title: Effectiveness of Salmon Carcass Analogs as a Form of Nutrient Enhancement for Juvenile Coho Salmon (Oncorhynchus kisutch) in Three Lower Columbia Watersheds
Creators: name: Sturza, Matthew T.
Publisher: name: Western Washington University
Year: 2017

description: <p>Adult Pacific salmon exhibit a form of parental care after spawning and perishing by depositing a subsidy of marine derived nutrients (MDN) that may be incorporated into the stream food web and feed juvenile salmon. Adult salmon populations have significantly declined since the late 19th century, thereby reducing the amount of MDN within Pacific Northwest Streams. This loss in nutrients within stream food webs may be limiting the growth and survival of juvenile salmon and therefore reducing the population sizes of adult salmon. One strategy to mitigate for nutrient deficiencies within a stream is the use of salmon carcass analogs (SCA), pellets composed of pulverized and pasteurized marine forage fish. We investigated the effectiveness of SCA in enhancing the size and abundance of juvenile coho salmon within a complex of three watersheds (Abernathy, Germany, and Mill Creek) that empty into the lower Columbia River near Cathlamet, WA. SCA applications occurred in the fall (2010-2013) on Germany Creek and in the spring (2013-2015) on Abernathy Creek, while Mill Creek served as a reference watershed and did not receive SCA applications. We periodically gathered samples of periphyton, macroinvertebrates, and juvenile coho (fin clips) before and after SCA application at approximately two month intervals. Juvenile coho were also sampled for fork length and weight. Samples were taken at three sites at the lower, middle, and upper extent of adult coho spawning within each watershed. During the final sampling event of each year, while juvenile coho were outmigrating, fin clips were taken at smolt traps located near each river’s confluence with the Columbia River. Data from smolt traps were used to estimate the average fork length and abundance of juvenile coho during each year of this project. To evaluate the timing and extent of nutrients from SCA being incorporated into the stream food web, samples were processed and analyzed for δ 15N, a measure of the abundance of the heavier isotope of nitrogen that occurs more abundantly in the marine environment. Seasonal trends of δ15N in periphyton, macroinvertebrates, and juvenile coho, as well as seasonal trends of juvenile coho fork length and weight were compared between fertilized and unfertilized watersheds. We detected SCA effects on seasonal trends of macroinvertebrate and juvenile coho δ15N for the fall and spring treatments, indicating SCA nutrients were incorporated by these communities. We detected SCA effects on the seasonal trends of juvenile coho fork length and weight for the spring treatment, but not for the fall treatment. We could not detect SCA effects on seasonal trends of periphyton δ15N for either the fall or spring treatment, potentially due to smaller than needed sample sizes. Overall the effect of fall SCA application was to disrupt the seasonal trend of δ15N values among trophic levels by causing an increase in δ15N during the late fall/early winter when values are normally decreasing. The effect of spring SCA application was to enhance the seasonal trend, causing increases in δ15N values greater than those seen in the absence of SCA applications. Comparing juvenile coho sizes and abundances between years with and without SCA application and between fertilized and unfertilized watersheds indicated that neither the fall or spring treatment had a significant effect on coho growth and survival. Where SCA are to be used as a salmonid recovery tool, we recommend that careful watershed selection and subsequent monitoring be employed to ensure investments are worthwhile.</p>