Juvenile Salmon Habitat Use, Growth, and Production In A Large Alaskan River Floodplain Dominated by Beavers

Beavers (Castor canadensis) may influence the ecology and production of juvenile salmon in large alluvial river floodplains. We hypothesized that beavers increase production of juvenile salmonids because they grow faster in beaver ponds than in other habitats. We selected five classes of floodplain habitat used by juvenile salmon on the Kwethluk river in Alaska: free-flowing and beaver-influenced spring brooks, and early-, mid-, and late-successional beaver ponds. We minnow trapped habitats and tagged juvenile salmon with elastomer and PIT tags to measure movement and growth rates. Densities were 4 times higher in early-successional ponds than mid-successional ponds (0.78/m² vs. 0.18/m²), and 28 times higher in early-successional vs. late-successional ponds (0.78/m² vs. 0.03/m²). Movement rates between habitats were restricted at base flow (0.37%) but possible during flood events. YOY coho grew significantly faster in early-successional ponds than both types of spring brooks (0.4mm/day vs. 0.26-0.32mm/day). Overall, larger juveniles were produced in early-successional beaver ponds than other habitats (85-93mm vs. 73-80mm). Our results show that beavers increased production because juvenile salmon grew faster and larger in early-successional ponds in the parafluvial zone. However, beavers also blocked orthofluvial habitat from use which may reduce production of juvenile salmon overall at the floodplain scale.