Type and timing of phosphorous addition for coho salmon and steelhead trout production

In-stream fertilization is recognized as an effective rehabilitation strategy for increasing fish production in nutrient poor streams. Past research has focused on timings and concentration levels, and short term trials. Several alternative management schemes are possible, but only a limited number have been field-tested. Sixteen nutrient replacement schemes were developed based on an imitation of historical nutrient inputs such as inorganic and organic components of fish carcass, leaves and background hydrology and geology. Further, six schemes (3 schemes one year and 3 others the next year, see below) were field tested over one year using nine artificial sub-alpine stream channels that received natural background water from a nearby spawning channel and two Oncorhynchus species, namely endangered coho salmon (O. kisutch) and steelhead trout (O. mykiss). Schemes tested include: 1) summer inorganic fertilizer addition to reach 3 |ig/L SRP, 2) organic fertilizer to reach 3 fig/L SRP, 3) no fertilizer addition, 4) organic fertilizer addition in the summer to reach 3 ug/L SRP, 5) organic fertilizer in the fall to reach 3 (Xg/L SRP and 6) no fertilizer addition. Background SRP conditions in schemes 1-3 were nil in the summer and above phosphorous growth saturating conditions due to upstream salmon carcass decomposition (SCD) in the fall, and in schemes 4-6, they were approximately phosphorous growth saturating in the summer and fall (due to SCD that fall and the previous fall). Schemes 1 and 6 were pseudo-replicates and their results were compared to investigate the effects on the response variables of having a variable amount of inorganic phosphorous (either from fertilizer or natural sources) available in the summer and dissolved phosphorous from SCD available in the fall. Juvenile fish length, weight, fat stores, and over-winter biomass were the primary response variables. Food web response to phosphorous inputs including algal standing biomass, stream macroinvertebrate numbers and biomass, and resident fatty acid profiles were investigated to elucidate the results. In general, the benefits were not universal in terms of fish species. Steelhead trout benefited the most from in-stream phosphorous augmentation. Year 1, steelhead trout over-winter sizes under schemes 1 and 2 were significantly larger than under scheme 3 (p<0.0001). Lipid levels were highest year one in steelhead trout under scheme 1 during the summer and under scheme 2 and 3 after winter. Coho salmon size years 1 and 2 was significantly larger after summer under treatments 1 and 4 than under the others conducted during the same time period, but the size advantage did not persist over winter. Total lipid content in coho salmon year 1 followed a similar trend. Year 2 after summer steelhead trout size obtained under scheme 4 was larger than obtained under scheme 5 and 6, but like year 1 coho salmon, this advantage did not persist over winter. Scheme 1 and 6 the pseudo replicates did not produce statistically similar results. Scheme 1 that received a higher input concentration of SRP during the fall than scheme 6 produced more after winter biomass. As well, in the summer coho salmon grew faster under scheme 6, than under scheme 1. This can be attributed to SRP being available for a longer time period in scheme 6. These results illustrate that future research should be conducted to investigate how concentration and duration of phosphorous inputs as well as source and timing can be manipulated to mimic historical stream phosphorous inputs and produce increased benefits to freshwater resident juvenile salmonids. More tests on other schemes and species compositions could lead to improved management practices and higher fish yields.