Age Estimates of Wild Pallid Sturgeon in the Upper Missouri and Yellowstone Rivers Based on Fin Rays, Otoliths and Bomb Radiocarbon

The lower Yellowstone River and Missouri River between Fort Peck Dam and Lake Sakakawea contain a wild stock of approximately 125 adult pallid sturgeon (Scaphirhynchus albus).  No detectible recruitment has occurred in this portion of the Missouri River basin for several decades as catch records indicate that the wild stock is comprised almost exclusively of large (> 1,000 mm) apparently old individuals.  However, limited age information is available to discern birth years and draw inferences on past recruitment episodes that produced the extant stock.  Pectoral fin rays and otoliths were obtained from 11 upper basin pallid sturgeon mortalities (1997-2007) that were frozen and maintained at the University of Alabama whole-fish repository. Ages were estimated based on counts of presumed annuli. In addition, otolith core material was assayed for bomb radiocarbon (¹⁴C) using accelerator mass spectrometry.  Atmospheric detonation of thermonuclear bombs beginning in the 1950s produced a globally detected pattern of initially increasing (~ 1958), maximum (~ mid to late 1960s) and decreasing radiocarbon concentrations, and the radiocarbon can be retained in hard parts to serve as a marker for age determinations in long-lived fishes.  Among-reader age estimates based on counting presumed annuli varied from 35 to 61 years for fin rays and from 42 to 61 years for otoliths.  Otolith cores did not contain bomb radiocarbon, indicating that the sample population must have been born prior to 1958 when initial increases in bomb radiocarbon first occurred.  Results highlight the longevity of pallid sturgeon in northern latitudes, and provide inferences on spawning and recruitment events following closure of mainstem dams.