A watershed-specific formula to predict coho salmon reproduction using
functional flow metrics
Abstract
In many rural areas of arid and semi-arid 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. In particular, the timing and magnitude of flow needed to
sustain key ecological functions remain poorly quantified in many
regions. This work aims to quantify hydrologic conditions that support
persistence of key ecosystem species using a functional flows framework.
We use the coho ( Oncorhynchus kisutch) and Chinook (
Oncorhynchus tshawytscha) salmon run in Scott Valley, a 2,109 km
2 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 used
lasso regression to build predictive models of coho and Chinook salmon
reproductive success based on hydrologic metrics. To control for cohort
effects, we chose normalized ecological response metrics for coho and
Chinook (number of outmigrating smolt per spawning adult or spawning
adult female). For both species, we calculated optimal prediction models
using a cross-validation bootstrapping approach to resample and test on
unsampled observations. Lambda values, a key fitting parameter in the
lasso models, were selected based on an average relative test error
threshold of 1.0. Selected lambda values were used to calculate a final
predictive model, or Hydrologic Benefit function, using the full dataset
for each species. Hydrology could explain a greater degree of variance
in relative coho reproduction than in Chinook. The hydrologic metrics
that explain the greatest variance in coho reproduction values occur
during the window of their parents’ spawning and, to a lesser extent, in
the spring and fall of their year of rearing in freshwater. This
supports an interpretation that spawning conditions may exert a
significant influence on the mortality rates of the hatching juveniles.
Robustness of the results indicate that this method for empirically
deriving hydrologic metrics with the highest ecological benefit for a
threatened species may be useful in other watersheds, where sufficient
ecological data records are available, to evaluate trade-offs and
support water management decisions in human-altered novel ecosystems.