Pre-spawn mortality is a phenomenon where adult coho salmon, Oncorhynchus kisutch, die before spawning when returning to freshwater streams to spawn.Feist BE, Buhle ER, Arnold P, Davis JW, and Scholz NL. 2011. Landscape ecotoxicology of Coho salmon spawner mortality in urban streams. PLOS ONE 6(8): 1-11.Spromberg JA, and Scholz NL. 2011. Estimating the future decline of wild Coho salmon populations resulting from early spawner die-offs in urbanizing watersheds of the Pacific Northwest, USA. Integrated Environmental Assessment and Management 9999: 1-9. It is also known as Urban Runoff Mortality Syndrome in more recent studies.Chow, M. I., Lundin, J. I., Mitchell, C. J., Davis, J. W., Young, G., Scholz, N. L., & McIntyre, J. K. (2019). An urban stormwater runoff mortality syndrome in juvenile coho salmon. Aquatic Toxicology, 214, 105231.Peter, K. T., Tian, Z., Wu, C., Lin, P., White, S., Du, B., ... & Kolodziej, E. P. (2018). Using high-resolution mass spectrometry to identify organic contaminants linked to urban stormwater mortality syndrome in coho salmon. Environmental science & technology, 52(18), 10317-10327. This occurrence has been observed in much of the Puget Sound region of the Pacific Northwest.McCarthy SG, Incardona JP, and Scholz NL. 2008. Coastal storms, toxic runoff, and the sustainable conservation of fish and fisheries. American Fisheries Society Symposium 64: 1-21. During fall migration, salmonids (trout and salmon) pass through urban watersheds which are contaminated with stormwater runoff. As the coho salmon pass through these waters, many will show symptoms of lethargy, loss of equilibrium and disorientation, and die within a few hours of showing these symptoms. These symptoms and behaviors are prevalent after rain events. Mortality often occurs before salmon have the opportunity to spawn, which is determined by cutting open female carcasses and observing for unfertilized eggs. Rates of pre-spawn mortality could impact the local salmon populations. Based on model projections, if rates continue, populations of coho salmon could become extinct within the next few decades.
Weight of evidence suggests coho salmon pre-spawn mortality is due to exposure to stormwater runoff. A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon.Scholz NL, Myers MS, McCarthy SG, Labenia JS, McIntyre JK, Ylitalo GM, Rhodes LD, Laetz CA, Stehr CM, French BL, McMillan B, Wilson D, Reed L, Lynch KD, Damm S, Davis JW, and Collier TK. 2011. Recurrent die-offs of adult Coho salmon returning to spawn in Puget Sound lowland urban streams. PLOS ONE 6(12): 1-12. A negative correlation exists between the incidence of pre-spawn mortality and urban land use, specifically with the impervious surfaces, commercial property type, and local roads. Exposure to metals and petroleum hydrocarbons displayed some response to pre-spawn mortality, while exposure to pesticides, diseases, and water quality variables did not elicit this effect. This trend has not been observed in any other life history stage of the coho salmon nor in other salmon species in the Puget Sound region.
History Pre-spawn mortality was first observed in the Puget Sound after efforts had gone into restoring urban watersheds in hopes of drawing salmon back to these areas. Restoration consisted of removing culverts, debris, and any other barriers.NOAA Coastal Storm Programs. Available online at: Stormwater and Salmon- Pre-spawn Mortality of Coho Salmon in Restored Urban Streams Accessed on 4 May 2013.Katz SL, Barnas K, Hicks R, Cowen J, and Jenkinson R. 2007. Freshwater habitat restoration actions in the Pacific Northwest: a decade’s investment in habitat improvement. Restoration Ecology 15(3): 494-505. Restoration projects were successful in that salmon returned to these runs, however, in initial surveys of restoration success, the pre-spawn mortality symptoms were observed. It is now recognized that in restoring watersheds, water quality and contamination must also be considered for successful spawning events of coho salmon if the population is to persist.
Upon displaying specific symptoms (below), mortality of the coho salmon can be expected within a few hours. Mortality usually occurs before these fish have had a chance to spawn. In fact, in observing the female coho salmon carcasses, most of the fish have retained greater than 90 percent of their eggs. Pre-spawn mortality occurs in both male and female salmon, however, it is difficult to determine if male salmon have spawned, thus, pre-spawn mortality is usually signified by egg retention of female salmon. Pre-spawn mortality has also been observed in both wild and hatchery fish alike.
Symptoms Salmon of pre-spawn mortality display unusual behaviors and symptoms, such as swimming in circles or exhibiting other erratic swimming patterns. Other symptoms include lethargy, disorientation, loss of equilibrium, gaping, and fin splaying. Death of the coho salmon occurs within hours after observing these symptoms.
Fish behavior and swimming patterns Coho salmon that are affected by pre-spawn mortality show behavioral symptoms that are easily identified. The fish displayed behaviors like swimming near the surface of the water, swimming in circular patterns, or consistently running into the river bank. This may be due to a loss of equilibrium and orientation of the fish.
Gaping This symptom consists of the salmon repeatedly opening and closing its mouth. This action in some species of fish (e.g. eels) is used to push water over the gills to meet respiratory demands. For coho salmon, this could be an indication that the fish is not getting enough dissolved oxygen, signifying stress to the respiratory system or that the muscular system has been affected and this gaping is a forced action.Balebona MC, Krovacek K, Morinigo MA, Mansson I, Faris A, and Borrego JJ. 1998. Neurotoxic effect on two fish species and a PC12 cell line of the supernate of Vibrio alginolyticus and Vibrio anguillarum. Veterinary Microbiology 63: 61-69.
Fin splaying Fin splaying occurs when the pectoral fin of the salmon is rigid and extended perpendicular to the body of the fish instead of laying flat against the fish.Eaton RC, Bombardieri RA, and Meyer DL. 1977. The mauthner-initiated startle response in teleost fish. Journal of Experimental Biology 66: 65-81. In other fish, e.g. kelp bass, this action is a startle response.
Cause A study indicated that road runoff and water samples collected at coho salmon URMS cases were chemically similar with tire tread wear particle leachate. Further investigation on URMS and tire wear leachate narrowed down the scope from thousands of chemicals to one unknown chemical, which was found as an ozonation product of the common antiozonant 6PPD. This toxic transformation product, called 6PPD-quinone, kills coho salmon at nanogram per liter concentrations. It has been detected in road runoff and creek stormwater when URMS occurred.
Nonpoint source pollution
Urban and stormwater runoff Many of the contaminants that are responsible for degrading urban watersheds come from nonpoint source pollution. This consists of a wide array of contaminants coming from a variety of sources and locations.
The contaminants that have been found in stormwater runoff include metals, such as aluminum, barium, cobalt, iron, arsenic, cadmium, chromium, copper, lead, nickel, and zinc,Lough GC, Schauer JJ, Park JS, Shafer MM, Deminter JT, and Weinstein JP. 2005. Emissions of metals associated with motor vehicle roadways. Environmental Science & Technology 39: 826-836. polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (motor oil and fuel), and pesticides (including insecticides).Davis AP. 2005. Green engineering promote low: applying the principles of green engineering can help create more sustainable development. Environmental Science and Technology 338-344.
Anthropogenic sources are responsible for many of the contaminants in stormwater runoff in urban areas. These anthropogenic sources include motor vehicles, which contribute metals, antifreeze or coolants, and petroleum hydrocarbons from brake pads and leaking fluids. Many of these contaminants will collect on impervious surfaces (e.g. roads and parking lots) and with rain, will wash into streams, rivers, and the ocean. Other sources of runoff include pesticides and fertilizers used on residential and commercial properties, which can be washed into storm drains during rain events.
Effects of runoff There are a number of possibilities as to the cause of pre-spawn mortality. It is possible that this mortality is due to acute cardiorespiratory toxicity to the fish. The symptoms (discussed above) and rapid rate of mortality are consistent with this type of toxicity. The exact chemical combination or mixture responsible for this early mortality is not known. However, a few possible explanations are discussed below.
Narcotic effects Narcosis is a phenomenon that occurs when a combination of organic toxicants, each at low concentrations, have a toxic effect on an organism.Veith GD, and Broderius SJ. 1990. Rules for distinguishing toxicants that cause Type I and Type II narcosis syndromes. Environmental Health Perspectives 87: 207-211. Narcosis has a nonspecific mode of action, where it does not target one specific site of action, but instead affects the membranes and protoplasm. These individual organic toxicants affect the fish at sublethal levels, however, the combination of contaminants can lead to mortality of the organism. However, if the stressor is removed narcotic effects can be reversed. Fish influenced by narcosis may display lethargic symptoms. Narcosis can also result in respiratory-cardiovascular responses to fish.McKim JM, Schmieder PK, Carlson RW, Hunt GJ, Niemi GJ. 1987. Use of respiratory-cardiovascular responses of rainbow trout (Salmo gairdneri) in identifying acute toxicity syndromes in fish: Part 1. Pentachlorophenol, 2,4-dinitrophenol, tricaine methanesulfonate and 1-octanol. Environmental Toxicology and Chemistry 6: 295-312.
Mixtures Pre-spawn mortality may be attributed to a mixture of contaminants. In nonpoint source runoff, there is often more than one chemical, thus organisms may be influenced by the combination or mixture of different contaminants. Mixtures of chemicals may have the same mode of action, which can increase toxicity due to two or more chemicals influencing the same site of an organism.Laetz CA, Baldwin DH, Collier TK, Herbert V, Stark JD, and Scholz NL. 2009. The synergistic toxicity of pesticide mixtures: Implications for risk assessment and the conversation of endangered Pacific salmon. Environmental Health Perspectives 117(3): 348-353. The mixture of two or more contaminants may have a number of different effects to an organism, such as additive or synergistic effects. Additive toxicity occurs when the toxicity of individual contaminants (with a similar mode of action) can be summed to determine the final toxicity. Synergism occurs when the sum of two or more toxicants is greater than the sum of the individual components.