Dispersal mechanisms of macroinvertebrates colonizing salmon flesh in a developing Alaskan stream

Patterns of carcass colonization by macroinvertebrates suggest deterministic resource tracking. As salmon carcasses decay in riverine ecosystems, exudates and abraded particles enter the water column accompanied by qualitative changes in benthic biofilms. We used a combination of implanted substrates, controlled exposure experiments and descriptive surveys to examine dispersal and colonization behaviour of macroinvertebrates in a developing stream in the presence/absence of decaying salmon flesh. Macroinvertebrate colonization of implanted trays containing salmon flesh showed no relationship with their sequential downstream position and (potential) increasing gradient of decay products. Dispersal to trays was principally due to drift with colonization patterns indicative of resource evaluation by direct contact, as opposed to remote detection. Macroinvertebrates in enclosed exposure chambers switched between artificial channels with and without salmon flesh demonstrated no significant difference related to exposure history; chamber opening was followed by a net migration from both treatment and controls. In a companion experiment, macroinvertebrates freely colonizing rock baskets switched between channel treatments, demonstrated no significant difference related to exposure history. Although the distribution of macroinvertebrates in the stream corresponded to habitats where carcasses accumulated, macroinvertebrate density in respective habitats was unrelated to carcass densities. The absence of a behavioural dispersal response to the presence of decaying salmon flesh in this study might be explained by the spatio-temporal design of the experiments, the use of previously frozen flesh as opposed to fresh carcasses, and the relatively early developmental stage of this stream and its associated macroinvertebrate community.