Fate of juvenile salmonids stranded in off-channel pools: implications for nutrient transfers

Fish stranding is a complex phenomenon largely attributed to anthropogenic causes in regulated rivers. Although our knowledge of the frequency of stranding and fate of stranded fish in unregulated rivers is limited, this phenomenon may be widespread and important for the transfer of nutrients from aquatic to terrestrial ecosystems. Using a combination of observational data, an experimental manipulation, and a nitrogen subsidy model, we investigated the fate and implications of fish stranded in off-channel pools created by the last spring flood, in the Kadashan River watershed on Chichagof Island, Alaska, USA. Of fish stranded in pools (exclusively juvenile Dolly Varden Char [Salvelinus malma] and Coho Salmon [Oncorhynchus kisutch]), smaller individuals (age-0; < 74 mm) were more common than larger ones (age-1; > 74 mm). Mortality rate was mainly influenced by cover availability, and larger fish tended to disappear at a higher rate than smaller ones. These observations, together with detection of predator activity, suggest that predation was the main cause of mortality for stranded fish. We estimate that fish stranding occurred during 66% of the years between 1980 and 2015, and that in a single stranding event approximately 1.62 kg of nitrogen is available to predators in the 0.24 km² floodplain of the Kadashan watershed surveyed. Thus, fish stranding likely has implications for cross ecosystem connectivity via aquatic nutrient transfers to terrestrial food webs. With projected increases in extreme precipitation and flood events in Southeast Alaska the incidence of fish stranding in unregulated rivers will likely increase. Our results suggest by ensuring that cover (e.g. large wood and artificial structures) is available in off-channel habitats to benefit species or populations of conservation concern.