Larval Delta Smelt Research Articles

Evaluating the Performance of a Small‐Scale Culture System for Delta Smelt

AbstractSince 2008, a refuge population of Delta Smelt Hypomesus transpacificus has been cultured in a conservation hatchery to help protect the imperiled species from extinction. Culture techniques for this population have thus far focused on maximizing genetic diversity in conjunction with large‐scale operations (40 larval tanks, 5,954‐L larval system, 224,000 larvae capacity), which includes rearing multiple family groups within the same tanks. However, Delta Smelt production and associated research projects would benefit from also being able to rear fish on a smaller scale, such as in individual family groups. Recently, a new culture system and modified culture practices were developed for rearing small groups of larval Delta Smelt. We (1) determined the performance of the small‐scale “experimental” rearing system from embryo to 80 d posthatch (dph) and (2) tested whether the performance of the experimental system met the standard set by the large‐scale “refuge” rearing system at critical culturing life stages in terms of embryo hatching rate (10 d postfertilization) and larval survival, length, and weight (40 and 80 dph). From 1,000 embryos, the experimental system produced a mean ± SE of 376 ± 44 late‐stage larvae. The hatching rate of embryos did not differ between systems. Early‐stage larvae (0–40 dph) reared in the experimental system did not differ in survival rate but were longer and heavier than those reared in the refuge system. In contrast, late‐stage larvae (41–80 dph) reared in the experimental system had poorer survival and were shorter and lighter than refuge‐reared larvae. Thus, the experimental system met the refuge‐system benchmark for embryo hatching and exceeded the benchmark in early‐larval production, but it could be improved for late‐larval rearing. Nevertheless, this small‐scale rearing system will provide much‐needed flexibility to enhance Delta Smelt culture practices and facilitate future research.

Exposure to permethrin or chlorpyrifos causes differential dose- and time-dependent behavioral effects at early larval stages of an endangered teleost species.

Pyrethroid and organophosphate pesticides are two of the most commonly used classes of insecticide worldwide. At sublethal concentrations, permethrin (a pyrethroid) and chlorpyrifos (an organophosphate) impact behavior in model fish species. We investigated behavioral effects of environmentally relevant concentrations of permethrin or chlorpyrifos on early larval delta smelt Hypomesus transpacificus, a Critically Endangered teleost species endemic to the San Francisco Bay Delta, California, USA. Using a photomotor behavioral assay of oscillating light and dark periods, we measured distance moved, turn angle, meander, angular velocity, rotations, thigmotaxis (time spent in the border versus center), and swim speed duration and frequency. The lowest concentrations of permethrin used in the tests (0.05 and 0.5 μg l−1) caused significant increases in distance moved at 72 and 96 h, respectively. At 48, 72, and 96 h of exposure, 5 μg l−1 of permethrin caused a hyperactive state in which the larvae significantly decreased thigmotaxis, quickly turning in short bouts of activity, characterized by significant increases in rotations and freezing events. Larvae exposed to 0.05 μg l−1 chlorpyrifos significantly increased thigmotaxis at 72 and 96 h. In response to 5 μg l−1 chlorpyrifos, larvae significantly increased velocity at 72 h exposure, and significantly increased freezing events at 96 h. Behavioral data on larval delta smelt exposed to contaminants present in their limited habitat have the potential to aid evaluations of the suitability of spawning and rearing habitats for this endangered species, thus improving conservation management strategies focused on this sensitive life stage.

Effects of light and turbidity on feeding, growth, and survival of larval Delta Smelt (Hypomesus transpacificus, Actinopterygii, Osmeridae)

Understanding how environmental factors—here, light intensity and turbidity—influence the feeding, growth, and survival of larval Delta Smelt is critical for optimizing the culture of this endangered fish species. Three sets of rearing trials were conducted where larvae were cultured under different levels of turbidity and light intensity. Delta Smelt larvae feeding activity was observed throughout the initial adjustment of light and turbidity levels to ascertain feeding ability and behavior at the different levels. It was found that there is a minimum threshold of combined light and turbidity to generate their exogenous feeding response. At low turbidity conditions, early-stage larvae begun feeding and growing earlier when light levels were higher. The condition factor and survival of the late-stage larvae was increased with increased turbidity. The results suggest Delta Smelt larvae may be cultured under lower turbidity and light conditions rather than at the currently used levels. We conclude that the interaction between turbidity and light intensity plays an important role in the feeding activity, growth, and survival of larvae. The results here may inform fish culture lab methodologies, but may also provide useful baseline knowledge for light and turbidity preferences of Delta Smelt larvae in the wild.

Generation of Quantitative Polymerase Chain Reaction Detectability Half‐Lives and Comparison of Sampling Protocols for Genetic Diet Studies of San Francisco Estuary Fishes

AbstractSeveral factors affect the probability of genetic analyses to detect prey in predator gut contents, including biological differences in the prey and predator species as well as differences in sampling and laboratory methodologies. Understanding these biases allows researchers to more appropriately put genetic prey detections in an ecological context. In this study, we determined the detectability half‐lives of DNA from two prey species in the guts of two predators. The half‐life detectability of juvenile Chinook Salmon Oncorhynchus tshawytscha in Striped Bass Morone saxatilis was 66.2 h, and that of larval Delta Smelt Hypomesus transpacificus in Mississippi Silverside Menidia audens was 26.4 h. Additionally, we performed a series of laboratory trials to examine the effects of variables in sample collection and preservation methodologies on the detectability of prey. Differences between methodologies were minimal, providing confidence that laboratory analyses will not be greatly affected by inconsistencies in field sampling procedures. Injecting a 95% solution of ethanol into the stomach via the esophagus immediately following collection and placing the fish on ice in the field prior to freezing at −20°C in the laboratory is a protocol readily applied in the field that will provide consistent results.Received August 19, 2015; accepted December 3, 2015 Published online April 11, 2016

Physiological stress biomarkers reveal stocking density effects in late larval Delta Smelt (Hypomesus transpacificus)

Suboptimal fish stocking densities in experimental systems may elicit stress responses that can affect experimental results. Fish species, age and size, water chemistry and flow, and physical characteristics of the experimental system (e.g., tank, cage) are among the parameters to be considered when determining stocking densities. However, systematic studies to define fish densities minimizing stress in experimental systems are rarely performed. This is particularly true when working with species of low aquaculture value or a non-model test species such as the Delta Smelt (Hypomesus transpacificus). The aim of this study was to use physiological stress biomarkers to determine suitable fish densities for specific experimental vessels routinely used for this species. We maintained late larval Delta Smelt (60days post-hatch; dph) over a period of 24h, at five different densities: 7, 14, 28, 42, and 56 fish per 8L circular fish tank. We assessed whole body cortisol and transcriptomic biomarkers that lead to cortisol production to quantify stress levels. Both marker types delivered similar results. Cortisol levels were lowest at densities of 28 and 42 fish per tank, whereas lowest fish densities (7 and 14 fish per tank) evoked the highest stress levels. Genes such as Mineralocorticoid Receptor 1 and Glucocorticoid Receptor 2, as well as 11-Beta-Hydroxysteroid-Dehydrogenase-2 depicted the lowest expression levels at stocking densities 28 and 42, and elevated expression levels for stocking densities 7 and 14. Our data support the observations that late larval Delta Smelt should be exposed, acclimated, and cultured in groups rather than as individuals or in low numbers. This study indicates the importance of adequately defining experimental conditions that minimize stress, specifically when stress is measured as an endpoint. In addition to classical cortisol measurements, responses of the transcriptome also appear suitable in assessing stress responses in fish, and in determining optimal holding conditions, particularly if short-term responses are the study focus.Statement of relevanceThe study highlights the importance of evaluating stress in order to determine species-specific stocking densities. The results are thus relevant to a wide audience in the field of aquaculture and experimental biology.

The transcriptome-wide effects of exposure to a pyrethroid pesticide on the Critically Endangered delta smelt Hypomesus transpacificus

Research into the effects of contaminants on fishes is often conducted on well- studied model test species, whose responses may be different than those of endangered species. We developed an oligonucleotide microarray consisting of 12 595 genes to examine the effects of the pesticide permethrin on an endangered fish in California, USA, the delta smelt Hypomesus transpacificus. The microarray was used in combination with quantitative PCR (qPCR) assess- ments. We exposed larval delta smelt to permethrin for 96 h at concentrations of 0.69, 1.37, 2.56, 4.84, 12.88 and 24.94 µg l �1 , and compared the responses to a control group. The 96 h 50% lethal concentration (LC50) was 4.07 µg l �1 , which is lower than those reported for model test species and similar to other endangered species. With increasing exposure concentration, we detected the dif- ferential expression of 3342 microarray features with elevated expression of genes involved in protein degradation and apoptosis, and decreased expression of immune function genes. Func- tional analysis indicated that genes involved in protein degradation, immune function, an unfolded protein response, meta bolism and cell signaling cascades were affected by exposure to permethrin. Many of the gene responses in the 0.69 µg l �1 treatment group differed in their direc- tional change in expression from those at higher exposure concentrations, suggesting a potential mechanistic threshold of sub-lethal toxicity at concentrations below the lowest observed effect concentration (LOEC) of 2.56 µg l �1 . These results are consistent with non-monotonic response patterns to contaminants, and demonstrate the effects on a sensitive fish species of a widely used pyrethroid pesticide at concentrations below those that affect model test species.

Transcriptomic profiling permits the identification of pollutant sources and effects in ambient water samples

Contaminant exposure is one possible contributor to population declines of endangered fish species in the Sacramento–San Joaquin Estuary, California, including the endangered delta smelt (Hypomesus transpacificus). Herein we investigated transcriptional responses in larval delta smelt resulting from exposure to water samples collected at the Department of Water Resources Field Station at Hood, a site of concern, situated upstream of known delta smelt habitat and spawning sites and downstream of the Sacramento Regional Wastewater Treatment Plant (SRWTP). Microarray assessments indicate impacts on energy metabolism, DNA repair mechanisms and RNA processing, the immune system, development and muscle function. Transcription responses of fish exposed to water samples from Hood were compared with exposures to 9% effluent samples from SRWTP, water from the Sacramento River at Garcia Bend (SRGB), upstream of the effluent discharge, and SRGB water spiked with 2mg/L total ammonium (9% effluent equivalent). Results indicate that transcriptomic profiles from Hood are similar to 9% SRWTP effluent and ammonium spiked SRGB water, but significantly different from SRGB. SRGB samples however were also significantly different from laboratory controls, suggesting that SRWTP effluent is not solely responsible for the responses determined at Hood, that ammonium exposure likely enhances the effect of multiple-contaminant exposures, and that the observed mortality at Hood is due to the combination of both effluent discharge and contaminants arising from upstream of the tested sites.

Linking mechanistic and behavioral responses to sublethal esfenvalerate exposure in the endangered delta smelt; Hypomesus transpacificus (Fam. Osmeridae).

BackgroundThe delta smelt (Hypomesus transpacificus) is a pelagic fish species listed as endangered under both the USA Federal and Californian State Endangered Species Acts and considered an indicator of ecosystem health in its habitat range, which is limited to the Sacramento-San Joaquin estuary in California, USA. Anthropogenic contaminants are one of multiple stressors affecting this system, and among them, current-use insecticides are of major concern. Interrogative tools are required to successfully monitor effects of contaminants on the delta smelt, and to research potential causes of population decline in this species. We have created a microarray to investigate genome-wide effects of potentially causative stressors, and applied this tool to assess effects of the pyrethroid insecticide esfenvalerate on larval delta smelt. Selected genes were further investigated as molecular biomarkers using quantitative PCR analyses.ResultsExposure to esfenvalerate affected swimming behavior of larval delta smelt at concentrations as low as 0.0625 μg.L-1, and significant differences in expression were measured in genes involved in neuromuscular activity. Alterations in the expression of genes associated with immune responses, along with apoptosis, redox, osmotic stress, detoxification, and growth and development appear to have been invoked by esfenvalerate exposure. Swimming impairment correlated significantly with expression of aspartoacylase (ASPA), an enzyme involved in brain cell function and associated with numerous human diseases. Selected genes were investigated for their use as molecular biomarkers, and strong links were determined between measured downregulation in ASPA and observed behavioral responses in fish exposed to environmentally relevant pyrethroid concentrations.ConclusionsThe results of this study show that microarray technology is a useful approach in screening for, and generation of molecular biomarkers in endangered, non-model organisms, identifying specific genes that can be directly linked with sublethal toxicological endpoints; such as changes in expression levels of neuromuscular genes resulting in measurable swimming impairments. The developed microarrays were successfully applied on larval fish exposed to esfenvalerate, a known contaminant of the Sacramento-San Joaquin estuary, and has permitted the identification of specific biomarkers which could provide insight into the factors contributing to delta smelt population decline.

English

Movements of pelagic organisms in the tidal freshwater regions of estuaries are sensitive to the movements of water. In the Sacramento-San Joaquin Delta—the tidal freshwater reach of the San Francisco Estuary—such movements are key to losses of fish and other organisms to entrainment in large water-export facilities. We used the Delta Simulation Model-2 hydrodynamic model and its particle tracking model to examine the principal determinants of entrainment losses to the export facilities and how movement of fish through the Delta may be influenced by flow. We modeled 936 scenarios for 74 different conditions of flow, diversions, tides, and removable barriers to address seven questions regarding hydrodynamics and entrainment risk in the Delta. Tide had relatively small effects on fate and residence time of particles. Release location and hydrology interacted to control particle fate and residence time. The ratio of flow into the export facilities to freshwater flow into the Delta (export:inflow or EI ratio) was a useful predictor of entrainment probability if the model were allowed to run long enough to resolve particles’ ultimate fate. Agricultural diversions within the Delta increased total entrainment losses and altered local movement patterns. Removable barriers in channels of the southern Delta and gates in the Delta Cross Channel in the northern Delta had minor effects on particles released in the rivers above these channels. A simulation of losses of larval delta smelt showed substantial cumulative losses depending on both inflow and export flow. A simulation mimicking mark–recapture experiments on Chinook salmon smolts suggested that both inflow and export flow may be important factors determining survival of salmon in the upper estuary. To the extent that fish behave passively, this model is probably suitable for describing Delta-wide movement, but it is less suitable for smaller scales or alternative configurations of the Delta.

English

English