Age estimations of wild pallid sturgeon (Scaphirhynchus albus, Forbes & Richardson 1905) based on pectoral fin spines, otoliths and bomb radiocarbon: inferences on recruitment in the dam-fragmented Missouri River

An extant stock of wild pallid sturgeon Scaphirhynchus albus persists in the fragmented upper Missouri River basin of Montana and North Dakota. Although successful spawning and hatch of embryos has been verified, long-term catch records suggest that recruitment has not occurred for several decades as the extant stock lacks juvenile size classes and is comprised exclusively of large, presumably old individuals. Ages of 11 deceased (death years 1997-2007) wild S.albus (136-166cm fork length) were estimated based on pectoral fin spines, sagittal otoliths and bomb radiocarbon (C-14) assays of otoliths to test the hypothesis that members of this stock are old and to provide inferences on recruitment years that produced the extant stock. Age estimations based on counts of presumed annuli were about 2years greater for otoliths (mean=51years, range=43-57years) than spines (mean=49years, range=37-59years). Based on C-14 assays, confirmed birth years for all individuals occurred prior to 1957, thus establishing known longevity of at least 50years. Estimated age based on presumed otolith annuli for one S.albus was validated to at least age 49. Although C-14 assays confirmed pre-1957 birth years for all S.albus, only 56% of estimated ages from spines and 91% of estimated ages from otoliths depicted pre-1957 birth years. Both ageing structures were subject to under-ageing error (up to 15years). Lack of or severe curtailment of S.albus recruitment in the upper Missouri River basin since the mid-1950s closely parallels the 1953-1957 timeframe when a mainstem reservoir was constructed and started to fill. This reservoir may function as a system-wide stressor to diminish recruitment success of S.albus in the upper Missouri River basin.