Pre-spawn mortality is a phenomenon where adult coho salmon, Oncorhynchus kisutch, die before spawning when returning to freshwater streams to spawn. It is also known as Urban Runoff Mortality Syndrome in more recent studies. This occurrence has been observed in much of the Puget Sound region of the Pacific Northwest. 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. 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. 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.
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. In other fish, e.g. kelp bass, this action is a startle response.