Laboratory‐bred Longfin Smelt produced offspring in the first year in captivity.

Objective: This study aims to develop the culture methods for Longfin Smelt Spirinchus thaleichthys in a laboratory setting, achieving the first production of a fully captive second generation (F2). The objective includes understanding the critical factors influencing the breeding and maturation of this species in captivity, contributing to conservation efforts and potential revival of this threatened species. Methods: Longfin Smelt broodstock were collected from the San Francisco Estuary and reared in controlled conditions at the University of California Davis Fish Conservation and Culture Laboratory (FCCL). The study involved spawning of wild broodstock, rearing of first generation (F1) progeny to adulthood, and subsequent spawning to produce F2 offspring. Key variables such as salinity, temperature, and diet were carefully managed throughout the process. Fertilization and hatching rates were calculated, and the larvae were reared using the same methods to closely monitor and understand their growth and development. Result: First generation Longfin Smelt successfully matured and spawned at age 1, earlier than commonly observed in the wild. From five crosses, four produced viable offspring. The study recorded a range of fertilization rates (10–40%) and high hatching rates (75–97%). Salinity was identified as a critical factor in both larval development and adult maturation. While initial larval survival was challenging, the larval rearing system used in this study proved effective. Conclusion: The study represents a major breakthrough in the cultivation of Longfin Smelt, showing that it is possible to complete their life cycle under controlled conditions. It has provided valuable understanding of the maturation and breeding processes in these fish, with an emphasis on the influence of salinity. These outcomes are crucial for conservation strategies, offering the potential to help establish a refuge population and laying the groundwork for further research aimed at refining captive breeding methods for this species. Impact statementThis research achieves a significant milestone in protecting Longfin Smelt, a threatened fish vital to both river and ocean environments. Successful breeding in labs takes us a step closer to their conservation, enhancing ecosystem health and aiding local fisheries. This work shows the power of dedicated conservation efforts in preserving at‐risk species and sustaining the balance of our natural habitats. [ABSTRACT FROM AUTHOR]