The Effects of Temperature and Salinity on the Endocrinology in Two Life Stages of Juvenile Rainbow/Steelhead Trout (Oncorhynchus mykiss).

The San Francisco Bay Delta is experiencing seasonally warmer waters and salt water intrusion into historically freshwater ecosystems due to climate change. Steelhead/rainbow trout (Oncorhynchus mykiss) are resident in the Bay-Delta from juvenile development through the smoltification process. Due to sea level rise, premature seawater acclimation may co-occur with increased temperatures on pre-smolt juveniles. To evaluate the interactive effects of salinity and temperature on juvenile life stages of salmonids, rainbow trout alevin (3 days post-hatch) were exposed to 13o C, 16.4o C and 19o C temperatures for 10 days and then challenged for 24 hours to 18 ppt seawater (SW). Similarly, fry (4 weeks post-hatch) were exposed to 13o C, 16.4o C and 19o C temperatures for two weeks (14 days) and then challenged to SW. Estradiol-17β (E2 ), cortisol, triiodothyronine (T3 ), and thyroxine (T4 ) were measured in whole animal homogenates and muscle tissue using Enzyme Linked Immunnosorbent Assays. Transcripts of gill Na+ /K+ ATPase β (NKAα1b), brain Growth Hormone I (gh1), and brain Gonadotropin-Releasing Hormone Receptor 2 (gnrh2) were also measured. Alevin exhibited a significant temperature-dependent decrease in survival and fry showed a temperature-dependent decrease in condition factor. Gene expression of NKAα1b, gh1, and gnrh2 was significantly decreased in all SW challenged alevin, and a significant decrease in gnrh2 expression was observed in fry with temperature. Alevin T3 and T4 concentrations were significantly increased with increasing temperature. There was a temperature-dependent increase in E2 of fry, but not in alevin. The results of this study demonstrate that increasing temperature and SW exposure may adversely affect the survival and SW acclimation of alevin and fry stages of salmonids, and that the tolerances of younger juvenile stages should be considered when assessing the response of salmonid populations to climate change stressors. This article is protected by copyright. All rights reserved.