Oil pollution toxicity to marine fish has been observed from oil spills such as the Exxon Valdez disaster, and from nonpoint sources, such as surface runoff, which is the largest source of oil pollution in marine waters.

Crude oil entering waterways from spills or runoff contain polycyclic aromatic hydrocarbons (PAHs), the most toxic components of oil. The route of PAH uptake into fish depends on many environmental factors and the properties of the PAH. The common routes are ingestion, ventilation of the gills, and dermal uptake. Fish exposed to these PAHs exhibit an array of toxic effects including genetic damage, morphological deformities, altered growth and development, decreased body size, inhibited swimming abilities and mortality.Carls, MG, Rice, SD, Hose, JE. 1999. Sensitivity of fish embryos to weathered crude oil: Part I. Low‐level exposure during incubation causes malformations, genetic damage, and mortality in larval Pacific herring (Clupea pallasi). Environmental Toxicology and Chemistry, 18 (3): 481–493.Heintz, RA, Short, JW, Rice, SD. 1999 Sensitivity of fish embryos to weathered crude oil: Part II. Increased mortality of pink salmon (Oncorhynchus gorbuscha) embryos incubating downstream from weathered Exxon Valdez crude oil. Environmental Toxicology and Chemistry, 18 (3): 494–503.Incardona, JP, Carls, MG, Teroaka, H, Sloan, CA, Collier, TK, Scholz, NL. 2005. Aryl Hydrocarbon Receptor-Independent Toxicity of Weathered Crude Oil during Fish Development. Environmental Health Perspectives, 113 (12): 1755–1762. The morphological deformities of PAH exposure, such as fin and jaw malformations, result in significantly reduced survival in fish due to the reduction of swimming and feeding abilities. While the exact mechanism of PAH toxicity is unknown, there are four proposed mechanisms. The difficulty in finding a specific toxic mechanism is largely due to the wide variety of PAH compounds with differing properties.

History

Research on the environmental impact of the petroleum industry began in earnest, during the mid to late 20th century, as the oil industry developed and expanded.Shigenaka, G. 2014. Twenty-Five Years After the Exxon Valdez Oil Spill: NOAA’s Scientific Support, Monitoring and Research. Seattle: NOAA Office of Response and Restoration. Large scale transport of crude oil increased as a result of the increasing worldwide demand for oil, subsequently increasing the number of oil spills. Oil spills provided perfect opportunities for scientists to examine the in situ effects of crude oil exposure to marine ecosystems, and collaborative efforts between the National Oceanic and Atmospheric Administration (NOAA) and the United States Coast Guard resulted in improved response efforts and detailed research on oil pollution's effects. The Exxon Valdez oil spill in 1989, and the Deepwater Horizon oil spill in 2010, both resulted in increased scientific knowledge on the specific effects of oil pollution toxicity to marine fish.

Exxon Valdez oil spill Focused research on oil pollution toxicity to fish began in earnest in 1989, after the Exxon Valdez tanker struck a reef in Prince William Sound, Alaska and spilled approximately 11 million gallons of crude oil into the surrounding water.Skinner, SK, Reilly, WK. 1989. The Exxon Valdez Oil Spill: A Report to the President. United States National Response Team. At the time, the Exxon Valdez oil spill was the largest in the history of the United States. There were many adverse ecological impacts of the spill including the loss of the loss of billions of Pacific herring and pink salmon eggs. Pacific herring were just beginning to spawn in late March when the spill occurred, resulting in nearly half of the population's eggs being exposed to crude oil. Pacific herring spawn in the intertidal and subtidal zones, making the vulnerable eggs easily exposed to pollution.

Deepwater Horizon oil spill After April 20, 2010, when an explosion on the Deepwater Horizon Macondo oil drilling platform triggered the largest oil spill in US history, another opportunity for oil toxicity research was presented.Adams, A. 2015. Summary of Information concerning the Ecological and Economic Impacts of the BP Deepwater Horizon Oil Spill Disaster. National Resource Defense Council. IP:15-04-A Approximately 171 million gallons of crude oil flowed from the seafloor into the Gulf of Mexico, exposing the majority of the surrounding biota. The Deepwater Horizon oil spill also coincided directly with spawning window of various ecologically and commercially important fish species, including yellowfin and Atlantic bluefin tuna.Incardona JP, Gardner LD, Linbo TL, Brown TL, Esbaugh AJ, Mager E, Stieglitz JD, French BL, Labenia JS, Laetz CA, Tagal M, Sloan CA, Elizur A, Benetti DD, Grosell M, Block BA, Scholz NL. 2014. Deepwater Horizon crude oil impacts the developing hearts of large predatory pelagic fish. Proceedings of the National Academy of Sciences of the United States of America, 111(15): 1510–1518. The oil spill directly affected Atlantic bluefin tuna, as approximately 12% of larval tuna were located in oil-contaminated waters,Muhling, BA, Roffer, MA, Lamkin, JT, Ingram Jr., GW, Upton, MA, Gawlikowski, G, Muller-Karger, F, Habtes, S, Richards, WJ. 2012. Overlap between Atlantic bluefin tuna spawning grounds and observed Deepwater Horizon surface oil in the northern Gulf of Mexico. Marine Pollution Bulletin. 64(4): 679–687. and Gulf of Mexico is the only known spawning grounds for the western population of bluefin tuna.

Exposure to oil Oil spills, as well as daily oil runoff from urbanized areas, can lead to polycyclic aromatic hydrocarbon (PAHs) entering marine ecosystems. Once PAHs enter the marine environment, fish can be exposed to them via ingestion, ventilation of the gills, and dermal uptake.Snyder, Susan, Erin Pulster, Dana Wetzel, Steven Murawski. 2015. PAH exposure in Gulf of Mexico demersal fishes, post- deepwater horizon. Environmental Science and Technology 49: 8786–8795. The major route of uptake will depend on the behavior of the species of fish and the physicochemical properties of the PAH of concern. Habitat can be a major deciding factor for the route of exposure. For example, demersal fish or fish that consume demersal fish are highly likely to ingest PAHs that have sorbed to the sediment, whereas fish that swim at the surface are at a higher risk for dermal exposure. Upon coming in contact with a PAH, bioavailability will affect how readily the PAH is taken up. The EPA identifies 16 major PAHs of concern and each of these PAHs has a different degree of bioavailability. For instance, PAHs with lower molecular weight are more bioavailable because they dissolve more readily in water and are therefore more bioavailable for fish within the water column. Similarly, hydrophilic PAHs are more bioavailable for uptake by fish. For this reason, usage of oil dispersants, like Corexit, to treat oil spills can increase the uptake of PAHs by increasing their solubility in water and making them more available for uptake via the gills. Once a PAH is taken up, the fish's metabolism can affect the duration and intensity of the exposure to target tissues. Fish are able to readily metabolize 99% of PAHs to a more hydrophilic metabolite through their hepato-biliary system. This allows for the excretion of PAHs. The rate of metabolism of PAHs will depend on the sex and size of the species. The ability to metabolize PAHs into a more hydrophilic form can prevent bioaccumulation and halt PAHs from being passed on to organisms further up the food web. Because oil can persist in the environment long after oil spills via sedimentation, demersal fish are likely to be continually exposed to PAHs many years after oil spills. This has been proven by looking at the biliary PAH metabolites of bottom-dwelling fish. For instance, bottom-dwelling fish still showed elevated levels of low molecular weight PAH metabolites 10 years after Exxon Valdez oil spill.

Crude oil components Crude oil is composed of more than 17,000 compounds.Simanzhenkov, V., & Idem, R. (2003). Crude Oil Chemistry (1st ed.). CRC Press. https://doi.org/10.1201/9780203014042 Among these 17,000 compounds are PAHs, which are considered the most toxic components of oil. PAHs are formed by pyrogenic and petrogenic processes. Petrogenic PAHs are formed by the elevated pressure of organic material. In contrast, pyrogenic PAHs are formed through the incomplete combustion of organic material. Crude oil naturally contains petrogenic PAHs and these PAH levels are increased significantly through the burning of oil which creates pyrogenic PAHs. The level of PAHs found in crude oil differs with the type of crude oil. For example, crude oil from the Exxon Valdez oil spill had PAH concentrations of 1.47%, while PAH concentrations from the North Sea have much lower PAH concentrations of 0.83%.

Sources of crude oil pollution Crude oil contamination in marine ecosystems can lead to both pyrogenic and petrogenic PAHs entering these ecosystems. Petrogenic PAHs can enter waterways through oil seeps, major oil spills, creosote and fuel oil runoff from urban areas.Hussein, Abdel, and Mona Mansour. 2015. A review on polycyclic aromatic hydrocarbons: Source, environmental impact, effect on human health and remediation. Egyptian Journal of Petroleum 25: 107–123 Pyrogenic PAH sources consist of diesel soot tire rubber and coal dust.Burgess, RM, Ryba, S, Cantwell, M, Perron, MM, Tien, R, Thibideau, LM. 2001. Bioavailability of PAHs from pyrogenic and petrogenic sources using glass fish. Society of Environmental Toxicology and Chemistry Annual Meeting, Baltimore, MD. Although there are natural sources of PAHs such as volcanic activity and seepage of coal deposits, anthropogenic sources pose the most significant input of PAHs into the environment. These anthropogenic sources include residential heating, asphalt production, coal gasification, and petroleum usage. Petrogenic PAH contamination is more common from crude oil spills such as Exxon Valdez, or oil seeps; however, with runoff pyrogenic PAHs can also be prevalent. Although major oil spills such as Exxon Valdez can introduce a large amount of crude oil to a localized area in a short time span, daily runoff comprises most of the oil pollution to marine ecosystems. Atmospheric deposition can also be a source of PAHs into marine ecosystems. The deposition of PAHs from the atmosphere into a water body is largely influenced by the gas-particle partitioning of the PAH.

Effects