Thiamine (vitamin B1) is required by all living organisms because it is a cofactor for enzymes in multiple metabolic pathways. It is scarce in natural systems, and deficiency can lead to reproductive failure, neurological issues, and death. A long-term mystery has been the lack of recruitment of critically endangered shortnose (Chasmistes brevirostris) and Lost River suckers (Deltistes luxatus) of the Klamath Basin that lies on the border of California and Oregon. We investigated whether thiamine deficiency may be a factor. In 2020 and 2021 we measured thiamine content within eggs, age-0 fish, and adults. We found high interannual and among species variation thiamine content (nmol/g) of eggs, but no distinct differences among adults. Interestingly, there was a marked decrease in thiamine content of age-0 suckers as they grew that coincided with lake conditions that degrade thiamine. We will present evidence that thiamine deficiency may be part of the reason the suckers are failing to recruit and discuss avenues of future research.