Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

<em>Abstract.</em>—Oligtrophic streams are ubiquitous throughout coastal British Columbia, and thereby, significant nutrient influx can be provided externally via salmon carcasses. At the Keogh River on northern Vancouver Island, experimental nutrient addition was conducted from 1983 to 1986 to examine if potential increases in trophic productivity may augment growth and production of salmonid smolts. Subsequently, an applied treatment was conducted over the past decade at the infertile Salmon River to offset intensive logging impacts and to accelerate colonization of steelhead trout <em>Oncorhynchus mykiss </em>of headwater reaches above a hydroelectric diversion. The two rivers were treated with agricultural (dry, later liquid) fertilizers, while upstream control reaches were untreated. At Keogh, inorganic P and N were introduced to produce target soluble phosphorus concentrations of 10–15 mg per L, and N loadings of 50–100 mg per L over the four years of nutrient addition. Average peak algal biomass as chlorophyll <em>a </em>increased 5–10-fold in response to nutrient addition. Geometric mean weights of steelhead trout and coho salmon <em>O. kisutch </em>fry within several treated reaches were 1.4–2.0-fold higher than the control, and mean weights of steelhead parr were 30–130% greater in the three treated reaches. Average steelhead smolt yield in three brood years increased 62% (peak, 2.5-fold in 1987) over prefertilization years; yet there was no increase in average smolt size because mean smolt age was reduced by about one year. There were corresponding increases in returning adults and reported catches by steelhead anglers at the Keogh River, compared with trends at an adjacent river fishery. The response of coho smolts to nutrient addition was less marked, or a suggested 21% increase in numbers (<EM>P </EM>< 0.1) with no change in size, although results were moderated by production of coho smolts from several untreated tributaries and small lakes. At the upper Salmon River, where nutrient targets were reduced to one-third that of the Keogh, nutrient addition was associated with 3–7-fold higher benthic insect density in treated reaches than controls, and 2–3-fold greater mean weights and biomass of steelhead and rainbow trout in treated index sites than upstream, unfertilized sites. Over the decade, estimated numbers of steelhead parr and smolt migrants at the Salmon River diversion increased from about 1,500 to 8,000. The results at the Salmon River confirmed those of the Keogh and indicated that lower-level nutrient addition can produce a similar positive trophic response.