Turbidity and temperature effects on growth and gene transcription of threatened juvenile Longfin Smelt (Spirinchus thaleichthys)

The Longfin Smelt (LFS, Spirinchus thaleichthys) population within the San Francisco Estuary, California, has experienced a substantial reduction, diminishing to <1% of their historical abundance. This decline has culminated in their classification as a threatened species under the purview of the California Endangered Species Act. Understanding their physiology and stress response in relation to varying environmental conditions, such as temperature and turbidity, is crucial for LFS culturing, management, and conservation. In this study, we assessed juvenile LFS (age range during exposure: 181 to 228 days post hatch, dph) performance as measured by growth and gene expression following four weeks at two temperatures (11 °C and 14 °C) and three turbidity levels (1, 4, and 11 nephelometric turbidity units (NTU)). At the end of the 4-week exposure period, we conducted assessments encompassing fork length, wet weight, condition factor, and examined alterations in the transcription of 12 genes. The selection of these genes aimed at determining responses associated with osmoregulation, growth, metabolism, and general stress, as all of which are potentially influenced by temperature and/or turbidity. Weight and condition factor was significantly higher at lower temperature, whereas turbidity had no effect on growth, condition factor, and transcriptomic stress-response. Instead, the lower expression levels of Catalase, Citrate Synthase and Growth Factor Receptor Bound Protein 10 at 14 °C were indicative of metabolic and growth-related changes governed by temperature. This suggests that rearing of LFS at 11 °C and low turbidity (<11 NTU) is suitable for the juvenile stage, whereas growth as well as metabolic capacity is limited at slightly warmer temperatures.