A Watershed-Specific Approach to Identify Key Functional Flow Metrics Supporting Salmon Reproduction

Abstract

In many rural areas of arid and semiarid regions, balancing agricultural and environmental water needs is a key challenge facing resource managers. This is complicated by the tendency for the water needs of cultivated crops to be better understood than those of aquatic ecosystems. This work aims to quantify hydrologic conditions that support the persistence of key ecosystem species using functional flows, not in the context of a prescribed flow regime, but in terms of identifying features of the hydrograph that are empirically correlated with specific ecological outcomes. We use the coho (Oncorhynchus kisutch) and Chinook (Oncorhynchus tshawytscha) salmon runs in Scott Valley, a 2 109 km² undammed rural watershed in northern California, USA, as a case study. Taking advantage of a nearly two-decade ecological monitoring dataset and long-term stream gauge measurements, we first examined hydrological–ecological correlations; then compared six different statistical modelling structures, using two techniques: LASSO and MARSS. In LASSO regressions, to balance the explanatory power of the models with the risk of overfitting, we used k-fold cross-validation to find the lowest error value of the tuning parameter lambda. In MARSS, we calculated models for each individual hydrologic metric and compared them using AICc values. Correlation coefficients indicate that hydrologic factors and spawner abundance both exert influence on juvenile fish production. Statistical models generally agreed that the hydrologic metrics with the highest coefficients are earlier river reconnection and greater fall flow magnitude during parents' spawning for coho, and lower wet season median flow and slower spring recession rate for Chinook (though this could change with additional years of data, especially for the smaller coho dataset). The influence of some metrics, notably fall flow difference from dry season, was positive or negative depending on the fish life stage in which the flow occurred. This approach for empirically identifying hydrologic metrics with high ecological importance for a threatened species may be useful in other watersheds, where sufficient ecological data are available; it could be used to evaluate trade-offs and support water management decisions in human-altered novel ecosystems.