Behavioral responses of skipjack tuna to diluted waters from the Yangtze River in the East China Sea

This study investigated changes in the swimming behavior of skipjack tuna (Katsuwonus pelamis) in the low-salinity waters of the East China Sea (ECS), influenced by nutrient-rich freshwater from the Yangtze River, to elucidate their response to riverine freshwater. Behavioral data from 18 individuals collected through archival tag release surveys conducted around Japan's Nansei Islands were analyzed. Five individuals that experienced sea surface salinity (SSS) below 33.5 psu were identified as low-salinity-exposed individuals, and their relationships with marine environmental parameters (e.g., temperature, salinity, chlorophyll, sea surface height, and dissolved oxygen [DO]) were examined at the individual level. Furthermore, energy intake was estimated from body temperature changes, enabling comparison of swimming behavior between low-salinity environments and the open ocean from both physical and physiological perspectives. The five low-salinity-exposed individuals showed different distribution and behavioral patterns between spring (three individuals: Sp1, Sp2, and Sp3) and summer (two individuals: Su1 and Su2). In spring, skipjack tuna were observed in waters with temperatures no less than 18°C. In summer, individuals were observed in environments with DO levels mainly higher than 3.6 mL L-1 and SSS of no less than 31.3 psu. Additionally, two individuals (Sp1 and Su2) showed higher estimated energy intake in low-salinity waters compared with the open ocean (Sp1: 139.9 ± 46.0 vs. 92.3 ± 43.8 kcal/day; Su2: 210.5 ± 52.2 vs. 176.6 ± 59.3 kcal/day), while the remaining three individuals showed no significant difference or could not be compared due to very short residence time in low salinity condition. Although these results were obtained from only five individuals, the findings suggest that diluted water in the ECS may affect feeding opportunities and distribution of skipjack tuna through variability in temperature, salinity and DO.