Proportion of Pacific saury spawned during their first spawning season estimated by the histological observations of ovary tissues

Pacific saury experience spawning season two times during their lifetime. In this study, we estimated the proportion of individuals that spawned during the first spawning season (PSFS) by a histological approach. Ovaries were sampled by surveys covering the distribution area of Pacific saury widely during the end of spawning season (June and July) in 2013-2019. Spawning histories of individuals were determined by Victoria blue (VB)-positive ovarian arterioles, postovulatory follicles, yolked oocytes, and/or atresia from yolked oocytes. Combining these histological results with the biomass estimates derived from a swept area method, we estimated the annual PSFS, which varied from 13.0% to 63.1%, with a mean of 31.1% and a median of 28.3%. In addition, we validated the spawning-related mortality, a potential factor of the underestimation of PSFS, by following two ways: 1) radiuses of otolith annual rings, as the indicator of the size at the beginning of first spawning season, were compared between the main spawning season and the end of the spawning season; 2) sex ratios were compared between the pre-spawning and the end of spawning seasons. We found no evidence of spawning-related mortality by these validations and thus our estimates of PSFS are considered plausible so far.