Riparian insect response to experimental in-stream salmon additions

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Terrestrial and emerging aquatic insects incorporate marine-derived nutrients from salmon carcasses either directly by consuming carcass material, or indirectly when salmon nutrients stimulate primary and secondary producers. We hypothesized that addition of salmon carcasses would increase abundance, alter species composition, and affect distribution of insects in riparian areas. In summer 2008 we initiated a large-scale field experiment across 9 streams, consisting of 500-m reaches treated with salmon carcasses (n=3), pelletized salmon (n=3), and un-treated reference reaches (n=3). Sticky traps were deployed at 0, 5, and 25 m intervals at each stream to track patterns. From direct counts, 91% of aquatic insects captured were within 5 m of the stream. No differences were observed in adult aquatics between treatments, though we anticipate a lag in response when larval insects in the stream emerge next season. In contrast, our data suggests terrestrial dipterans were attracted to reaches receiving carcass and analog treatments, resulting in altered insect assemblage composition in the stream corridor. Such attraction could affect predator-prey dynamics in linked stream-riparian food webs, as these insects are prey for terrestrial consumers and stream fishes.

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North American Benthological Society 2009 Meeting