These data was collected in conjuction with Upper Mississippi River Restoration-Long Term Resource Monitoring (UMRR-LTRM) element data. The goal of this data collection was to assess age structure of smallmouth buffalo Ictiobus bubalus in the Upper Mississippi River system. Through ongoing UMRR-LTRM efforts, we collected fish with a goal 10 smallmouth buffalo from 20mm length groups, beginning at 100mm of total length. Upon collection fish were measured, weighed and frozen, then transported to the Illinois River Biological Station in Havana Illinois. Upon completion of fish collection, fish were thawed and both lapillus otoliths were removed and a sex determination was made via visual inspection of the gonads. Lapillus otoliths were then thin sectioned, polished on 1000 and 2000 grit sandpaper until annuli were clearly visible. Fish age was then estimated independently by 3 readers using the polished thin sections, any disagreements were discussed and a final consensus age estimation was determined for every fish. When consensus could not be reached that sample was discarded from final dataset. Annuli for all aged fishes were then measured from the focus of the otolith for later analysis. This was done by measuring from the focus, or center of otolith formed near fish's birth, to each identified annuli. Every measurement began at the focus, so each successive annuli will have a larger length than preceeding annuli as fish grows over time.