Effects of turbidity, temperature and predation cue on the stress response of juvenile delta smelt

Abstract

The San Francisco Estuary (SFE) is one of the most degraded ecosystems in the United States, and organisms that inhabit it are exposed to a suite of environmental stressors. The delta smelt (Hypomesus transpacificus), a small semi-anadromous fish endemic to the SFE and considered an indicator species, is close to extinction in the wild. The goal of this study was to investigate how environmental alterations to the SFE, such as reductions in turbidities, higher temperatures and increased prevalence of invasive predators affect the physiology and stress response of juvenile delta smelt. Juvenile delta smelt were exposed to two temperatures (17 and 21°C) and two turbidities (1-2 and 10-11 NTU) for 2 weeks. After the first week of exposure, delta smelt were exposed to a largemouth bass (Micropterus salmoides) predator cue at the same time every day for 7 days. Fish were measured and sampled on the first (acute) and final (chronic) day of exposures to predator cues and later analyzed for whole-body cortisol, glucose, lactate, and protein. Length and mass measurements were used to calculate condition factor of fish in each treatment. Turbidity had the greatest effect on juvenile delta smelt and resulted in reduced cortisol, increased glucose and lactate, and greater condition factor. Elevated temperatures reduced available energy in delta smelt, indicated by lower glucose and total protein, whereas predator cue exposure had negligible effects on their stress response. This is the first study to show reduced cortisol in juvenile delta smelt held in turbid conditions and adds to the growing data that suggest this species performs best in moderate temperatures and turbidities. Multistressor experiments are necessary to understand the capacity of delta smelt to respond to the multivariate and dynamic changes in their natural environment, and results from this study should be considered for management-based conservation efforts.

Keywords: San Francisco Estuary; climate change; conservation; delta smelt; supplementation.

Figure 1

Whole-body cortisol measurements (ng·g⁻¹) in juvenile delta smelt during an acute and chronic predator cue exposure (NC = no cue; PC = predator cue) held at two temperatures (light grey = 17°C; dark grey = 21°C) and two turbidities (1–2 NTU & 10–11 NTU). Fish were sampled 15 min after exposure to a largemouth bass predator cue (vertical angled bars) or tank water (blank bars). Cortisol levels were significantly lower in fish held in turbid compared with low-turbidity conditions. Data (n = 14–20) are presented as mean ± SEM.

Figure 2

Whole-body glucose measurements (μg·g⁻¹) in juvenile delta smelt during an acute and chronic predator cue exposure (NC = no cue; PC = predator cue) held at two temperatures (light grey = 17°C; dark grey = 21°C) and two turbidities (1–2 NTU & 10–11 NTU). Fish were sampled 15 min after exposure to a largemouth bass predator cue (vertical angled bars) or tank water (blank bars). Glucose levels were significantly greater in fish held at lower temperatures and greater turbidities. Data (n = 12–20) are presented as mean ± SEM.

Figure 3

Whole-body lactate measurements (μg·g⁻¹) in juvenile delta smelt during an acute and chronic predator cue exposure (NC = no cue; PC = predator cue) held at two temperatures (light grey = 17°C; dark grey = 21°C) and two turbidities (1–2 NTU & 10–11 NTU). Fish were sampled 15 min after exposure to a largemouth bass predator cue (vertical angled bars) or tank water (blank bars). Lactate levels were significantly greater in fish held in turbid conditions and during the chronic predator stress. Data (n = 14–20) are presented as mean ± SEM.

Figure 4

Protein-to-mass ratios (μg·mL⁻¹·g⁻¹) in juvenile delta smelt during an acute and chronic predator cue exposure (NC = no cue; PC = predator cue) held at two temperatures (light grey = 17°C; dark grey = 21°C) and two turbidities (1–2 NTU & 10–11 NTU). Fish were sampled 15 min after exposure to a largemouth bass predator cue (vertical angled bars) or tank water (blank bars). Protein-to-mass ratios were significantly greater in fish held at 17°C compared with those held at 21°C. Data (n = 15–20) are presented as mean ± SEM.