Timing of paddlefish spawning in the Upper Missouri River, Montana, USA in relation to river conditions

Abstract

Summary Spawning activity of paddlefish Polyodon spathula in the Missouri River, Montana in 2008–2009 was examined to delineate spawning sites and times in relation to discharge, water temperature and turbidity. One hundred thirty-six eggs were collected at water temperatures ranging from 12.0 to 20.7� C (mean, 16.3� C; SD, 2.5). Only 12 of 89 (13%) congregations of radio-tagged adults observed during the spawning period coincided with egg captures. Six larvae were collected at water temperatures ranging from 19.1 to 21.7� C (mean, 20.5� C; SD, 0.86). Peak discharge in 2008 (903 m 3 s )1 on 14 June) was approximately 30% greater in magnitude and occurred 11 days later than peak discharge in 2009 (612 m 3 s )1 on 3 June). Despite these differences in the hydrograph, no significant differences in egg CPUE were found between years (ANOVA, F = 0.69, P = 0.56). Logistic regression identified no significant river condition variables associated with the presence or absence of eggs (P > 0.14 for all variables). However, in both years maximum egg CPUE was recorded within 3 days of the hydrograph peak and at similar water temperatures (17.5� C in 2008, 16.8� C in 2009). These results suggest an overall association of peaking discharge and seasonally warming water temperatures with egg deposition. Higher catches of eggs and larvae than observed in this study may be necessary to clarify short-term (day-to-day) effects of environmental changes on spawning activity. Continued investigation of the relationship between short-term changes in river conditions and paddlefish spawning activity is needed to understand the mechanics underlying the reproductive success of this species.