Maintaining a sustainable egg bank through rapid maturation and the production of large clutches is an important strategy for large branchiopods surviving in ephemeral habitats, and such egg banks can only be replenished by the recruitment of hatched eggs as a function of fitness (F). The ability to reach a threshold F represents a key adaptation to selection pressures. We investigated daily changes in the population density/size and hatching numbers of fairy shrimp, Branchinella kugenumaensis, and estimated their F values in an ephemeral pool, Siangtian Pond, in northern Taiwan during 2005, and four cohorts were observed. The number of hatchings per cohort ranged from to , representing a hatching density between 1.42 and 2.83 , and the number of newly produced eggs per cohort ranged from to , representing a high level of recruitment, ranging from 10,631 to 37,425 in the sediment, which yielded a mean fitness of 2.05 produced eggs per hatching (CI  =  1.82 ∼ 2.35). The threshold F for replenishing the hatched portion of the egg bank was derived theoretically and ranges from 1.39 to 2.36, varying with the probability of reproduction failure and the rate of egg mortality. Because only a small fraction of the egg bank hatches during each inundation episode, the observed F may not be sufficient for replenishing the remaining egg bank. Therefore, additional strategies for long-term persistence are required, which may be achieved by exhibiting occasional indeterminate growth in addition to the regular determinate growth pattern of this species.