Watershed hydrogeomorphology drives freshwater productivity of anadromous salmonids: Implications for habitat conservation and restoration

Abstract

Considering the spatial omnipresence of the threat to biodiversity and limited resources and time for conservation and restoration, it is crucial to prioritize conservation and restoration activities to maximize benefits. By transporting marine-derived nutrients to freshwater and surrounding ecosystems, anadromous salmonids contribute greatly to biodiversity maintenance; however, their abundance has been decreased by human activities in many regions. Salmon populations are mainly governed by their productivity in the freshwater life stage; therefore, freshwater productivity, namely, the number of juveniles migrating to the ocean per reproducing parent, should be investigated to maintain healthy populations. Given that productivity decreases dramatically in response to flooding, the flood disturbance intensity controlled by hydrogeomorphology at a watershed scale may strongly influence the freshwater productivity of salmonids. In this study, we evaluated the effect of watershed hydrogeomorphology on the productivity of pink salmon (Oncorhynchus gorbuscha). We surveyed the escapement and number of fry migrants of pink salmon and measured environmental factors, including the average watershed slope and stream power index, as parameters of hydrogeomorphology. The freshwater productivity of pink salmon differed among the streams investigated and was negatively affected by average watershed slope, stream power, and average watershed maximum daily precipitation. These results indicated that flood disturbance reduces the freshwater productivity of pink salmon and that salmon productivity in an individual stream can be predicted by watershed hydrogeomorphology. Our approach can be applied to other anadromous salmonids that have spawning behaviour similar to that of pink salmon, which bury eggs in gravel. Predicting highly productive habitats based on the present study can contribute to planning and prioritizing habitat conservation and restoration for anadromous salmonids.