Comparative effects of spawning salmon and their carcasses in a southeast Alaska stream ecosystem

Background/Question/Methods

Pacific salmon (Oncorhynchus spp.) exhibit anadromy and semelparity, two life-history traits that result in the massive transfer of marine-derived nutrients from the ocean to their natal stream ecosystems. These nutrients – released by living salmon through excretion, and leached from salmon carcass as they decompose – can increase the abundance of stream organisms as they are transferred through food webs. Conversely, extensive nest digging in stream sediments by spawning salmon can markedly reduce stream organism abundance. We conducted manipulative and observational field studies in Maybeso Creek, Prince of Wales Island, Southeast Alaska, that allowed us to evaluate the ecological effects of (1) nutrients excreted from live salmon, (2) nutrients leached from their carcasses, and (3) disturbance to stream sediments. We increased the abundance of salmon carcasses by installing 140, 1.5m-long carcass-retention devices, made of re-bar and conduit pipe, in a 110m length of stream channel. Ecological responses in this reach were compared to a non-manipulated upstream control reach using a BACI design. Within these two reaches we also installed 2 x 2m salmon exclosures and procedural control plots. Exclosures prohibited salmon disturbance to stream sediments, but allowed the entry of salmon-derived nutrients dissolved in streamwater. We sampled dissolved nutrients, benthic algal biomass (as chlorophyll a), macroinvertebrate community composition, isotopic composition of juvenile coho salmon before, during, and after the salmon run.

Results/Conclusions

Concentrations of ammonium, soluble reactive phosphorus, and nitrate increased more than 15-fold with the arrival of migrating salmon. Algal biomass in exclosure plots increased by a factor of 11 relative to pre-run values. In plots that experienced spawning disturbance, algal biomass increased by only a factor of five. Later in the salmon run, retention devices resulted in an approximately five-fold increase in carcass abundance. Despite this significant increase in carcass abundance, dissolved nutrients and benthic algal biomass increased only slightly in the carcass-retention reach relative to the reach where carcasses were not experimentally retained. Our results illustrate that nutrients excreted by migrating and spawning salmon can have an immediate and strong positive influence on algal biomass. However, these effects are largely offset by salmon spawning disturbance. From this study we conclude that (1) the ecological effects of salmon are primarily through nutrient excretion and salmon spawning disturbance, while (2) those of salmon carcasses are relatively modest. These results illustrate that the diverse behavior and life-history traits of salmon can have a complex influence on stream ecosystems.