Hydrophobic organic contaminants (HOCs) pose risk to aquatic organisms at high concentrations and have been implicated in the declining health of the endangered fish species, Delta smelt (Hypomesus transpacificus). Legacy and current-use pesticides, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) were quantified in a surrogate fish species (Wakasagi; Hypomesus nipponensis), sediments, zooplankton, and suspended solids throughout the Sacramento Deep Water Ship Channel (SDWSC). Forty-four of 63 analytes were detected across the tested media, with legacy pesticides (LPs), PAHs, and PCBs being the most prevalent contaminants measured in Wakasagi. Pyrethroids, a class of current-use insecticides, were rarely detected in Wakasagi tissue. Pattern analysis via cosine similarity showed that contaminant concentrations associated with suspended solids best reflected the concentrations measured within the Wakasagi, suggesting that suspended solids may represent a relevant exposure pathway. While only DDE and DDD concentrations exceeded the threshold effect concentration for sediment (TEC), contaminants associated with suspended solids exceeded most sediment quality guidelines. In addition, PAH concentrations in suspended solids exceeded multiple lesion thresholds for a variety of other fish species. These results suggest potential ecological concern from suspended solid-associated contaminant exposure. Future studies need to be performed to assess the relationship between HOCs associated with suspended solids and their potential harm to pelagic organisms for the purpose of contaminant mitigation in the SDWSC.

Chinook Salmon (Oncorhynchus tshawytscha) populations in the Sacramento-San Joaquin Delta have declined likely due to multiple environmental stressors. Among the candidate factors contributing to these declines are habitat alteration, hydrologic modification, and pesticide exposure which can disrupt salmonid physiology and behavior. This study assessed pesticide exposure by whole-body residue analysis of hatchery-origin field-caught juvenile Chinook Salmon during their early ocean outmigration (2022-2025) to evaluate how residency duration, migration route, and release origin influence contaminant accumulation. In parallel, juvenile Chinook Salmon were exposed in cages at five sites within migration pathways to confirm pesticide accumulation and compare residues. Target analytes, including the insecticides bifenthrin, fipronil, 4,4'-dichlorodiphenyltrichloroethane (DDT) and common degradation products 4,4'-dichlorodiphenyldichloroethylene (DDE), 4,4'-dichlorodiphenyldichloroethane (DDD), fipronil sulfone, fipronil sulfide, and fipronil desulfinyl were quantified in whole-body homogenates. Consistent pesticide exposure was observed in the field-caught salmon, with DDE residues being found in all salmon samples as high as 4.80 nmol/g lipid (average = 0.671 nmol/g lipid), and bifenthrin residues were detected in most samples (91%) as high as 0.43 nmol/g lipid (average = 0.046 nmol/g lipid). DDT, DDD, fipronil, and fipronil degradation products were detected less frequently and at lower concentrations, indicating additional, but lower exposure (23% DDT + DDD; 8.5% fipronil (sum)). Pesticide residues increased significantly with residency time, and a six-fold increase in bifenthrin and four-fold increase in DDE were observed. A multivariate analysis indicated that the route of outmigration was also a significant predictor for bifenthrin and DDE accumulation. Cage-exposed salmon deployed for 2 weeks at five Delta sites showed consistent accumulation, with a similar residue pattern as the field-caught salmon. These results demonstrate that pesticide accumulation in juvenile Chinook Salmon during outmigration is driven by the combined effects of residency duration, migration timing, and watershed characteristics, with implications for contaminant exposure and risk assessments in the Delta ecosystem.

Pyrethroid insecticides are ubiquitous in waterways throughout the United States. Consequently, species have evolved resistance to these insecticides via receptor site mutations with unknown impacts to their fitness. The goal of this study was to determine if pyrethroid-resistant Hyalella azteca populations (Escondido and Mosher) exhibited disrupted phototaxis compared to a non-resistant population. We tested phototactic response to determine the sensitivity of H. azteca to chemicals that cause neurobehavioral impacts. H. azteca were exposed to non-dosed, permethrin (pyrethroid), and fipronil (phenylpyrazole) treatments 24 h before assessing phototaxis using light stimulus. Each population was genotyped for mutations in the voltage-gated sodium channel (VGSC) and the γ-aminobutyric acid (GABA) receptor. Non-resistant H. azteca had a faster response to light stimulus than resistant populations following the non-dosed and fipronil-dosed exposures. When exposed to pyrethroids, the Escondido population responded more rapidly than the non-resistant population. Genotyping results revealed all populations were wild-type for rdl, the gene encoding the GABA-gated chloride channel and the target for phenylpyrazole insecticides. In addition, the Mosher population shifted from homozygous for the L925I substitution in the VGSC gene (vgsc) to a relatively weaker I936F for pyrethroid resistance. The exact cause in the reduction in negative phototaxis and general fitness is unknown but may stem from reduced protein efficiency from the vgsc mutations or possible results of genetic bottlenecking, hence reducing genetic diversity. Benefits of adaptation may be limited to improved survivorship under harsh conditions of insecticide exposure but at a cost to overall fitness including phototactic behavior.

Despite its better accuracy and applicability, the critical body residue (CBR) approach in toxicity assessments is not utilized as often as external concentrations, and even less often in mixture toxicity tests. In this project, we exposed juvenile Chinook salmon (Oncorhynchus tshawytscha) to two current-use pesticides (bifenthrin and fipronil) and a legacy pesticide degradation product (4,4'- dichlorodiphenyldichloroethylene, DDE) that are commonly found in migration waterways, and acute toxicity was related to pesticides residues in salmon tissue. An equipotent mixture of bifenthrin and fipronil was tested, and 50 % lethality (LR50, determined as toxic units, TU) after 96 h of exposure was determined in the presence and absence of added DDE. The results suggested that bifenthrin and fipronil joint toxicity was likely additive though the LR50 values were slightly greater than 1 (LR50 = 1.52 (1.18-1.83)), and the addition of DDE did not significantly affect acute toxicity (LR50 = 1.48 (1.25-1.75)). Residues that caused acute lethality were much higher than the maximum environmentally observed residues in field-caught salmon, which suggests little risk of a fish kill scenario at these exposure levels. This does not exclude the chance of sublethal effects occurring at these exposure levels, and the CBR approach could also be used to assess sublethal effects of pesticide mixtures. This work demonstrates the utility of the CBR approach in assessing mixture toxicity and the risk of exposure in field-caught salmon.

A response spectrum model (RSM) was developed that relates sublethal effects to a whole-body residue dose metric to better assess the risk of fipronil exposure to outmigrating Chinook salmon. The use of whole-body residues obviates the need for bioavailability estimates of hydrophobic pesticides, it enables exposure estimates from all pesticide sources, and it is suitable for pre-existing fish monitoring programs. After 10-d of exposure to fipronil, sublethal endpoints were measured, which included swim performance, growth inhibition, routine swimming behavior, and response to light stimuli. Parent fipronil and fipronil metabolites (sulfone, sulfide, and desulfinyl) internal residues were analyzed. The RSM indicated that anxiety-like and shoaling behaviors were the most sensitive endpoints resulting from accumulated fipronil and fipronil metabolites, while swim performance was the least sensitive. The residues for the most sensitive endpoint on the RSM were a factor of 49-66 greater than the fipronil residues found in field caught juvenile Chinook salmon from the Yolo Bypass and Sacramento River (California, USA), in 2019 and 2020. This suggested that the risk of negative effects on outmigration due to fipronil exposure was low for those populations. The newly developed fipronil RSM can be applied to other salmon populations to help guide future management decisions regarding the effects of fipronil on sensitive Chinook salmon runs.

Long-term declines in salmonid populations observed in California Central Valley have prompted efforts to enhance the understanding of how environmental stressors impact sensitive species. Bifenthrin, a current-use insecticide, has been consistently detected throughout the Sacramento-San Joaquin River Delta (Delta) and has been linked to detrimental effects in salmon. Traditionally, aqueous concentration is used in toxicological studies to evaluate the effects of pesticides on aquatic organisms, which assumes that concentration of the toxicant in water is a valid surrogate for dose. The critical body residue approach was established as an improved technique for assessing toxicity of hydrophobic contaminants, but there is a lack of data to support the application of this method in assessing risk of contaminant exposure in the environment. The current study creates a response spectrum model (RSM) demonstrating the relationship between internal residue and effects observed in Chinook Salmon from laboratory-based exposures. To develop the RSM, a series of behavioral and physiological endpoints were measured using bifenthrin-dosed Chinook Salmon to use with previously generated sublethal and mortality data for incorporation in the model. The most sensitive endpoints were locomotion and shoaling behavior, followed by anxiety, growth, swim performance, upper thermal sensitivity, olfactory response, and lethality. The RSM endpoints were compared to bifenthrin residues in field-collected juvenile Chinook Salmon collected in 2019-2020 as part of our earlier studies. We found bifenthrin tissue residues were at similar levels to the most sensitive endpoints featured in the RSM, suggesting that bifenthrin exposure in the field could cause behavioral effects to salmon as they out-migrate through the Delta. The developed RSM is a tool that could be used by water quality managers to evaluate the extent to which bifenthrin exposure may impact behavior and performance in juvenile salmon, providing a field-based verification of its effects on outmigration.

The use of internal body residues has the potential to improve toxicological assessments of hydrophobic pesticides. The acute toxicity of three classes of pesticides were assessed in juvenile Chinook salmon using internal body residues. Chinook salmon were exposed to two current-use pesticides bifenthrin and fipronil, and 4,4 '- dichlorodiphenyldichloroethylene (DDE), which is a degradation product of the legacy pesticide dichlorodiphenyltrichloroethane (DDT). After 96-h of aqueous exposure to each pesticide, the pesticide content in whole-body Chinook salmon homogenates was measured using gas chromatography/mass spectrometry with methane negative chemical ionization. The wet-weight (ww) normalized lethal residue at 50% mortality (LR50) was lowest for bifenthrin (0.654 nmol/g ww), followed by fipronil (7.17 nmol/g ww) and the sum of fipronil and its sulfone, sulfide, and desulfinyl degradation products (8.72 nmol/g ww). No lethality was observed for DDE, even at the highest body residue (>116 nmol/g ww). LR50 estimates were also normalized to dry weight and lipid content and compared to field-caught fish to assess risk. The use of a risk quotient approach indicated that bifenthrin imparts the highest risk of acute toxicity in juvenile Chinook salmon among the three pesticides tested. In comparison to external dose metrics, the use of internal body residues has the potential to improve risk assessment by providing a more direct link between pesticide concentration at the receptor site and toxicological effects.

Stable isotopes (SI) and fatty acid (FA) biomarkers can provide insights regarding trophic pathways and habitats associated with contaminant bioaccumulation. We assessed relationships between SI and FA biomarkers and published data on concentrations of two pesticides [dichlorodiphenyltrichloroethane and degradation products (DDX) and bifenthrin] in juvenile Chinook Salmon (Oncorhynchus tshawytscha) from the Sacramento River and Yolo Bypass floodplain in Northern California near Sacramento. We also conducted SI and FA analyses of zooplankton and macroinvertebrates to determine whether particular trophic pathways and habitats were associated with elevated pesticide concentrations in fish. Relationships between DDX and both sulfur (δ34S) and carbon (δ13C) SI ratios in salmon indicated that diet is a major exposure route for DDX, particularly for individuals with a benthic detrital energy base. Greater use of a benthic detrital energy base likely accounted for the higher frequency of salmon with DDX concentrations > 60 ng/g dw in the Yolo Bypass compared to the Sacramento River. Chironomid larvae and zooplankton were implicated as prey items likely responsible for trophic transfer of DDX to salmon. Sulfur SI ratios enabled identification of hatchery-origin fish that had likely spent insufficient time in the wild to substantially bioaccumulate DDX. Bifenthrin concentration was unrelated to SI or FA biomarkers in salmon, potentially due to aqueous uptake, biotransformation and elimination of the pesticide, or indistinct biomarker compositions among invertebrates with low and high bifenthrin concentrations. One FA [docosahexaenoic acid (DHA)] and DDX were negatively correlated in salmon, potentially due to a greater uptake of DDX from invertebrates with low DHA or effects of DDX on FA metabolism. Trophic biomarkers may be useful indicators of DDX accumulation and effects in juvenile Chinook Salmon in the Sacramento River Delta.

The Sacramento Deep Water Ship Channel (SDWSC) in the San Francisco Estuary, which is an active commercial port, is critical habitat for pelagic fish species including delta smelt (Hypomesus transpacificus), longfin smelt (Spirinchus thaleichthys), and Sacramento perch (Archoplites interruptus). Pelagic organism decline has been attributed to covarying factors such as manipulation of habitat, introduction of invasive species, decrease in food production, and contaminant exposure. Quantification of bioavailable toxicant loads in the SDWSC is limited despite previous surveys that have detected elevated contaminant concentrations in the sediments. Therefore, the focus of the present study was to characterize the bioavailability of the contaminants in the SDWSC from six sites along the channel. At each site, organochlorine pesticides (OCPs), pyrethroid insecticides, polyaromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) were quantified in sediment, zooplankton, and suspended solids. In addition, Tenax extraction was used to measure the bioaccessible fraction of sediment-associated contaminants freely dissolved in the water. Bioaccessible contaminants in the sediment provided an uptake route for these stressors into invertebrates and fish with bioaccessible OCPs being found at all sites, particularly 4,4'-dichlorodiphenyldichloroethylene (DDE). Bifenthrin was the only pyrethroid detected in the chosen matrices and it was found at concentrations below levels of concern. Bioaccessible PAHs were found at all sites, with highest detections for phenanthrene and pyrene. No PCBs were detected in sediments, but were detected in both suspended solids and zooplankton. Contaminant concentrations overall were significantly higher in suspended solids, followed by zooplankton and sediments. The highest sediment concentrations of DDE, fluoranthene, pyrene, and dibenzo[a,h]anthracene exceeded sediment quality benchmarks indicating potential risk to sediment-dwelling species. Finally, elevated contaminant levels were found in both suspended solids and zooplankton, suggesting additional risk to pelagic species in the SDWSC.

Hyalella azteca are epibenthic amphipods that have developed resistance to pyrethroid and organophosphate insecticides due to single amino acid substitutions in the voltage-gated sodium channel and the acetylcholinesterase-1 gene, respectively. Aquatic systems are often contaminated with several different types of insecticides, therefore there is a possibility that H. azteca have also developed resistance to other classes of in-secticides. The aims of the current study were to verify that pyrethroid-and organophosphate-resistant H. azteca have retained their resistance after being cultured in the absence of selective pressure for 5 years (Escondido Creek population) and 9 years (Mosher Slough population), to determine if these populations have cross-resistance to carbaryl (carbamate) and 4,4 '-dichlorodiphenyltrichloroethane (DDT; organochlorine), and deter-mine whether previous field exposure to fipronil (phenylpyrazole) and imidacloprid (neonicotinoid) caused resistance in cultured pyrethroid-and organophosphate-resistant H. azteca populations. Escondido Creek and Mosher Slough H. azteca populations both maintained high tolerances for bifenthrin due to L925I and I936F amino acid substitutions. Resistance was also found for chlorpyrifos in the Escondido Creek and Mosher Slough populations with lower genotype frequencies of the G119S substitution, indicating that additional factors may be responsible for organophosphate resistance in this study. Mosher Slough H. azteca were moderately resistant to DDT, and Escondido Creek and Mosher Slough H. azteca were moderately resistant to carbaryl, suggesting cross-resistance. No differences were observed in acute toxicity values across the three populations of H. azteca for fipronil and imidacloprid, and this is possibly due to the lack of exposure to toxic concentrations of these in-secticides in the field and lack of similar modes of action to pyrethroids and organophosphates. Resistance is known to be associated with fitness costs that can place insecticide-resistant populations at risk for decline through decreased survival and reduced fecundity.

Projected water temperature increases based on predicted climate change scenarios and concomitant pesticide exposure raises concern about the responses of aquatic organisms. To better understand the effect of pesticide mixtures and influence of water temperature to fish, juvenile Chinook salmon (Oncorhynchus tshawytscha) were dietarily exposed to a mixture of legacy and current use pesticides (p,p'-DDE, bifenthrin, chlorpyrifos, esfenvalerate, and fipronil) at concentrations detected from field-collected prey items in the Sacramento-San Joaquin Delta, California (Delta) and exposed under current and predicted future water temperature scenarios, 11, 14, or 17 °C, for 14 days. The expression of a subset of genes (deiodinase 2-dio2, gonadotropin releasing hormone 2-gnrh2, and catechol-o-methyltransferase-comt) involved in neuroendocrine, dopaminergic, and olfactory function previously shown to be altered by individual pesticide exposures germane to this study were determined and olfactory function assessed using a Y-maze behavioral assay. When total body burdens of pesticides were measured, a significant decrease in dio2 expression was observed in Chinook salmon exposed at 14 °C compared to fish kept at 11 °C. Increases in gnrh2 expression were also observed in fish exposed to 14 °C. Similarly, increases in comt expression was noted at 14 and 17 °C. Additionally, altered expression of all transcripts was observed, showing interactions between temperature and individual pesticide concentrations. Chinook salmon spent significantly more time actively avoiding the odorant arm at baseline conditions of 11 °C in the Y-maze. At higher temperatures, Chinook spent significantly more time not making a choice between the odorant or clean arm following exposure to the low pesticide mixture, relative to 11 °C. These results suggest that dietary exposure to pesticide mixtures can potentially induce neuroendocrine effects and behavior. Impaired olfactory responses exhibited by Chinook salmon could have implications for predator avoidance in the wild under increased temperature scenarios and impact populations in the future.

Juvenile Chinook salmon (Oncorhynchus tshawytscha) of the Sacramento River system encounter many anthropogenically-induced stressors while rearing and migrating to the Pacific Ocean. Located in a prominent agricultural region, the watershed serves as a source of notable contaminants including pesticides. Salmon rearing in riverine and floodplain areas are potentially exposed to these compounds via dietary exposure, which can vary based on selected food webs. Previous studies have suggested that juvenile Chinook salmon rearing in riverine and floodplain environments of the Sacramento River watershed are characterized by different dietary preferences, with potential for contrasting pesticide exposure between habitats. To examine the potential for pesticide exposure, juvenile Chinook salmon and known dietary items were collected in the mainstem Sacramento River and an adjacent floodplain, the Yolo Bypass, in 2019 and 2020, and analyzed for 33 pesticides, including degradates and isomers. Organochlorine pesticides including the DDX group (p,p'-DDT, p,p'-DDD and p,p'-DDE) were prevalent in all examined biota. There was a significantly greater number of total pesticide detections across all classes in zooplankton compared to macroinvertebrates, coupled with higher bifenthrin concentrations in zooplankton across regions and years, which may indicate different exposure potential depending on fish dietary preferences. Detection frequencies and concentrations of organochlorines were higher in prey items during flooding than in drought conditions, suggesting resuspension of legacy compounds. Significantly higher concentrations of organochlorines were recorded in floodplain rearing fish compared to the Sacramento River. These findings suggest that within these habitats, juvenile Chinook salmon feeding primarily on zooplankton within the water column may be exposed to a greater range of pesticides than those feeding on benthic macroinvertebrates, and that the benefits of floodplain rearing may come at a cost of increased organochlorine exposure.

Two commonly used insecticides, bifenthrin and fipronil, can accumulate in the prey of juvenile Chinook salmon, yet the effects of dietary exposure are not understood. Therefore, to better characterize the effect of a dietary exposure route, juvenile Chinook salmon were fed chironomids dosed with a concentration of 9 or 900 ng/g of bifenthrin, fipronil, or their mixture for 25 days at concentrations previously measured in field-collected samples. Chinook were assessed for maximum swimming performance (Umax) using a short-duration constant acceleration test and biochemical responses related to energetic processes (glucose levels) and liver health (aspartate aminotransferase (AST) activity). Chinook exposed to bifenthrin and bifenthrin and fipronil mixtures had a significantly reduced swimming performance, although not when exposed to fipronil alone. The AST activity was significantly increased in bifenthrin and mixture treatments and glucose levels were increased in Chinook following a mixture treatment, although not when exposed to fipronil alone. These findings suggest that there are different metabolic processes between bifenthrin and fipronil following dietary uptake that may influence toxicity. The significant reductions in swimming performance and increased levels of biochemical processes involved in energetics and fish heath could have implications for foraging activity and predator avoidance in wild fish at sensitive life stages.

Aquatic organisms are exposed to complex mixtures of pesticides in the environment, but traditional risk assessment approaches typically only consider individual compounds. In conjunction with exposure to pesticide mixtures, global climate change is anticipated to alter thermal regimes of waterways, leading to potential co-exposure of biota to elevated temperatures and contaminants. Furthermore, most studies utilize aqueous exposures, whereas the dietary route of exposure may be more important for fish owing to the hydrophobicity of many pesticides. Consequently, the current study aimed to determine the effects of elevated temperatures and dietary pesticide mixtures on swimming performance and lipid metabolism of juvenile Chinook salmon, Oncorhynchus tshawytscha. Fish were fed pesticide-dosed pellets at three concentrations and three temperatures (11, 14 and 17 °C) for 14 days and swimming performance (Umax) and expression of genes involved in lipid metabolism and energetics were assessed (ATP citrate lyase, fatty acid synthase, farnesoid x receptor and liver x receptor). The low-pesticide pellet treatment contained five pesticides, p,p’-DDE, bifenthrin, esfenvalerate, chlorpyrifos and fipronil at concentrations based on prey items collected from the Sacramento River (CA, USA) watershed, with the high-pesticide pellet treatment containing a six times higher dose. Temperature exacerbated effects of pesticide exposure on swimming performance, with significant reductions in Umax of 31 and 23% in the low and high-pesticide pellet groups relative to controls at 17 °C, but no significant differences in Umax among pesticide concentrations at 11 or 14 °C. At 14 °C there was a significant positive relationship between juvenile Chinook salmon pesticide body residues and expression of ATP citrate lyase and fatty acid synthase, but an inverse relationship and significant downregulation at 17 °C. These findings suggest that temperature may modulate effects of environmentally relevant pesticide exposure on salmon, and that pesticide-induced impairment of swimming performance may be exacerbated under future climate scenarios.

Recent studies demonstrated pyrethroid resistance associated with voltage-gated sodium channel mutations in populations of the epibenthic amphipod, Hyalella azteca. Resistant populations were able to tolerate and bioconcentrate pyrethroids at concentrations significantly higher than toxic levels for non-resistant populations. In conjunction with elevated bioconcentration potential, environmental alteration particularly as a result of global climate change is anticipated to significantly alter abiotic parameters including temperature and salinity. These changes are expected to influence uptake and biotransformation of contaminants. Thus, the aims of the current study were a) to examine the bioconcentration potential of permethrin in two pyrethroid-resistant clades of H. azteca and b) assess the influence of temperature and salinity changes on toxicokinetic parameters. Two pyrethroid-resistant clades of H. azteca were exposed to 14C-permethrin at three salinities (0.2, 1.0 and 6.0 practical salinity units (PSU)) and temperatures (18, 23 and 28 °C). Tests were conducted for up to 36 h and uptake, elimination and biotransformation rates were calculated. Both populations demonstrated bioconcentration factors (BCFs) between five and seven times greater than published data for non-resistant H. azteca, with significant differences between clades. Calculated BCF values were comparable to field populations of resistant H. azteca, emphasizing the potential for elevated pyrethroid bioconcentration in the natural environment and increased exposure for predators consuming pyrethroid-resistant aquatic invertebrates. Alterations to temperature and salinity had no statistically significant effect on uptake or parent compound half-life in either population, though biotransformation was elevated at higher temperatures in both populations. Salinity had a variable effect between the two populations, with lower BCF values at 1.0 PSU in clade D H. azteca and greater BCFs at 6.0 PSU in clade C H. azteca. This is the first study to demonstrate the potential for future climate scenarios to influence toxicokinetics in pyrethroid-resistant aquatic organisms.

The Sacramento River watershed, California, provides important rearing and migratory habitat for several species of conservation concern. Studies have suggested significant benefits for juvenile fish rearing in floodplain habitats of the watershed compared to the mainstem Sacramento River. However, the potential for contaminant exposure in each of these two habitats is poorly understood. Consequently, the present study aimed to determine the distribution and occurrence of bioavailable pesticides within two known salmon habitats using a suite of approaches including exhaustive chemical extraction, single-point Tenax extraction (SPTE) and ex situ passive sampling. Sediment samples were collected from sites within both habitats twice annually in 2019 and 2020, with inundation of the floodplain and high flows for both areas in 2019 and low flow conditions observed in 2020. Sediment characteristics including total organic carbon, black carbon and particle size distribution were determined to elucidate the influence of physical characteristics on pesticide distribution. Using exhaustive extractions, significantly greater sediment concentrations of organochlorines were observed in the floodplain compared to the Sacramento River in both years, with bioaccessible organochlorine concentrations also significantly greater in the floodplain (ANOVA, p < 0.05). Using both SPTEs and exhaustive extractions, significantly fewer pesticides were detected across both sites under low flow conditions as compared to high flow conditions (Poisson regression, p < 0.05). Sediment characteristics including percent fines and black carbon had significant positive relationships with total and bioaccessible pyrethroid and organochlorine concentrations. Fewer analytes were detected using low-density polyethylene (LDPE) passive samplers as compared to SPTEs, suggesting greater sensitivity of the Tenax technique for bioavailability assessments. These findings suggest that threatened juvenile fish populations rearing on the floodplain may have greater exposure to organochlorines than fish inhabiting adjacent riverine habitats, and that pesticide exposure of resident biota may be exacerbated during high-flow conditions.

The increased use of pyrethroid insecticides raises concern for exposure to non-target aquatic species, such as Chinook salmon (Oncorhynchus tshawytscha). Cypermethrin, a type II pyrethroid, is frequently detected in surface waters and sediments at concentrations that exceed levels that induce toxicity to several invertebrate and salmonid species. To better understand the effects of cypermethrin to salmonids following dietary exposure, juvenile Chinook salmon were dietarily exposed to a 0, 200, or 2000 ng/g cypermethrin diet for a duration of 7, 14, or 21 days and assessed for body burden residues, swimming performance, lipid content, and lipid homeostatic gene expression. The average cypermethrin concentrations in fish dietarily exposed to cypermethrin for 21 days were 155.4 and 952.1 ng cypermethrin/g lipid for the 200 and 2000 ng/g pellet treatments, respectively. Increased trends of fatty acid synthase (fasn, r2 = 0.10, p < 0.05) and ATP citrate lyase (acly, r2 = 0.21, p < 0.001) mRNA expression were found in the fish livers relative to increasing cypermethrin body burden residues, though no significant changes in the mRNA expression of farnesoid X receptor or liver X receptor were observed. Furthermore, Chinook salmon dietarily exposed to cypermethrin did not have a significantly altered burst swimming performance (Umax). These results support studies that have suggested Umax may not be a sensitive endpoint when assessing the effects of certain pesticide classes, such as pyrethroids, but that dysregulation of fasn and acly expression may alter lipid homeostasis and energy metabolism in the liver of fish dietarily exposed to cypermethrin.

Given extensive use of pesticides in agriculture, there is concern for unintended consequences to non-target species. The non-target freshwater amphipod, Hyalella azteca has been found to show resistance to the organophosphate (OP) pesticide, chlorpyrifos, resulting from an amino acid substitution in acetylcholinesterase (AChE), suggesting a selective pressure of unintended pesticide exposure. Since resistant organisms can survive in contaminated habitats, there is potential for them to accumulate higher concentrations of insecticides, increasing the risk for trophic transfer. In the present study, we estimated the uptake and elimination of chlorpyrifos in non-resistant US Lab, and resistant Ulatis Creek (ULC Resistant), H. azteca populations by conducting 24-h uptake and 48-h elimination toxicokinetic experiments with 14C-chlorpyrifos. Our results indicated that non-resistant H. azteca had a larger uptake clearance coefficient (1467 mL g(-1) h(-1)) than resistant animals (557 mL g(-1 )h(-1)). The half-life derived from the toxicokinetic models also estimated that steady state conditions were reached at 13.5 and 32.5 h for US Lab and ULC, respectively. Bioaccumulation was compared between nonresistant and resistant H. azteca by exposing animals to six different environmentally relevant concentrations for 28 h. Detection of chlorpyrifos in animal tissues indicated that resistant animals exposed to high concentrations of chlorpyrifos were capable of accumulating the insecticide up to 10-fold higher compared to non-resistant animals. Metabolite analysis from the 28-h concentration experiments showed that between 20 and 50 % parent compound was detected in H. azteca. These results imply that bioaccumulation potential can be more significant in chlorpyrifos resistant H. azteca and may be an essential factor in assessing the full impacts of toxicants on critical food webs, especially in the face of increasing pesticide and chemical runoff.

Chronic exposure to pyrethroid insecticides can result in strong selective pressures on non-target species in aquatic systems and drive the evolution of resistance and population-level changes. Characterizing the underlying mechanisms of resistance is essential to better understanding the potential consequences of contaminant-driven microevolution. The current study found that multiple mechanisms enhance the overall tolerance of Hyalella azteca to the pyrethroid permethrin. In H. azteca containing mutations in the voltage-gated sodium channel (VGSC), both adaptation and acclimation played a role in mitigating the adverse effects of pyrethroid exposures. Pyrethroid resistance is primarily attributed to the heritable mutation at a single locus of the VGSC, resulting in reduced target-site sensitivity. However, additional pyrethroid tolerance was conferred through enhanced enzyme-mediated detoxification. Cytochrome P450 monooxygenases (CYP450) and general esterases (GE) significantly contributed to the detoxification of permethrin in H. azteca. Over time, VGSC mutated H. azteca retained most of their pyrethroid resistance, though there was some increased sensitivity from parent to offspring when reared in the absence of pyrethroid exposure. Permethrin median lethal concentrations (LC50s) declined from 1809 ng/L in parent (P0) individuals to 1123 ng/L in the first filial (F1) generation, and this reduction in tolerance was likely related to alterations in acclimation mechanisms, rather than changes to target-site sensitivity. Enzyme bioassays indicated decreased CYP450 and GE activity from P0 to F1, whereas the VGSC mutation was retained. The permethrin LC50s in resistant H. azteca were still two orders-of-magnitude higher than non-resistant populations indicating that the largest proportion of resistance was maintained through the inherited VGSC mutation. Thus, the noted variation in tolerance in H. azteca is likely associated with inducible traits controlling enzyme pathways. A better understanding of the mechanistic and genomic basis of acclimation is necessary to more accurately predict the ecological and evolutionary consequences of contaminant-driven change in H. azteca.