Juvenile salmonid growth, survival, and production in a large river floodplain modified by beavers (Castor canadensis)

Beavers (Castor canadensis) may strongly influence juvenile salmon production by damming spring brooks that are primary rearing habitats on expansive floodplains of large Pacific Rim salmon rivers. We studied three floodplain rearing habitats in the Kwethluk River, Alaska: free-flowing (beaver-free, n = 3) and beaver-influenced (below beaver dams, n = 4) spring brooks and early-successional beaver ponds (n = 4). We analyzed juvenile coho (Oncorhynchus kisutch) and Chinook (Oncorhynchus tshawytschwa) salmon movement, survival, densities, and growth using a multistate robust capture-mark-recapture design. Survival (46% to 80%) and densities (0.9 fish·m⁻²) were highest in beaver-free spring brooks. Ponds had lower salmon densities, producing less biomass per unit area than beaver-influenced or beaver-free spring brooks (1.87 ± 0.57 g·m⁻² vs. 2.98 ± 1.22 and 3.23 ± 0.73 g·m⁻²). However, ponds covered 2× greater area than either type of spring brook and therefore produced more salmon biomass at the floodplain scale than either type of spring brook (175 kg vs. 149 kg in beaver-influenced spring brooks and 140 kg in beaver-free spring brooks). We conclude that beaver damming of floodplain spring brooks produces bigger juveniles and more total biomass, but spring brooks produce significantly more, albeit smaller, coho and Chinook juveniles. Thus, the presence of beavers on the floodplain increases habitat variation, which provides a larger range of growth opportunities for juvenile salmon.