Warm, dry winters truncate timing and size distribution of seaward‐migrating salmon across a large, regulated watershed

Abstract

Ecologists are pressed to understand how climate constrains the timings of annual biological events (phenology). Climate influences on phenology are likely significant in estuarine watersheds because many watersheds provide seasonal fish nurseries where juvenile presence is synched with favorable conditions. While ecologists have long recognized that estuaries are generally important to juvenile fish, we incompletely understand the specific ecosystem dynamics that contribute to their nursery habitat value, limiting our ability to identify and protect vital habitat components. Here we examined the annual timing of juvenile coldwater fish migrating through a seasonally warm, hydrologically managed watershed. Our goal was to (1) understand how climate constrained the seasonal timing of water conditions necessary for juvenile fish to use nursery habitats and (2) inform management decisions about (a) mitigating climate‐mediated stress on nursery habitat function and (b) conserving heat‐constrained species in warming environments. Cool, wet winters deposited snow and cold water into mountains and reservoirs, which kept the lower watershed adequately cool for juveniles through the spring despite the region approaching its hot, dry summers. For every 1°C waters in April were colder, the juvenile fish population (1) inhabited the watershed 4–7 d longer and (2) entered marine waters, where survival is size selective, at maximum sizes 2.1 mm larger. Climate therefore appeared to constrain the nursery functions of this system by determining seasonal windows of tolerable rearing conditions, and cold water appeared to be a vital ecosystem component that promoted juvenile rearing. Fish in this system inhabit the southernmost extent of their range and already rear during the coolest part of the year, suggesting that a warming climate will truncate rather than shift their annual presence. Our findings are concerning for coldwater diadromous species in general because warming climates may constrain watershed use and diminish viability of life histories (e.g., late springtime rearing) and associated portfolio benefits over the long term. Lower watershed nurseries for coldwater fish in warming climates may be enhanced through allocating coldwater reservoir releases to prolong juvenile rearing periods downstream or restorations that facilitate colder conditions.

Individual‐based simulations suggest mixed impacts of warmer temperatures and a nonnative predator on Chinook salmon

“…Earlier emergence and outmigration are likely to become more frequent since incubation is accelerated during warmer winters (Mundy and Evenson 2011). Multiple studies have found that the median migration date of salmonids shifted earlier as water temperature increased in the migration corridor: spawning Chinook and sockeye salmon in the Columbia River, Oregon (Crozier et al 2008 a ); outmigrating Chinook salmon in the Sacramento‐San Joaquin River, California (Munsch et al 2019); outmigrating wild Atlantic salmon ( Salmo salar ) in the River Bush, Northern Ireland (Kennedy and Crozier 2010); and five species of outmigrating salmon in Auke Creek, Alaska (Kovach et al 2013). The effects of this shift on later life stages are unknown, but could include changes in which year salmon migrate to sea (Beakes et al 2011), susceptibility to size‐selective predation (Weitkamp et al 2015), mismatched resources (Crozier et al 2008 a , Satterthwaite et al 2014), altered population age structure (Tattam et al 2015), and decreased survival to adulthood (Thompson and Beauchamp 2014).…”

“…In addition to phenological shifts in a warmer thermal regime, we observed a decrease in simulated subyearling mass. Although higher temperatures boost simulated growth potential (Falke et al 2019), this potential may not be realized due to truncated growing seasons; for example, Munsch et al (2019) found that body size of outmigrating Chinook salmon was directly and negatively related to warmer thermal regimes in a California watershed where salmon outmigrated earlier. Lower mass could also be due to density‐dependent impacts on growth.…”

Striped bass (Morone saxatilis) migration timing driven by estuary outflow and sea surface temperature in the San Francisco Bay-Delta, California

“…1 ). Although the tributary populations of the Sacramento River exhibit less temporal overlap, climate warming could truncate the seaward-migration period for juvenile Chinook salmon 44 . Striped bass commonly predate on juvenile Chinook salmon with potentially catastrophic consequences.…”

Introduction of Bluefin Killifish (Lucania goodei) into the Sacramento–San Joaquin Delta

“…The DJFMP currently samples over 60 beach seine sites throughout the estuary and the lower Central Valley of California, either weekly or every 2 weeks year-round ( Figure 1). Because of the extensive spatio-temporal coverage of this sampling effort, data from the beach seine survey has been used over the years to better understand fish habitat and community changes within the shallow, near-shore habitat of the Delta (Sommer et al 2001;Feyrer et al 2005;Brown and May 2006;Mahardja et al 2016;Mahardja et al 2017;Munsch et al 2019). DJFMP beach seine sampling was conducted by hauling a single 15.2-m x 1.3-m beach seine net with 3-mm 2 mesh and a 1.3-m � 1.3-m bag into shore.…”