PHYSICAL AND CHEMICAL IMPACTS OF SPAWNING SALMON ON ALASKAN STREAM ECOSYSTEMS

It is now widely recognized that migrations of Pacific salmon move substantial quantities of marine-derived nutrients and energy to coastal freshwater ecosystems. Less recognized are the roles that salmon play as modifiers of the physical habitat of ecosystems used for spawning. We investigated bioturbation caused by nest-digging sockeye as a potential disturbance of salmon spawning habitats. Sockeye salmon (Oncorhynchus nerka) dig nests (redds) in stream and lake gravel that average over 2 m2 and are 20 cm deep. In areas with healthy populations of salmon, sockeye often spawn at extremely high densities, with networks of redds covering entire stream and river reaches. During spawning, sockeye displaced silt from areas where they spawn. An experimental exclusion of spawning salmon indicated that dislodged silt accumulates locally in areas without spawning activity. In addition, disturbance by salmon leads to the stream-wide export of substantial quantities of silt and particulate nutrients, as demonstrated by large increases in the particulate, total phosphorus, and total nitrogen loads in water flowing out of creeks during peak salmon spawning. Thus, salmon redd-digging can be an important component of aquatic disturbance regimes--affecting a variety of biota as well as modifying the physical characteristics and ecosystem processes of coastal streams, lakes, and rivers.