We presume that the shovelnose sturgeon (Scaphirhynchus platorynchus) has evolved to spawn in the springtime when environmental conditions are at some optimum, but this state has not yet been defined. In this study physiological readiness to spawn in shovelnose sturgeon was examined to define more closely when spawning could occur and thus identify and evaluate prevailing environmental conditions that could cue spawning during that period. Reproductive assessments of Lower Missouri River shovelnose during 4 years (2005-2008) and at two locations (Gavins Point Dam, South Dakota and Boonville, Missouri) were used to identify shovelnose sturgeon spawning periods. Initiation of the spawning period, as defined by the presence of reproductively ready fish, was a highly predictable yearly event and extended over several weeks at each reach. The spawning period occurred earlier in the lower reach than in the upper reach and environmental conditions during the periods varied between locations and among years. Shovelnose sturgeon collected during the presumed spawning periods were at varying degrees of readiness to spawn as indicated by oocyte polarization index and blood reproductive hormones. Evaluation of the influence of environmental factors on readiness to spawn using stepwise multiple regression analysis indicated photoperiod followed by temperature were the best candidate variables overall to explain the trend. However, within geographically distinct populations gravid females are not all reproductively synchronized. Assuming that this apparent asynchrony in readiness is normal and not an artifact of the disturbed Missouri River system, we infer that individual sturgeon can persist in a reproductively ready state until conditions appropriate for spawning occur. Taken together, our results lead us to hypothesize that gravid females early in the reproductive cycle (post-vitellogenesis) respond to day length, a reliable annual cue, become increasingly more ready to spawn in response to temperature, and that another set of cues, short-term and specific for localized environmental conditions or events, serve to signal ovulation and release of gametes.