California steelhead trout (Oncorhynchus mykiss) populations only modestly affected by a recent drought and marine heat wave

Haley A Ohms, Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA, Haley A Ohms, David A Boughton and Brian C Spence, Southwest Fisheries Science Center, NOAA, Santa Cruz, CA

Background/Question/Methods
Increased variability and intensity of climate extremes are expected to have dire consequences for some species. Anadromous fishes are uniquely susceptible to climate extremes because they occupy both marine and freshwater environments during their life cycle. While much of the recent research has necessarily focused on predictive modeling, recent climate extremes have provided an opportunity to directly measure population responses. We examined how the recent extreme California drought (2012–2015) and North Pacific marine heat wave (2014–2016) affected productivity of nine north-central California steelhead populations. We used dynamic factor analysis to measure the long-term synchrony among the populations, assess the relationships between population productivity and climate drivers, and characterize the productivity during these recent extreme events.
Results/Conclusions
Much to our surprise, the effects of the freshwater drought and the marine heat wave on steelhead productivity were modest overall. Steelhead productivity was highly correlated among populations, except for populations in the three larger rivers. Both spring outmigration flow and amount of thermally favorable marine habitat available during ocean residency were correlated with steelhead productivity over the timeseries. However, productivity remained within -1.5 standard deviations of the mean during the drought and marine heat wave years in nearly all the study populations. The exception was the two largest river populations, which had anomalously low productivity (less than -2 standard deviations of the mean) in a single year associated with low summer rearing flows, high ocean temperatures, and high spawner flows. The greater sensitivity of larger-river populations to extreme conditions suggests that comparing population demographics and dynamics between small and large rivers could provide important insight into future population responses. Further, while the asynchronous responses between small- and larger-river populations did buffer the species response to the recent extreme conditions, the strong synchrony among small river populations could still result in widespread declines during future environmental extremes.