Pacific salmon (Oncorhynchus spp.) can have substantial ecological effects on the stream ecosystems in which they spawn and die. During spawning runs, salmon simultaneously excrete nutrients and disturb sediments, effects that have earned salmon labels as resource subsidies and ecosystem engineers. These dual impacts can enhance or reduce the growth and abundance of aquatic biota, but the mechanisms driving variation in responses remain poorly understood. In a meta-analysis of Pacific salmon research in their native range, I found that dissolved nutrients and benthic communities respond positively to salmon overall, but that variability is driven by methodological and environmental factors, such as salmon abundance and sediment size. These finding raise questions about the ecological impacts of salmon in introduced regions, where environmental conditions can vary considerably from the native range.
The remainder of my dissertation research focuses on the ecological impacts of introduced Pacific salmon in spawning streams of the Laurentian Great Lakes. I found that salmon can influence stream ecosystems through interactions with other fish species, biological transport of contaminants, nutrient enrichment, and benthic disturbance. For example, in a Lake Michigan tributary, over half of the stream-resident brook trout monitored over the course of a salmon run moved upstream upon the arrival of spawners, demonstrating that resident fish may alter their behavior during salmon spawning runs. I also found chemical effects related to salmon. Across 10 tributary streams, pollutant concentrations (e.g., PCBs) in stream-resident fish were strongly correlated with inputs from spawning salmon. Salmon effects on dissolved nutrients, however, were inconsistent and generally small compared to native ranges. In contrast, disturbance effects were large where salmon were abundant, sometimes reducing the abundance of benthic organisms by an order of magnitude.
Overall, I found evidence that responses to salmon spawning in the Great Lakes were strongly driven by environmental factors. Weak nutrient responses and strong benthic disturbance appear to be driven by high background nutrients and small sediments, respectively. Comparisons of responses between native and introduced ranges suggest that the ecological relevance of animal subsidies can be strongly dependent on environmental conditions. In the Great Lakes, sediment disturbance, interspecific interactions, and pollutant dispersal are important considerations in assessing the overall ecological impacts of Pacific salmon introductions.