Riverward migration associated with food acquisition in adult Japanese seabass Lateolabrax japonicus: evidence from stable sulphur isotope ratios and otolith Sr:Ca ratios.

Abstract

The Japanese seabass (Lateolabrax japonicus) spawns offshore in winter, and certain adults exhibit riverward migration during the pre-spawning period (spring to autumn). However, the significance of this migration remains unclear. This study aimed to assess the dietary and migration histories of adult Japanese seabass between marine and riverine habitats using stable sulphur isotope ratios (δ³⁴S) and otolith strontium:calcium (Sr:Ca) ratios. In addition, we examined the effects of riverine prey consumption on body condition by comparing the δ³⁴S values with body condition factors. The δ³⁴S analyses of gonads and red blood cells estimated that riverine prey comprised 38.5% (95% credible interval = 24.2-64.3%) of the diet of Japanese seabass during 6 months before the spawning season. The δ³⁴S values of gonads tended to decrease as the number of otolith Sr:Ca ratios meeting the river residence criterion (4.4 × 10^-3) increased. In adult Japanese seabass, no significant relationship between the δ³⁴S values of gonads and body condition factors was observed, indicating that Japanese seabass with a high consumption rate of riverine prey could maintain a body condition similar to that of marine prey. The use of both marine and riverine habitats by adult Japanese seabass is likely to be a beneficial behaviour that increases feeding opportunities for the population.