Fish and Other Aquatics

Impacts of Pesticides on Fish

Fish can be directly or indirectly impacted by pesticides. Some long-term exposures cause abnormalities or mutations in developing fish larvae, while acute exposure can cause immediate fish die-offs. The liver, kidney, brain and gills of exposed fish are extremely vulnerable to chemical exposure.  Linking pesticides to be the cause of harm to fish can be difficult because they are highly mobile animals, and the effects may not show until much later in life.

  • A 2015 study showed that when fish larvae are exposed to pesticides through water contamination from runoff, they can develop swimming abnormalities as they grow, making them an easy target for prey and impacting their survival rate.
  • Wild Steelhead
    Wild Steelhead. Photo by USFWS - Pacific Region.
    In 2012, thousands of fish were found dead in Prince Edward Island, Canada, due to pesticide runoff from nearby agricultural fields after heavy rain. In 2013, dozens more dead fish washed ashore, again after heavy rains and possible pesticide exposure.
  • In 2012, the National Marine Fisheries Service (NMFS) drafted a biological opinion to the Environmental Protection Agency (EPA) concluding that three herbicides (oryzalin, pendimethalin, and trifluralin) pose a direct threat to approximately 50% of endangered Pacific salmon and Puget Sound steelhead species, and adversely impact their habitat.
  • Fish species are also sensitive to endocrine disruptors. In 2005, researchers in British Columbia, Canada, demonstrated that biological changes induced by sublethal exposure to pesticides include inhibition of important enzymes and growth delay.

[See More Scientific Studies Below]

Economic Cost

Fisheries are valuable resources that are enjoyed by millions of Americans. Fish provide food services for humans and other wildlife. They also provide benefits for citizens through direct financial gain or recreational enjoyment. For example, the seafood industry provides jobs for commercial fishers and retailers, while the other aquatic areas provide the opportunity for recreational activities such as sport fishing.  

One estimate for the economic cost of the impacts of pesticides on fish uses information provided by EPA’s fining of Coors Beer for river pollution ($10 per fish). With this information, it is assumed that the economic value of fish killed by pesticides each year is estimated to be $10-25 million. This is most likely a vast underestimate, as fish kills due to pesticides are hard to trace (see David Pimentel’s 2005 study for more information). A separate study has estimated that the entire value of recreational fishing is worth $27.9 billion annually. From this estimate, one can assume that as fish kills due to pesticides increase, there will be less available fish for recreational fishing. As the supply decreases, the cost will increase, leading to increased spending by citizens who partake in this activity.

Litigations & Lawsuits

In 2008, more than 13 organizations filed a legal petition demanding that EPA regulate novel nanomaterial pesticides. EPA remained silent, prompting Beyond Pesticides and other organizations to sue the EPA in 2014. Silver nanoparticles are often impregnated into a wide variety of consumer products. These nanoparticles are released when washed, where they exit down the drain and enter into the environment. These products have been found to be toxic and potentially lethal to fish. In early 2015, the EPA finally responded and agreed to regulate these novel nanomaterials as pesticides.

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Juvenile Coho Salmon
Juvenile Coho Salmon. Photo by USFWS - Pacific Region

In 2010, Earthjustice, representing the Pacific Coast Federation of Fishermen’s Associations, the Northwest Coalition for Alternatives to Pesticides, and Defenders of Wildlife, filed litigation that called for EPA adoption of reasonable fish protections from insecticides. Following the Lawsuit, EPA restored stream buffers to protect salmon from pesticides. The buffers apply to salmon habitat throughout California, Oregon, and Washington to prohibit aerial spraying of broad-spectrum pesticides diazinonchlorpyrifosmalathion, carbaryl, and methomyl within 300 feet of salmon habitat and prohibit ground-based applications within 60 feet.

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Scientific Studies: 

  • Behavioral Effects of the Mixture and the Single Compounds Carbendazim, Fipronil, and Sulfentrazone on Zebrafish (Danio rerio) Larvae
    Pesticides are often detected in freshwater, but their impact on the aquatic environment is commonly studied based on single compounds, underestimating the potential additive effects of these mixtures. Even at low concentrations, pesticides can negatively affect organisms, altering important behaviors that can have repercussions at the population level. This study used a multi-behavioral approach to evaluate the effects of zebrafish larvae exposure to carbendazim (C), fipronil (F), and sulfentrazone (S), individually and mixed. Five behavioral tests, thigmotaxis, touch sensitivity, optomotor response, bouncing ball test, and larval exploratory behavior, were performed to assess potential effects on anxiety, fear, and spatial and social interaction. Significant changes were observed in the performance of larvae exposed to all compounds and their mixtures. Among the single pesticides, exposure to S produced the most behavioral alterations, followed by F and C, respectively. A synergistic effect between the compounds was observed in the C + F group, which showed more behavioral effects than the groups exposed to pesticides individually. The use of behavioral tests to evaluate pesticide mixtures is important to standardize methods and associate behavioral changes with ecologically relevant events, thus creating a more realistic scenario for investigating the potential environmental impacts of these compounds.
    [Gomes, S. da S. et al. (2024) Behavioral effects of the mixture and the single compounds carbendazim, fipronil, and sulfentrazone on zebrafish (danio rerio) larvae, Biomedicines. Available at: https://www.mdpi.com/2227-9059/12/6/1176.]
  • Identification of DDT+ in Deep Ocean Sediment and Biota in the Southern California Bight
    The recent rediscovery of offshore DDT waste dumping in the Southern California Bight (SCB) has led to questions about the extent and type of pollution in deep ocean environments. We used a nontargeted analysis to identify halogenated organic compounds (HOCs), including DDT+, in sediment in the San Pedro Basin. Additionally, we examined the chemical profiles of deep ocean biota inhabiting the SCB to assess the bioavailability of DDT+ and HOCs to the deep ocean food web. We detected 49 HOCs across all samples, including 15 DDT+ compounds in the sediment and 10 DDT+ compounds in the biota. Compounds included tris(4-chlorophenyl)methane (TCPM) and its isomers and three unknown DDT-related compounds previously identified in marine mammals. No clear trends were identified regarding DDT+ distribution in sediments. High DDT+ body burdens were found in biota irrespective of collection location, indicating widespread DDT+ contamination in the deep ocean of the SCB. TCPMs were detected in all biota samples except a single surface species, indicating that deep ocean sediment may be a source of DDT+ to the marine food web. This study demonstrates that the analysis of the larger suite of DDT+ is critical to trace deep ocean pollution of DDT in the SCB.
    [Stack, M.E. et al. (2024) ‘Identification of DDT+ in deep ocean sediment and biota in the Southern California bight’, Environmental Science & Technology Letters, 11(5), pp. 479–484. Available at: https://pubs.acs.org/doi/full/10.1021/acs.estlett.4c00115.]
  • Bioaccumulation of pesticides in fish resulting toxicities in humans through food chain and forensic aspects.
    A crucial component for agricultural productivity is pesticide application. Increased usage of pesticides has significantly increased agricultural output, reduced grain losses in storage, and overall enhanced human wellbeing. Globally, every year approximately 3 billion kg of pesticides are used which budgets around 40 billion USD. Pesticide use can leave behind unwanted residues that can contaminate food, the environment, and living tissues. They are known to spread from agricultural regions that have been treated into the wider environment, where they affect non-target creatures. All tiers of biological organisms, directly impacted by this exposure. Pesticides at sub-lethal levels alter every aspect of a fish's physiology, including histology, haematology, defence mechanisms, and behaviour. The same topic of pesticide toxicology is the emphasis of this article, which also addresses some important induced chronic toxicological effects of pesticides in fish and the extent of their bioaccumulation in fish tissues. The data represents the largest bodies of water, such as rivers and lakes, that have been contaminated by pesticides, notably due to pesticide drift. It has been discussed how readily pesticides are absorbed into fish bodies and how this enters the food chain inducing harmful impacts on human health when consumed.
    [Ray, S. and Shaju, S.T., 2023. Journal of Survey in Fisheries Sciences, pp.2223-2242.]
  • Characterization of Sulfoxaflor and Its Metabolites on Survival, Growth, Reproduction, Biochemical Markers, and Transcription of Genes of Daphnia magna
    Sulfoxaflor is a promising neonicotinoid. However, the negative implications of sulfoxaflor on nontarget aquatic organisms have been rarely studied. In this study, the risks of sulfoxaflor and its main metabolites X11719474 and X11519540 on Daphnia magna were characterized, including acute toxicity, reproduction, swimming behavior, biochemical markers, and gene transcription. Acute toxicity measurements indicated that X11719474 and X11519540 have high toxicity than the parent compound sulfoxaflor. Chronic exposure reduced reproduction and delayed the birth of the firstborn D. magna. Swimming behavior monitoring showed that exposure to three compounds stimulated swimming behavior. The induction of catalase, superoxide dismutase, and acetylcholinesterase activities was observed with oxidative stress, whereas malondialdehyde content was remarkably increased with exposure to sulfoxaflor, X11719474, and X11519540. Moreover, transcriptomics profiles showed that sulfoxaflor, X11719474, and X11519540 induced KEGG pathways related to cellular processes, organismal systems, and metabolisms. The findings present valuable insights into the prospective hazards of these pesticides and emphasize the critical importance of conducting a systematic evaluation of combining antecedents and their metabolites.
    [Yuan, T., Jiao, H., Ai, L., Chen, Y., Hu, D. and Lu, P., 2023. Journal of Agricultural and Food Chemistry, 71(16), pp.6424-6433.]
  • Haematological and biochemical toxicity in freshwater fish Clarias gariepinus and Oreochromis niloticus following pulse exposure to atrazine, mancozeb, chlorpyrifos, lambda-cyhalothrin, and their combination
    Transient exposures to high or low concentrations of a single or mixture of pesticides are common in aquatic organisms. Routine toxicity tests disregard transient exposures and the influence of time when examining the toxicity of contaminants. This study investigated the haematological and biochemical responses of juvenile C. gariepinus and O. niloticus to pesticide pulse exposure using three exposure patterns. The patterns include 4-hour pulse exposure to a high pesticide concentration, then 28 days of depuration, continuous exposure to a low pesticide concentration for 28 days, and 4-hour pulse exposure to a high concentration followed by continuous exposure to a low pesticide concentration for 28 days. On days 1, 14, and 28, fish samples were collected for haematological and biochemical analysis. Results showed that red blood cell count, packed cell volume, haemoglobin, platelet count, total protein, and sodium ion decreased, while white blood cell count, total cholesterol, bilirubin, urea, and potassium ion increased in both fish species after pulse, continuous and pulse & continuous exposure to the pesticides (p < 0.05). However, pulse exposure to the pesticides did not significantly affect alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase activity, and creatinine levels. The changes in these biomarkers indicate that 4-hour pulse exposure to high concentration was as hazardous as 24-hour continuous exposure to low pesticide concentration (p > 0.05). The toxic effects of pulse exposure were largely reversible by day 14. Using C. gariepinus and O. niloticus, this study shows that brief exposure to high pesticide pesticides was as hazardous as continuous pesticide exposure.
    [Kanu, K.C., Okoboshi, A.C. and Otitoloju, A.A., 2023. Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology, 270, p.109643.]
  • Pesticides and Parabens Contaminating Aquatic Environment: Acute and Sub-Chronic Toxicity towards Early-Life Stages of Freshwater Fish and Amphibians
    Pesticides and personal care products are two very important groups of contaminants posing a threat to the aquatic environment and the organisms living in it.. Therefore, this study aimed to describe the effects of widely used pesticides and parabens on aquatic non-target biota such as fish (using model organisms Danio rerio and Cyprinus carpio) and amphibians (using model organism Xenopus laevis) using a wide range of endpoints. The first part of the experiment was focused on the embryonal toxicity of three widely used pesticides (metazachlor, prochloraz, and 4-chloro-2-methyl phenoxy acetic acid) and three parabens (methylparaben, propylparaben, and butylparaben) with D. rerio, C. carpio, and X. laevis embryos. An emphasis was placed on using mostly sub-lethal concentrations that are partially relevant to the environmental concentrations of the substances studied. In the second part of the study, an embryo-larval toxicity test with C. carpio was carried out with prochloraz using concentrations 0.1, 1, 10, 100, and 1000 µg/L. The results of both parts of the study show that even the low, environmentally relevant concentrations of the chemicals tested are often able to affect the expression of genes that play either a prominent role in detoxification and sex hormone production or indicate cell stress or, in case of prochloraz, to induce genotoxicity.
    [Medkova, D., Hollerova, A., Riesova, B., Blahova, J., Hodkovicova, N., Marsalek, P., Doubkova, V., Weiserova, Z., Mares, J., Faldyna, M. and Tichy, F., 2023. Toxics, 11(4), p.333.]
  • Antagonistic effects and mechanisms of carbendazim and chlorpyrifos on the neurobehavior of larval zebrafish
    Residues from multiple pesticides are frequently detected on vegetables, which may produce combined toxicity not predicted by individual toxicity data. As these combined effects present additional dangers to food safety, we have compared individual to combined effects for a variety of pesticides. Carbendazim and chlorpyrifos are the two most commonly detected pesticides in vegetables, and previous studies reported that combined exposure results in synergistic developmental toxicity to zebrafish embryos. In this study, individual and combined effects on zebrafish motor activity were examined following individual and combined exposure to assess nervous system toxicity. Further, transcriptomics methods were used to identify potential molecular mechanisms for individual and combined toxicity. Carbendazim alone induced a disorganized swim pattern characterized by increased angular velocity, turn angle, meander, and acceleration during light-dark transition, while chlorpyrifos alone reduced average swim speed and light-dark acceleration. Combined treatment significantly reduced average swim velocity and total distance traveled. Combination indices indicated strong antagonism between compounds for average speed and light-dark acceleration. Transcriptomics (RNA-seq) showed that carbendazim significantly altered the expression of genes involved in antigen processing and presentation, apoptosis, autophagy, and metabolism, including ctslb, cyp7a1, hsp70l, and ugt1a1. Alternatively, chlorpyrifos significantly altered genes involved in various nervous system-related pathways, including glutamatergic, GABAergic, dopaminergic, and calcium signaling. Protein-protein interaction (PPI) network analysis suggested that chlorpyrifos significantly downregulated genes related to light transduction, resulting in decreased sensitivity to light-dark transitions, while antagonism mainly reflected divergent effects on phototransduction and retinol metabolism. Carbendazim had no significant effects on vision-related genes such as gnat1 and gngt1, while chlorpyrifos downregulated expression, an effect reversed by the combination. Comprehensive toxicity analyses must include joint effects of co-applied pesticides for enhanced food safety.
    [Zhang, W. et al. (2022) Antagonistic effects and mechanisms of Carbendazim and chlorpyrifos on the neurobehavior of larval zebrafish, Chemosphere. Available at: https://www.sciencedirect.com/science/article/abs/pii/S004565352200011X. ]
  • Emerging Midges Transport Pesticides from Aquatic to Terrestrial Ecosystems: Importance of Compound- and Organism-Specific Parameters
    Emerging aquatic insects have the potential to retain aquatic contaminants after metamorphosis, potentially transporting them into adjacent terrestrial food webs. It is unknown whether this transfer is also relevant for current-use pesticides. We exposed larvae of the nonbiting midge, Chironomus riparius, to a sublethal pulse of a mixture of nine moderately polar fungicides and herbicides (logKow 2.5–4.7) at three field relevant treatment levels (1.2–2.5, 17.5–35.0, or 50.0–100.0 μg/L). We then assessed the pesticide bioaccumulation and bioamplification over the full aquatic–terrestrial life cycle of both sexes including the egg laying of adult females. By applying sensitive LC–MS/MS analysis to small sample volumes (∼5 mg, dry weight), we detected all pesticides in larvae from all treatment levels (2.8–1019 ng/g), five of the pesticides in the adults from the lowest treatment level and eight in the higher treatment levels (1.5–3615 ng/g). Retention of the pesticides through metamorphosis was not predictable based solely on pesticide lipophilicity. Sex-specific differences in adult insect pesticide concentrations were significant for five of the pesticides, with greater concentrations in females for four of them. Over the duration of the adults’ lifespan, pesticide concentrations generally decreased in females while persisting in males. Our results suggest that a low to moderate daily dietary exposure to these pesticides may be possible for tree swallow nestlings and insectivorous bats.
    [Roodt, A.P., Röder, N., Pietz, S., Kolbenschlag, S., Manfrin, A., Schwenk, K., Bundschuh, M. and Schulz, R., Environmental Science & Technology.]
  • Health risks of sulfentrazone exposure during zebrafish embryo-larvae development at environmental concentration
    Knowledge about the negative effects and mechanism of sulfentrazone (SUL) on aquatic early life stages is still limited. Here we investigated the lethal and sub-lethal effects of SUL during zebrafish embryo-larvae development. Results demonstrated that the 96 h and 120 h-LC50 of SUL to embryonic zebrafish was 2.02 mg/L, and the 30 d-LC50 was 0.899 mg/L after embryos exposed to SUL for 30 d. High concentrations of SUL delayed yolk sac absorption, disordered the hatching and heart rate during zebrafish embryonic stage, while 0.0100–0.100 mg/L SUL had no phenotypic changes on embryonic development, but decreased the body weight of larvae after 30 d exposure. RNA-seq identified 321, 394 and 727 differentially expressed genes in larvae after embryos exposed to 0.0100 mg/L, 0.0400 mg/L and 0.400 mg/L SUL for 30 d, found that the transcriptional profiles involved in heart development and endocrine disruption were simultaneously influenced by different concentrations of SUL, such as adrenergic signaling in cardiomyocytes, cardiac muscle contraction, cell adhesion molecules and steroid biosynthesis. Biochemical analysis showed that SUL increased the levels of E2, T3 and TSH, induced the activities of mitochondrial complex IV, cytochrome c oxidase, Ca2+-ATPase, total Na+K+-ATPase and Ca2+Mg2+-ATPase, and decreased ATP formation after embryos exposed to SUL for 5 d and 30 d. Further comprehensive analysis demonstrated that SUL caused more significantly alteration on the transcript, level or activity of the key elements involved in heart development and endocrine disruption after 30 d exposure, indicated long-term SUL exposure might cause more negative effects on zebrafish at doses below the presumed no-observed-adverse-effect level during early life development. The results inferred the environmental concentration of SUL might cause potential cardiac and endocrine health risk in zebrafish later life stages, also facilitated a better understanding of the sub-lethal effects and molecular mechanism of SUL on aquatic organism.
    [Jiang, J. et al. (2022) Health risks of sulfentrazone exposure during zebrafish embryo-larvae development at environmental concentration, Chemosphere. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0045653521031040. ]
    The objective of this project was to assess the potential health risk to open-water swimmers in the vicinity of fish farms in Scotland in relation to medicinal treatments applied for the control of sea lice on salmon. The three substances assessed were azamethiphos, deltamethrin and hydrogen peroxide; these substances forming the active ingredients of products licensed for medicinal use on fish farms. The risk characterisation ratios for azamethiphos and deltamethrin were determined to be 0.8 and 0.0007, respectively. As these values were both below 1, it can be concluded that the concentrations of azamethiphos and deltamethrin used to treat fish are below the concentrations predicted by SWIMODEL to present no hazard to swimmers (on a worst-case basis). This demonstrates that the concentrations used to treat fish are safe for open-water swimmers, even before dilution and dispersion occurs in open waters. However, for hydrogen
    peroxide, the risk characterisation ratio was determined to be 27.7. As this value is above 1, this indicates a risk associated with the concentrations of hydrogen peroxide used in the fish treatment baths. Therefore, characterisation of dilution and dispersion factors are likely to be required to be taken into account to demonstrate that discharges of hydrogen peroxide are safe for open-water swimmers
    [WCA Environment Ltd.]
  • Bioaccumulation, Metabolism and the Toxic Effects of Chlorfenapyr in Zebrafish (Danio rerio)
    Chlorfenapyr is widely used as an insecticide/miticide. Tralopyril, the active metabolite of chlorfenapyr, is used as an antifouling biocide in antifouling systems, and negatively affects aquatic environments. However, it is unclear whether tralopyril is a metabolite of chlorfenapyr in aquatic vertebrates, and there is little data on the bioaccumulation and toxicity of chlorfenapyr to aquatic vertebrates. In this study, the bioaccumulation and elimination of chlorfenapyr in zebrafish were assessed, and tralopyril, the active metabolite of chlorfenapyr, was determined. The effects of chronic exposure to chlorfenapyr on zebrafish liver and brain oxidative damage, apoptosis, immune response, and metabolome were investigated. These results showed that chlorfenapyr has a high bioaccumulation in zebrafish, with bioaccumulation factors of 864.6 and 1321.9 after exposure to 1.0 and 10 μg/L chlorfenapyr for 21 days, respectively. Chlorfenapyr at these concentrations also rapidly accumulated in zebrafish, reaching 615.5 and 10336 μg/kg on the second and third days of exposure, respectively. Chlorfenapyr was degraded to tralopyril in zebrafish; therefore, both chlorfenapyr and tralopyril should be considered when evaluating the risk of chlorfenapyr to aquatic organisms. In addition, chronic exposure caused oxidative damage, apoptosis, and immune disorders in zebrafish liver. Chronic exposure also altered the levels of endogenous metabolites in liver and brain. After 9 days of depuration, some indicators of oxidative damage, apoptosis, and immunity returned to normal levels, but the concentration of endogenous metabolites in zebrafish liver was still altered. Overall, these results provide useful information for evaluating the toxicity and environmental fate of chlorfenapyr in aquatic vertebrates.
    [Chen, X. et al. (2021) Bioaccumulation, Metabolism and the Toxic Effects of Chlorfenapyr in Zebrafish (Danio rerio), J. Agric. Food Chem. Available at: https://pubs.acs.org/doi/10.1021/acs.jafc.1c02301. ]
  • Effects of Pesticides on Biodiversity and Climate Change
    Pesticides are the biological pollutants, which are being used by the man to kill the pests for increasing the yield of many crops and insect vectors to control the spread of disease. The tremendous use of pesticides has caused severe health hazards to organisms including human beings due to climate change. Excessive use of pesticides may lead to the destruction of biodiversity. Many birds, aquatic organisms and animals are under the threat of harmful pesticides for their survival. The pesticides effects can be lessen by organizing awareness program among the farmers, special training to them regarding consequences of pesticides, their screening and monitoring methods.
    [Chaudhary, V., Arya, S. and Singh, P. (2021) Effects of Pesticides on Biodiversity and Climate Change, International Journal on Environmental Sciences. Available at: https://doi.org/10.53390/ijes.v12i2.1. ]
  • Experimental evidence for neonicotinoid driven decline in aquatic emerging insects
    Survey data show a large-scale decline in insects. This global decline is often linked to human actions in intensive agricultural areas. To investigate whether this decline has a causal relationship with neonicotinoid insecticides, we performed an outdoor experiment with representative surface water concentrations of the neonicotinoid thiacloprid. We exposed naturally formed aquatic communities to increasing neonicotinoid concentrations and monitored insect emergence during a 3-mo period. We show that increasing neonicotinoid concentrations strongly decreased the abundance and biomass of five major insect orders that together comprised >99% of the 55,574 collected insects as well as the diversity of the most species-rich freshwater family, thus showing a causal relation between insect decline and neonicotinoids.
    [Barmentlo, S.H., Schrama, M., De Snoo, G.R., Van Bodegom, P.M., van Nieuwenhuijzen, A. and Vijver, M.G. Proceedings of the National Academy of Sciences, 118(44).]
  • Neurotoxicity of fipronil affects sensory and motor systems in zebrafish
    Fipronil is a phenylpyrazole insecticide that may selectively inhibit gamma-aminobutyric acid receptors in insects. Although fipronil is the most widely used insecticide in aquatic environments, few studies have evaluated its neurotoxicity for the sensory and motor systems of aquatic vertebrates. We assessed the effects of acute fipronil exposure on the survival rate, number of hair cells of lateral lines, and neurotoxicity for zebrafish (Danio rerio). In addition, heat maps and the speed and distance of the swimming trajectory were compared between zebrafish subjected to the sham and fipronil treatments. Western blotting and immunohistochemistry were conducted separately to compare expressions of oxidative stress, inflammation, apoptosis, and neurotoxicity related proteins in the brain tissue between adult zebrafish with sham and fipronil treatments. Our results indicated that the survival rates and the speed and distance of the swimming trajectory significantly decreased for adult zebrafish exposed to fipronil. The results also suggested that the number of hair cells of lateral lines significantly reduced for zebrafish embryos exposed to fipronil. In histopathology and Western blotting tests, substantial oxidative stress, inflammation, and apoptosis were observed in the brain tissue of adult zebrafish exposed to fipronil. Our results revealed that fipronil toxicity may impair sensory and motor systems in zebrafish because of damage to lateral hair cells and brain tissue through oxidative stress, inflammation, and apoptosis, which in turn result in a significantly reduced survival rate and impaired locomotion. The behavioral responses of zebrafish exposed to fipronil toxicity should be determined for better understanding the reliability of behavioral biomarkers in the risk assessment of environmental toxicology.
    [Wu, C.H. et al. (2021) Neurotoxicity of fipronil affects sensory and motor systems in zebrafish, Pesticide Biochemistry and Physiology. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0048357521001279. ]
  • The silence of the clams: Forestry registered pesticides as multiple stressors on soft-shell clams
    Contaminants are ubiquitous in the environment, often reaching aquatic systems. Combinations of forestry use pesticides have been detected in both water and aquatic organism tissue samples in coastal systems. Yet, most toxicological studies focus on the effects of these pesticides individually, at high doses, and over acute time periods, which, while key for establishing toxicity and safe limits, are rarely environmentally realistic. We examined chronic (90 days) exposure by the soft-shell clam, Mya arenaria, to environmentally relevant concentrations of four pesticides registered for use in forestry (atrazine, 5 μg/L; hexazinone, 0.3 μg/L; indaziflam, 5 μg/L; and bifenthrin, 1.5 μg/g organic carbon (OC)). Pesticides were tested individually and in combination, except bifenthrin, which was tested only in combination with the other three. We measured shell growth and condition index every 30 days, as well as feeding rates, mortality, and chemical concentrations in tissue from a subset of clams at the end of the experiment to measure contaminant uptake. Indaziflam caused a high mortality rate (max. 36%), followed by atrazine (max. 27%), both individually as well as in combination with other pesticides. Additionally, indaziflam concentrations in tissue (61.70–152.56 ng/g) were higher than those of atrazine (26.48–48.56 ng/g), despite equal dosing concentrations, indicating higher tissue accumulation. Furthermore, clams exposed to indaziflam and hexazinone experienced reduced condition index and clearance rates individually and in combination with other compounds; however, the two combined did not result in significant mortality. These two compounds, even at environmentally relevant concentrations, affected a non-target organism and, in the case of the herbicide indaziflam, accumulated in clam tissue and appeared more toxic than other tested pesticides. These findings underscore the need for more comprehensive studies combining multiple compounds at relevant concentrations to understand their impacts on aquatic ecosystems.
    [Tissot, A.G., Granek, E.F., Thompson, A.W., Hladik, M.L., Moran, P.W. and Scully-Engelmeyer, K. Science of The Total Environment, p.152053.]
  • Widespread agrochemicals differentially affect zooplankton biomass and community structure
    Anthropogenic environmental change is causing habitat deterioration at unprecedented rates in freshwater ecosystems. Despite increasing more rapidly than many other agents of global change, synthetic chemical pollution—including agrochemicals such as pesticides—has received relatively little attention in freshwater community and ecosystem ecology. Determining the combined effects of multiple agrochemicals on complex biological systems remains a major challenge, requiring a cross-field integration of ecology and ecotoxicology. Using a large-scale array of experimental ponds, we investigated the response of zooplankton community properties (biomass, composition, and diversity metrics) to the individual and joint presence of three globally widespread agrochemicals: the herbicide glyphosate, the neonicotinoid insecticide imidacloprid, and nutrient fertilizers. We tracked temporal variation in zooplankton biomass and community structure along single and combined pesticide gradients (each spanning eight levels), under low (mesotrophic) and high (eutrophic) nutrient-enriched conditions, and quantified (1) response threshold concentrations, (2) agrochemical interactions, and (3) community resistance and recovery. We found that the biomass of major zooplankton groups differed in their sensitivity to pesticides: ≥0.3 mg/L glyphosate elicited long-lasting declines in rotifer communities, both pesticides impaired copepods (≥3 µg/L imidacloprid and ≥5.5 mg/L glyphosate), whereas some cladocerans were highly tolerant to pesticide contamination. Strong interactive effects of pesticides were only recorded in ponds treated with the combination of the highest doses. Overall, glyphosate was the most influential driver of aggregate community properties of zooplankton, with biomass and community structure responding rapidly but recovering unequally over time. Total community biomass showed little resistance when first exposed to glyphosate, but rapidly recovered and even increased with glyphosate concentration over time; in contrast, taxon richness decreased in more contaminated ponds but failed to recover. Our results indicate that the biomass of tolerant taxa compensated for the loss of sensitive species after the first exposure, conferring greater community resistance upon a subsequent contamination event; a case of pollution-induced community tolerance in freshwater animals. These findings suggest that zooplankton biomass may be more resilient to agrochemical pollution than community structure; yet all community properties measured in this study were affected at glyphosate concentrations below common water quality guidelines in North America.
    [Hébert, M.P., Fugère, V., Beisner, B.E., Barbosa da Costa, N., Barrett, R.D., Bell, G., Shapiro, B.J., Yargeau, V., Gonzalez, A. and Fussmann, G.F. Ecological Applications, 31(7), p.e02423.]
  • Anthropogenic Contaminants and Histopathological Findings in Stranded Cetaceans in the Southeastern United States, 2012–2018
    Anthropogenic contaminants in the marine environment often biodegrade slowly, bioaccumulate in organisms, and can have deleterious effects on wildlife immunity, health, reproduction, and development. In this study, we evaluated tissue toxicant concentrations and pathology data from 83 odontocetes that stranded in the southeastern United States during 2012–2018. Mass spectrometry was used to analyze blubber samples for five organic toxicants (atrazine, bisphenol-A, diethyl phthalates, nonylphenol monoethoxylate [NPE], triclosan), and liver samples were analyzed for five non-essential elements (arsenic, cadmium, lead, mercury, thallium), six essential elements (cobalt, copper, manganese, iron, selenium, zinc) and one toxicant mixture class (Aroclor1268). Resultant data considerably improve upon the existing knowledge base regarding toxicant concentrations in stranded odontocetes. Toxicant and element concentrations varied based on animal demographic factors including species, sex, age, and location. Samples from bottlenose dolphins had significantly higher average concentrations of lead, manganese, mercury, selenium, thallium, and zinc, and lower average concentrations of NPE, arsenic, cadmium, cobalt, and iron than samples from pygmy sperm whales. In adult female bottlenose dolphins, average arsenic concentrations were significantly higher and iron concentrations were significantly lower than in adult males. Adult bottlenose dolphins had significantly higher average concentrations of lead, mercury, and selenium, and significantly lower average manganese concentrations compared to juveniles. Dolphins that stranded in Florida had significantly higher average concentrations of lead, mercury, and selenium, and lower concentrations of iron than dolphins that stranded in North Carolina. Histopathological data are presented for 72 animals, including microscopic evidence of Campula spp. and Sarcocystis spp. infections, and results of Morbillivirus and Brucella spp. molecular diagnostic testing. Sublethal cellular changes related to toxicant exposure in free-ranging odontocetes may lead to health declines and, in combination with other factors, may contribute to stranding.
    [Page-Karjian, A., Lo, C.F., Ritchie, B., Harms, C.A., Rotstein, D.S., Han, S., Hassan, S.M., Lehner, A.F., Buchweitz, J.P., Thayer, V.G. and Sullivan, J.M., 2020. Frontiers in Marine Science, 7, p.630.]
  • Anthropogenic stressors impact fish sensory development and survival via thyroid disruption
    Larval metamorphosis and recruitment represent critical life-history transitions for most teleost fishes. While the detrimental effects of anthropogenic stressors on the behavior and survival of recruiting fishes are well-documented, the physiological mechanisms that underpin these patterns remain unclear. Here, we use pharmacological treatments to highlight the role that thyroid hormones (TH) play in sensory development and determining anti-predator responses in metamorphosing convict surgeonfish, Acanthurus triostegus. We then show that high doses of a physical stressor (increased temperature of +3 °C) and a chemical stressor (the pesticide chlorpyrifos at 30 µg L−1) induced similar defects by decreasing fish TH levels and affecting their sensory development. Stressor-exposed fish experienced higher predation; however, their ability to avoid predation improved when they received supplemental TH. Our results highlight that two different anthropogenic stressors can affect critical developmental and ecological transitions via the same physiological pathway. This finding provides a unifying mechanism to explain past results and underlines the profound threat anthropogenic stressors pose to fish communities.
    [Besson, M., Feeney, W.E., Moniz, I., François, L., Brooker, R.M., Holzer, G., Metian, M., Roux, N., Laudet, V. and Lecchini, D., 2020. Nature communications, 11(1), pp.1-10.]
  • Cascading effects of insecticides and road salt on wetland communities
    Novel stressors introduced by human activities increasingly threaten freshwater ecosystems. The annual application of more than 2.3 billion kg of pesticide active ingredient and 22 billion kg of road salt has led to the contamination of temperate waterways. While pesticides and road salt are known to cause direct and indirect effects in aquatic communities, their possible interactive effects remain widely unknown. Using outdoor mesocosms, we created wetland communities consisting of zooplankton, phytoplankton, periphyton, and leopard frog (Rana pipiens) tadpoles. We evaluated the toxic effects of six broad-spectrum insecticides from three families (neonicotinoids: thiamethoxam, imidacloprid; organophosphates: chlorpyrifos, malathion; pyrethroids: cypermethrin, permethrin), as well as the potentially interactive effects of four of these insecticides with three concentrations of road salt (NaCl; 44, 160, 1600 Cl- mg/L). Organophosphate exposure decreased zooplankton abundance, elevated phytoplankton biomass, and reduced tadpole mass whereas exposure to neonicotinoids and pyrethroids decreased zooplankton abundance but had no significant effect on phytoplankton abundance or tadpole mass. While organophosphates decreased zooplankton abundance at all salt concentrations, effects on phytoplankton abundance and tadpole mass were dependent upon salt concentration. In contrast, while pyrethroids had no effects in the absence of salt, they decreased zooplankton and phytoplankton density under increased salt concentrations. Our results highlight the importance of multiple-stressor research under natural conditions. As human activities continue to imperil freshwater systems, it is vital to move beyond single-stressor experiments that exclude potentially interactive effects of chemical contaminants.
    [Lewis, J.L., Agostini, G., Jones, D.K. and Relyea, R.A., 2020. Environmental Pollution, p.116006.]
  • Early Life Exposure to Environmentally Relevant Levels of Endocrine Disruptors Drive Multigenerational and Transgenerational Epigenetic Changes in a Fish Model
    The inland silverside, Menidia beryllina, is a euryhaline fish and a model organism in ecotoxicology. We previously showed that exposure to picomolar (ng/L) levels of endocrine disrupting chemicals (EDCs) can cause a variety of effects in M. beryllina, from changes in gene expression to phenotypic alterations. Here we explore the potential for early life exposure to EDCs to modify the epigenome in silversides, with a focus on multi- and transgenerational effects. EDCs included contaminants of emerging concern (the pyrethroid insecticide bifenthrin and the synthetic progestin levonorgestrel), as well as a commonly detected synthetic estrogen (ethinylestradiol), and a synthetic androgen (trenbolone) at exposure levels ranging from 3 to 10 ng/L. In a multigenerational experiment, we exposed parental silversides to EDCs from fertilization until 21 days post hatch (dph). Then we assessed DNA methylation patterns for three generations (F0, F1, and F2) in whole body larval fish using reduced representation bisulfite sequencing (RRBS). We found significant (α = 0.05) differences in promoter and/or gene body methylation in treatment fish relative to controls for all EDCs and all generations indicating that both multigenerational (F1) and transgenerational (F2) effects that were caused by strict inheritance of DNA methylation alterations and the dysregulation of epigenetic control mechanisms. Using gene ontology and pathway analyses, we found enrichment in biological processes and pathways representative of growth and development, immune function, reproduction, pigmentation, epigenetic regulation, stress response and repair (including pathways important in carcinogenesis). Further, we found that a subset of potentially EDC responsive genes (EDCRGs) were differentially methylated across all treatments and generations and included hormone receptors, genes involved in steroidogenesis, prostaglandin synthesis, sexual development, DNA methylation, protein metabolism and synthesis, cell signaling, and neurodevelopment. The analysis of EDCRGs provided additional evidence that differential methylation is inherited by the offspring of EDC-treated animals, sometimes in the F2 generation that was never exposed. These findings show that low, environmentally relevant levels of EDCs can cause altered methylation in genes that are functionally relevant to impaired phenotypes documented in EDC-exposed animals and that EDC exposure has the potential to affect epigenetic regulation in future generations of fish that have never been exposed.
    [Major, K.M., DeCourten, B.M., Li, J., Britton, M., Settles, M.L., Mehinto, A.C., Connon, R.E. and Brander, S.M., 2020. Frontiers in Marine Science, 7, p.471.]
  • The Combined Algae Test for the Evaluation of Mixture Toxicity in Environmental Samples
    The combined algae test is a 96-well plate-based algal toxicity assay with the green algae Raphidocelis subcapitata that combines inhibition of 24-h population growth rate with inhibition of photosynthesis detected after 2 and 24 h with pulse-amplitude modulated (PAM) fluorometry using a Maxi-Imaging PAM. The combined algae test has been in use for more than a decade but has had limitations due to incompatibilities of the measurements of the 2 biological endpoints on the same microtiter plates. These limitations could be overcome by increasing growth rates and doubling times on black, clear-bottom 96-well plates by application of dichromatic red/blue light-emitting diode illumination. Different robotic dosing approaches and additional data evaluation methods helped to further expand the applicability domain of the assay. The combined algae test differentiates between nonspecifically acting compounds and photosynthesis inhibitors, such as photosystem II (PSII) herbicides. The PSII herbicides acted immediately on photosynthesis and showed growth rate inhibition at higher concentrations. If growth was a similar or more sensitive endpoint than photosynthesis inhibition, this was an indication that the tested chemical acted nonspecifically or that a mixture or a water sample was dominated by chemicals other than PSII herbicides acting on algal growth. We fingerprinted the effects of 45 chemicals on photosynthesis inhibition and growth rate and related the effects of the single compounds to designed mixtures of these chemicals detected in water samples and to the effects directly measured in water samples. Most of the observed effects in the water samples could be explained by known photosystem II inhibitors such as triazines and phenylurea herbicides. The improved setup of the combined algae test gave results consistent with those of the previous method but has lower costs, higher throughput, and higher precision.
    [Glauch, L. and Escher, B.I., 2020. Environmental Toxicology and Chemistry, 39(12), pp.2496-2508.]
  • Occurrence and Ecological Impacts of Microplastics in Soil Systems: A Review
    Microplastics, as a group of emerging contaminants, are receiving growing attention. During the last decade, their occurrence and toxicity in aquatic ecosystems have been intensively studied and reviewed, but less attention has been paid on soil ecosystems. Given the importance of soil ecosystems and the call for increasing research on soil from scientific communities, it is predicted that relevant studies will boom in the following years. The present review intends to provide a comprehensive overview of current knowledge on microplastic pollution in soil environments. We critically summarize the source, contamination level and fate of microplastics in (industrial and arable) soils. Then, we thoroughly describe what effects have been observed on soil microbes, animals and plants, and analyze what insights we can get from available information. Finally, we identify knowledge gaps that need to be filled and give suggestions for future research.
    [Zhu, F. et al. (2019) Occurrence and ecological impacts of microplastics in soil systems: A Review, Bull Environ Contam Toxicol. Available at: https://pubmed.ncbi.nlm.nih.gov/31069405/. ]
  • Dysbiosis and early mortality in zebrafish larvae exposed to subclinical concentrations of streptomycin.
    Exposure to low concentrations of antibiotics found in aquatic environments can increase susceptibility to infection in adult fish due to microbiome disruption. However, little is known regarding the effect of antibiotic pollution on fish larvae. Here, we show that exposure to streptomycin, a common antibiotic used in medicine and aquaculture, disrupts the normal composition of zebrafish larvae microbiomes, significantly reducing the microbial diversity found in the fish. Exposure to streptomycin also significantly increased early mortality among fish larvae, causing full mortality within a few days of exposure at 10 μg/mL. Finally, we found that subclinical concentrations of streptomycin also increased the abundance of class 1 integrons, an integrase-dependent genetic system associated to the horizontal transfer of antibiotic resistance genes, in the larvae microbiomes. These results suggest that even low concentrations of streptomycin associated with environmental pollution could impact fish populations and lead to the creation of antibiotic resistance reservoirs.
    [Pindling, S., Azulai, D., Zheng, B., Dahan, D. and Perron, G.G., 2018. FEMS microbiology letters, 365(18), p.fny188.]
  • Environmental concentrations of triclosan activate cellular defence mechanism and generate cytotoxicity on zebrafish (Danio rerio) embryos.
    Triclosan (TCS, 5‑chloro‑2‑(2,4‑dichlorophenoxy) phenol) is becoming a major surface waters pollutant worldwide at concentrations ranging from ng L−1 to μg L−1. Up to now, the adverse effects on aquatic organisms have been investigated at concentrations higher than the environmental ones, and the pathways underlying the observed toxicity are still not completely understood. Therefore, the aim of this study was to investigate the toxic effects of TCS at environmental concentrations on zebrafish embryos up to 120 hours post fertilization (hpf). The experimental design was planned considering both the quantity and the exposure time for the effects on the embryos, exposing them to two different concentrations (0.1 μg L−1, 1 μg L−1) of TCS, for 24 h (from 96 to 120 hpf) and for 120 h (from 0 to 120 hpf). A suite of biomarkers was applied to measure the induction of embryos defence system, the possible increase of oxidative stress and the DNA damage. We measured the activity of glutathione‑S‑transferase (GST), P‑glycoprotein efflux and ethoxyresorufin‑o‑deethylase (EROD), the level of ROS, the oxidative damage through the Protein Carbonyl Content (PCC) and the activity of antioxidant enzymes. The genetic damage was evaluated through DNA Diffusion Assay, Micronucleus test (MN test), and Comet test. The results showed a clear response of embryos defence mechanism, through the induction of P-gp efflux functionality and the activity of detoxifying/antioxidant enzymes, preventing the onset of oxidative damage. Moreover, the significant increase of cell necrosis highlighted a strong cytotoxic potential for TCS. The overall results obtained with environmental concentrations and both exposure time, underline the critical risk associated to the presence of TCS in the aquatic environment.
    [Parenti, CC et al. 2018. Science of the Total Environment 650 (2019): 1752-1758.]
  • Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection
    The continuous increase in synthetic plastic production and poor management in plastic waste have led to a tremendous increase in the dumping into our aqueous environment. Consequently, microplastics commonly defined as sizes less than 5mm are produced and stay in both seawater and freshwater environment. The presence of microplastics as a new type of emerging contaminant has become a great issue of concerns from public and government authorities. The sources of microplastics to freshwater systems are many with the largest portion from wastewater treatment plants. The abundance of microplastics varies with the location, from above 1 million pieces per cubic meter to less than 1 piece in 100 cubic meters. Microplastics can cause several harmful physical effects on humans and living organisms through such mechanisms as entanglement and ingestion. The microplastics can act as carriers of various toxins such as additives from industrial production processes and persistent contaminants by the sorption in waters. Those toxins may cause great health problems to humans. A few studies on the fishes demonstrated that the microplastics and the associated toxins are bio-accumulated and cause such problems as intestinal damage and change in metabolic profiles. In studies of microplastics, fresh water is first sampled by the nets with typical mesh size of 330mm for collection of microplastics. After the volume reducing process, the samples will then go through the purification process including density separation by such inorganic salts as sodium chloride and digestion process by oxidizing agents or enzymes. The sequence of these two processes (namely purification and digestion) is dependent on the sample type. The purified samples can be studied by several analytical methods. The commonly used methods for the qualification studies are FTIR spectroscopy, Raman spectroscopy, pyrolysis-GC/MS, and liquid chromatography. A tagging method can be used in the quantification study. Our literature study finds that there is still no universal accepted quantification and qualification tools of microplastics in fresh waters. More work is anticipated so as to obtain accurate information on microplastics in freshwater, which can then be used for the better assessment of the environmental risk.
    [Li, J., Liu, H. and Paul Chen, J. (2018) ‘Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection’, Water Research, 137, pp. 362–374. Available at: http://www.jlakes.org/uploadfile/news_images/hpkx/2018-05-11/1-s2.0-S0043135417310515-un.pdf. ]
  • Partitioning of persistent organic pollutants between blubber and blood of wild bottlenose dolphins: implications for biomonitoring and health
    Biomonitoring surveys of wild cetaceans commonly utilize blubber as a means to assess exposure to persistent organic pollutants (POPs), but the relationship between concentrations in blubber and those in blood, a better indicator of target organ exposure, is poorly understood. To define this relationship, matched blubber and plasma samples (n = 56) were collected from free-ranging bottlenose dolphins (Tursiops truncatus) and analyzed for 61 polychlorinated biphenyl (PCB) congeners, 5 polybrominated diphenyl ether (PBDE) congeners, and 13 organochlorine pesticides (OCPs). With the exception of PCB 209, lipid-normalized concentrations of the major POPs in blubber and plasma were positively and significantly correlated (R(2) = 0.828 to 0.976). Plasma concentrations, however, significantly increased with declining blubber lipid content, suggesting that as lipid is utilized, POPs are mobilized into blood. Compound- and homologue- specific blubber/blood partition coefficients also differed according to lipid content, suggesting POPs are selectively mobilized from blubber. Overall, these results suggest that with the regression parameters derived here, blubber may be used to estimate blood concentrations and vice versa. Additionally, the mobilization of lipid from blubber and concomitant increase in contaminants in blood suggests cetaceans with reduced blubber lipid may be at greater risk for contaminant-associated health effects.
    [Yordy JE, Wells RS, Balmer BC, Schwacke LH, Rowles TK, Kucklick JR. 2010. Environ Sci Technol. 15;44(12):4789-95]
  • Aquatic toxicity of glyphosate-based formulations: comparison between different organisms and the effects of environmental factors
    Glyphosate-based herbicides (e.g. Roundup) are extensively used in the aquatic environment, but there is a paucity of data on the toxicity of the formulated products and the influences by environmental factors. In this study, the acute toxicity of technical-grade glyphosate acid, isopropylamine (IPA) salt of glyphosate, Roundup and its surfactant polyoxyethylene amine (POEA) to Microtox bacterium (Vibrio fischeri), microalgae (Selenastrum capricornutum and Skeletonema costatum), protozoa (Tetrahymena pyriformis and Euplotes vannus) and crustaceans (Ceriodaphnia dubia and Acartia tonsa) was examined and the relative toxicity contributions of POEA to Roundup were calculated. The effects of four environmental factors (temperature, pH, suspended sediment and algal food concentrations) on the acute toxicity of Roundup to C. dubia were also examined. Generally, the toxicity order of the chemicals was: POEA>Roundup>glyphosate acid>IPA salt of glyphosate, while the toxicity of glyphosate acid was mainly due to its high acidity. Microtox bacterium and protozoa had similar sensitivities towards Roundup toxicity (i.e. IC50 from 23.5 to 29.5 mg AE/l). In contrast, microalgae and crustaceans were 4-5 folds more sensitive to Roundup toxicity than bacteria and protozoa. Except photosynthetic microalgae, POEA accounted for more than 86% of Roundup toxicity and the toxicity contribution of POEA was shown to be species-dependent. Increase in pH (6-9) and increase of suspended sediment concentration (0-200 mg/l) significantly increased the toxicity of Roundup to C. dubia, but there were no significant effects due to temperature change and food addition.
    [Tsui, M. and Chu, L. (2003) Aquatic toxicity of glyphosate-based formulations: Comparison between different organisms and the effects of environmental factors, Chemosphere. Available at: https://pubmed.ncbi.nlm.nih.gov/12821000/. ]
  • Organochlorine chemical residues in fish from the Mississippi River basin, 1995.
    Fish were collected in late 1995 from 34 National Contaminant Biomonitoring Program (NCBP) stations and 13 National Water Quality Assessment Program (NAWQA) stations in the Mississippi River basin (MRB) and in late 1996 from a reference site in West Virginia. Four composite samples, each comprising (nominally) 10 adult common carp (Cyprinus carpio) or black bass (Micropterus spp.) of the same sex, were collected from each site and analyzed for organochlorine chemical residues. At the NCBP stations, which are located on relatively large rivers, concentrations of organochlorine chemical residues were generally lower than when last sampled in the mid-1980s. Residues derived from DDT (primarily p,p'-DDE) were detected at all sites (including the reference site); however, only traces of the parent insecticide (p,p'-DDT) were present, which indicates continued weathering of residual DDT from past use. Nevertheless, concentrations of DDT (as p,p'-DDE) in fish from the cotton-farming regions of the lower MRB were great enough to constitute a hazard to fish-eating wildlife and were especially high at the NAWQA sites on the lower-order rivers and streams of the Mississippi embayment. Mirex was detected at only two sites, both in Louisiana, and toxaphene was found exclusively in the lower MRB. Most cyclodiene pesticides (dieldrin, chlordane, and heptachlor epoxide) were more widespread in their distributions, but concentrations were lower than in the 1980s except at a site on the Mississippi River near Memphis, TN. Concentrations were also somewhat elevated at sites in the Corn Belt. Endrin was detected exclusively at the Memphis site. PCB concentrations generally declined, and residues were detected at only 35% of the stations, mostly in the more industrialized parts of the MRB.
    [Schmitt CJ. 2002. Arch Environ Contam Toxicol.;43(1):81-97]