in late July ranged from 3.76 to 4.55 and decreased each month to values in late November from 2.91 to 3.59. The reduction rate (%) of the condition factor varied from 15.5% to 26.6%. The slopes of the decreasing condition factor, which were calculated from the relationship between the monthly condition factor of individuals and Julian Days each year, showed no significant correlation with cumulative water temperature from the first sampling day in July to the last one in November. The condition factor of sand lance was generally maintained at the end of the estivation period if the condition factor was high at the beginning of the period. The results suggest that starting estivation with a high condition factor is the most important criterion to enable survival of sand lance in Harima-Nada.