Spawning salmon and the phenology of emergence in stream insects

Abstract

Phenological dynamics are controlled by environmental factors, disturbance regimes and species interactions that alter growth or mortality risk. Ecosystem engineers can be a key source of disturbance, yet their effects on the phenologies of co-occurring organisms are virtually unexplored. We investigated how the abundance of a dominant ecosystem engineer, spawning sockeye salmon (Oncorhynchus nerka), alters the emergence phenology of stream insects. In streams with high densities of salmon, peak insect emergence occurred in early July, immediately prior to salmon spawning. By contrast, peak insect emergence in streams with low densities of salmon was weeks later and more protracted. The emergence of specific taxa was also significantly related to salmon density. A common rearing experiment revealed that differences in emergence timing are maintained in the absence of spawning salmon. We hypothesize that these patterns are probably driven by predictable and severe disturbance from nestdigging salmon driving local adaptation and being a trait filter of insect emergence. Thus, salmon regulate the timing and duration of aquatic insect emergence, a cross-ecosystem flux from streams to riparian systems.

Species traits reveal effects of land use, season and habitat on the potential subsidy of stream invertebrates to terrestrial food webs

“…Carlson, unpublished data). Earlier emergence might even be triggered as a phenological response to the presence of predators, allowing vulnerable immature insects to escape exposure to predation in the aquatic environment (Moore and Schindler 2010). These alternative explanations have strongly contrasting implications for the availability of aquatic subsidy to terrestrial consumers.…”

Spatial variation in timing of marine subsidies influences riparian phenology through a plant-pollinator mutualism

“…For example, Meehan et al (2005) estimated that a single, second-order, salmon stream in our study region can support up to 10 million blowfly larvae in a typical year even after a substantial proportion (66%) of the returning adult salmon have been harvested in Bristol Bay commercial fisheries (Clark et al 2007). Disturbance by salmon during redd digging is also thought to have potential evolutionary effects on the emergence timing of aquatic insects residing in salmon streams (Moore and Schindler 2010). Thus, salmon can have major impact on the timing and abundance of both terrestrial and aquatic insects in riparian areas.…”

“…Salmon return to streams, rivers, and beaches throughout the entire system and spawn between mid-July and late-September . Within individual streams, salmon spawn timing and entry to each stream is very consistent among years but differs substantially between streams (Moore and Schindler 2010). Over the last 50 years, sockeye salmon (Oncorhynchus nerka) escapement (fish that escape the commercial fishery to spawn) to the Wood River system has averaged 1.1 million (Baker et al 2006).…”

“…In these systems, salmon generally initiate spawning within 2 days of entering streams and salmon entry to the streams is predictable to within 2-5 days each year (Moore and Schindler 2010).…”

Eco-evolutionary dynamics in Pacific salmon

“…By disturbing the local substrate, salmon nest digging results in short-term decreases in insect biomass (for example, Field-Dodgson, 1987;Moore and Schindler, 2008). The predictable disturbance also influences insect life histories, where emergence is timed to occur before salmon arrival, presumably to avoid mortality because of salmon nest digging activities (Moore and Schindler, 2010). The size of the nest excavated by a salmon female is related to her body size; larger females dig larger nests (Steen and Quinn, 1999).…”