Hypothesis: Selective sex-related mortality of the marine threespine stickleback in the White Sea causes the female-biased sex ratio observed on the spawning grounds. Organisms: Adult threespine stickleback (Gasterosteus aculeatus) spawning in the inshore zone, and three species of predatory fishes: Atlantic cod (Gadus morhua), shorthorn sculpin (Myoxocephalus scorpius), and fourhorn sculpin (Myoxocephalus quadricornis). Place and times: Kandalaksha Bay, White Sea, Russia; June to August 2012–2018. Methods: The following sampling methods were used: beach seining to determine fish density and local population size, hand collection of dead fish on the spawning grounds, gill netting of predatory fishes, analysis of predatory fish stomach contents to determine the sex of well-preserved stickleback, and morphological analysis of stickleback spines to determine the sex of decomposed stickleback. Results: The dynamics of stickleback abundance in the lagoon is explained by inshore migration of the fish to the spawning area at the beginning of the spawning period, and their subsequent departure at the end of the spawning season, with females leaving the grounds earlier than males. During spawning (5–30 June), total stickleback mortality reaches about 0.1%; the difference in the relative mortality rates of 0.0044% per day for males and 0.0030% for females is statistically significant. Mortality increases in the post-spawning period, when fish abundance in the inshore zone falls considerably, but remains at a very low level. Because of the low level of non-predation mortality, it cannot be the cause of the female-biased sex ratio observed in the White Sea stickleback population. Predation-associated mortality caused by Atlantic cod and sculpins was male-biased. Based on the stomach contents of predatory fish, the sex ratio of stickleback prey was as follows: cod, 61% males/39% females; sculpins, 82% males/18% females, which was significantly different from the population at sea (35% males/ 65% females). This factor alone, however, is unlikely to explain the prevalence of female stickleback in the lagoon. The eventual offshore male-biased mortality, caused by increased depletion of energy reserves during the spawning period, is probably the main reason for the observed female-biased sex ratio.