Characterizing the stress response in juvenile Delta smelt exposed to multiple stressors

The Delta Smelt (Hypomesus transpacificus), once an abundant fish endemic to the Sacramento-San Joaquin Es-tuary, is now on the brink of extinction. Due to the high sensitivity of this species, knowledge of their stress response will be vital to their future survival and sustainability. Understanding the magnitude and kinetics of cortisol induction in Delta Smelt will provide valuable information when interpreting the degree of environ-mentally relevant stressors, such as warming and predator exposure. As little is known about the primary stress response and cortisol dynamics in Delta Smelt, the first aim of this study was to measure basal and maximal whole-body cortisol prior to and following exposure to a sublethal and significant netting stress at 17 and 21 degrees C. Our findings reveal that juvenile Delta Smelt held at 21 degrees C display an exacerbated stress response and a reduction in available energy compared to fish held at 17 degrees C. There was no evidence of the secondary stress response to the netting stress as whole-body glucose and lactate levels in treatment groups remained similar to basal values. The second aim of this study was to investigate the effect of a largemouth bass (Micropterus salmoides) predator cue, which was found to induce a significant increase in cortisol relative to control levels in juvenile Delta Smelt. Indices such as cortisol can be used as bioindicators of stress in the field and results from this study suggest that moderate temperatures and reduced predation are optimal release conditions during hatchery-based supple-mentation to minimize stress to this highly sensitive species.

Automated summary

The Delta Smelt, once abundant in the Sacramento-San Joaquin Estuary, is now endangered. This study investigates the stress response and cortisol dynamics in Delta Smelt, and finds that higher temperatures exacerbate the stress response and reduce available energy. The study also shows that the presence of a predator cue significantly increases cortisol levels in Delta Smelt.