Spatial and temporal shifts of macroinvertebrates within spawning and non-spawning habitats during a salmon run in Southeast Alaska

Background/Question/Methods

The idea of the refuge is critically important and some argue that it should be considered an integrating concept in ecology and evolution, as it encompasses a variety of phenomena including enemy-free space, cover, crypsis, functional responses, competition for resources and shelter.  Pacific salmon have been shown to disturb benthic sediments and macroinvertebrates in riffles during nest construction and we asked whether displaced invertebrates use refugia habitats.  Our study explored the spatial and temporal changes of macroinvertebrates in spawning (riffle) and non-spawning (refugia) habitats during a salmon run in Twelve Mile Creek on Prince of Wales Island, Alaska, USA.  Potential refugia included: pools, margins and the hyporheic zone, and we also sampled invertebrate drift.  We predicted that macroinvertebrates would decline in riffles and increase in drift and refugia habitats during salmon spawning.  Macroinvertebrate samples were collected as five replicates each from riffles, pools, margins and hyporheic wells every 10 days from 27 June until 20 September 2008.  Drift nets were used to collect daytime invertebrate drift at the same time as habitat sampling.  The salmon run was dominated by pink salmon (Oncorhynchus gorbuscha), which were counted twice weekly from the start of the spawning run until only carcasses remained. 

Results/Conclusions

We observed significant macroinvertebrate distributional changes within the stream channel during salmon spawning.  The density (p<0.001), biomass (p<0.001) and richness (p=0.009) of macroinvertebrates declined in riffles during spawning.  In pools, there was no change in density or richness, but insect biomass (p=0.002) declined during salmon run.  In margins, there was no change in the invertebrate community and in the hyporheic zone, insect biomass and richness did not change, but density (p=0.019) was greater during spawning.  The density (p=0.049) and richness (p<0.001) of drifting invertebrates increased during the run.  We observed macroinvertebrate declines within spawning habitat; however in non-spawning habitat, there were less pronounced changes in invertebrate density and biomass.  Disturbance is a central organizing factor in stream communities and is fundamental to the concept of patch dynamics, whereby the spatial and temporal variability of ecosystems are established by disturbance impacts. We demonstrate that invertebrate assemblages undergo spatial shifts within the stream channel during spawning and suggest that adult salmon are the causal mechanism for the observed invertebrate reductions in patches (riffles) where spawning activities are greatest.  Certain in-stream habitats may offer refuge to macroinvertebrates and be a fundamental determinant of macroinvertebrate resilience to salmon spawning disturbances in Southeast Alaskan streams.