DOI: 10.5061/dryad.bk3j9kdc9
title: Threatened salmon rely on a rare life history strategy in a warming landscape

description: Rare phenotypes and behaviours within a population are often overlooked,
 yet they may serve a heightened role for species imperilled by rapid
 warming. In threatened spring-run Chinook salmon spawning at the southern
 edge of the species range, we show late-migrating juveniles are critical
 to cohort success in years characterized by droughts and ocean heatwaves.
 Late migrants rely on cool river temperatures over summer, increasingly
 rare due to the combined effects of warming and impassable dams. Despite
 the dominance of late migrants, other strategies played an important role
 in many years. Our results suggest that further loss of phenotypic
 diversity will have critical impacts on population persistence in a
 warming climate. Predicted thermally suitable river conditions for late
 migrants will shrink rapidly in the future and will be largely relegated
 above impassable dams. Reconnecting diverse habitat mosaics to support
 phenotypic diversity will be integral to the long-term persistence of this
 species.
description: These datasets comprise the part of the manuscript that defines
 thermally suitable habitat in a warming climate. Temperature strongly
 influences salmonid physiology, growth and survival. Thus populations with
 access to diverse water temperatures during incubation and natal rearing
 are predicted to exhibit increased phenotypic and phenological diversity.
 To support late migrants, stream temperatures need to remain suitably cool
 over the summer to accommodate the extended rearing period. Mill and Deer
 Creek watersheds, along with upstream reaches of the Battle and Clear
 Creeks and the Yuba River, are among the few accessible and populated
 spring-run streams in the system that still support all three
 phenotypes. To explore why Deer and Mill Creeks may
 exhibit multiple juvenile life histories and how life history expression
 may change with climate change, we compared current and future thermal
 conditions along every current and historical spring-run stream.
 Temperature was obtained from a mean monthly stream temperature model. In
 brief, FitzGerald et al. (2021) employed a spatial stream temperature
 model to predict mean monthly stream temperature for nearly every river km
 in the western U.S. In the Central Valley, the test sample r2 was 0.813
 and the mean absolute prediction error (MAPE) was 1.024°C. We first
 clipped this stream temperature dataset with the current and historical
 Central Valley spring-run distributions, and defined accessible reaches as
 those downstream of impassable barriers, such as dams. In general, the
 distribution and stream networks matched, but a few reaches with
 spring-run did not have stream temperature. We averaged the monthly
 temperature at each stream segment from 2005-2015, representing our study
 period. In the Central Valley, stream temperatures are predicted to
 increase by 0.6°C by 2040 and 1.0°C by 2080 (Isaak et al. 2017), so we
 applied these deltas to the temperature dataset. We
 then examined stream temperature suitability (i.e, &lt; 15°C) for
 juveniles rearing in May and August of 2005-2015, 2040, and 2080. Here we
 used a fixed temperature threshold of 15°C because temperatures greater
 than ~15°C result in decreased growth rates and increased mortality rates,
 yet we acknowledge that there is likely some variation in this threshold
 according to local water quality, food availability, and the life stage
 considered.
description: Attached are two shapefiles for data reproduction. The shapefiles
 are Central Valley spring-run Chinook salmon spatial distributions (1:
 current and historical distributions; 2: accessible distribution) merged
 with monthly stream temperature. We include the months (May and August)
 and years of interest from our paper (2005-2015 averaged, 2040, 2080). We
 also include other months for the above year categories (June, July,
 September, October, November) and each individual year from 1993-2015 for
 other research. For our paper, we applied a threshold of 15°C, but we
 include all temperatures in these files for other research. See associated
 README_CVSCStemps file.