Pre-spawn mortality in coho salmon
[5] Mortality often occurs before salmon have the opportunity to spawn, which is determined by cutting open female carcasses and observing for unfertilized eggs.[6] [7] 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.[1] 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.[7] 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.[11] A study indicated that road runoff and water samples collected at coho salmon URMS cases were chemically similar with tire tread wear particle leachate.These anthropogenic sources include motor vehicles, which contribute metals, antifreeze or coolants, and petroleum hydrocarbons from brake pads and leaking fluids.Other sources of runoff include pesticides and fertilizers used on residential and commercial properties, which can be washed into storm drains during rain events.However, these specific symptoms and resulting pre-spawn mortality are not observed in other anadromous fish, thus, the coho salmon may be a more sensitive species than other salmonids.Results from experiments analyzing the effects of metal contaminants on fish discovered an increase in cadmium, lead, and nickel in the gill tissue.[15] Bioretention practices include rain gardens, vegetated swales, decrease impervious surfaces (instead using Pervious concrete), and green roofs.The cause of pre-spawn mortality for the sockeye salmon in the Fraser River, BC, and Bristol Bay, Alaska, can be attributed to factors such as fish returning to spawn too early, warmer water temperatures, low dissolved oxygen and disease or parasites.[20][23] In the Klamath River, mortality before spawning was due to disease, thermal stress, a low water flow and increasing numbers of salmon migrating within the same time period.