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Simple Summary: Longfin smelt Spirinchus thaleichthys is an imperiled estuarine species in California, USA. A captive culture program is currently being developed; however, prior to this study, longfin smelt have only fed on live prey in the hatchery. Here, we report on our first successful attempt to wean cultured juveniles onto a dry commercial pellet feed. A subset of F1 fish was switched to a mixed diet of reduced Artemia and dry feed for 62 days, with growth, survival, and body condition compared between feeding treatments. Our results highlight that juvenile longfin smelt can utilize dry feeds while maintaining a healthy body condition. The rapid decline of longfin smelt Spirinchus thaleichthys, a threatened euryhaline forage fish in California, is a serious concern for scientists and resource managers. To recover and conserve this species, a captive culture program was initiated, focusing on the collection, captive rearing and breeding of wild broodstock, and the rearing of their offspring. Although progress has been made in the collection of broodstock and the production and culturing of larvae, no studies have evaluated the rearing of juvenile life stages in captivity. The present study examines methodological considerations for culturing F1 juvenile longfin smelt, specifically, the first efforts toward weaning juveniles to a dry commercial pellet feed. Cultured juvenile longfin smelt were fed live Artemia only or co-fed Artemia and dry feed for 62 days, and the effects of feed type on juvenile survival, growth, body condition, and fatty acid profiles were examined. No significant differences were observed between feeding treatments, despite an 80% reduction in Artemia in the co-feeding treatment. Furthermore, examination of fish stomach contents at the end of the trial confirmed the transition to dry feed. This is the first study to indicate successful feeding by longfin smelt on dry commercial pellets, and suggests that juvenile longfin smelt can be fully weaned onto dry feeds. Results of this study are critical for closing the lifecycle of longfin smelt in captivity and developing a successful conservation culture program for this imperiled species. [ABSTRACT FROM AUTHOR]