Evidence for Altered and Insufficient Spawning As a Causative Factor in the Decline of Pallid Sturgeon and As an Impediment to Recovery

Studies of pallid sturgeon (Scaphirhynchus albus) in the relatively unaltered Yellowstone River suggest that spawning occurs during a short, 2-3 week period in late spring and early summer.  Males aggregate at discrete spawning areas characterized by current velocities in excess of 1 m/s and by patches of sorted, coarse substrate juxtaposed with sand.  Females migrate considerable distances upstream and downstream, stopping to spawn at locations where male aggregations have formed.  Female pallid sturgeon rarely fail to mature and release their eggs.  In contrast, studies in the Lower Missouri River from 2007–2014 have not documented persistent aggregations of male sturgeon.  Females typically do not become reproductively ready when expected, and have failed to release their eggs.  Females that spawn in the highly-altered Lower Missouri River often engaged in complex migrations and spawned primarily in unnatural conditions on outside bends adjacent to revetted banks.  Spawning was not limited to a few discrete locations, but was spread widely throughout the river.  Alteration in expected reproductive patterns, limited evidence of successful spawning, and an increase in the incidence of hybridization in the Lower Missouri River are supportive of hypotheses linking diminished adult abundance, altered spawning habitat, and disrupted spawning cues to species declines.