Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams

Abstract

Recent droughts raise global concern over potential biodiversity loss and mitigating impacts to vulnerable species has become a management priority. However, drought impacts on populations are difficult to predict, in part, because habitat refuges can buffer organisms from harsh environmental conditions. In a global change context, more extreme droughts may turn previously suitable habitats into ecological traps, where vulnerable species can no longer persist. Here, we explore the impacts of California's recent record-breaking drought on endangered juvenile Coho salmon. We estimated the variability of cumulative salmon survival using mark-recapture of nearly 20,000 tagged fish in intermittent stream pools during a 7-year period encompassing drought and non-drought conditions. We then determined the relative importance of physical habitat, streamflow, precipitation, landscape, and biological characteristics that may limit survival during drought. Our most striking result was an increase in the number of pools with reduced or zero survival during drought years and a coincident increase in spatial variability in survival among study reaches. In nearly half of the stream pools, salmon survival during drought was similar to mean survival of pools assessed during non-drought years, indicating some pools had remarkable resistance (ability to withstand disturbance) to extreme drought. Lower survival was most attributable to longer duration of disconnection between upstream and downstream habitats, a consequence of increasing drought severity. Our results not only suggest that many pools sustain juvenile salmon in non-drought years transition into ecological traps during drought but also highlight that some pools serve as refuges even under extreme drought conditions. Projected increases in drought severity that lead to longer droughts and greater habitat fragmentation could transform an increasing proportion of suitable habitats into ecological traps. Predicting future impacts of drought on Coho salmon and other sensitive species will require identification and protection of drought refuges and management strategies that prevent further habitat fragmentation.

Keywords: Pacific salmon; abiotic; isolated pools; mixed models; mortality; river drying; threatened species; water abstraction.

FIGURE 1

Map of four tributaries and eight study reaches within the Russian River catchment (upper right inset), including a diagram (lower right inset) of an example study reach. Study reach example shows location of pool habitats (P), temperature (T) and stage (i.e., streamflow) loggers (S), and PIT‐tag antennas

FIGURE 2

Drought classification between 2011 and 2017 within the Russian River catchment according to the US Drought Monitor (https://droughtmonitor.unl.edu/)

FIGURE 3

Cumulative survival estimates during 2011–2017 at eight study reaches within the Russian River catchment. Boxplots show median (thick black line), upper and lower quartile, and highest and lowest values within 1.5× the interquartile range. Points represent pool‐level observations within reaches

FIGURE 4

Probability density function illustrating distribution of cumulative salmon survival estimates in stream pools during drought (2012–2016, yellow) and non‐drought (2011 and 2017, blue) years (a). Temporal variability (coefficient of variability within study reaches across years) in cumulative survival estimates at eight study reaches within the Russian River catchment (b). Spatial variability (coefficient of variability across study reaches (sites) during each of the study years) in cumulative survival estimates across study reaches (c). Presumed survival estimates of zero in dry stream reaches were included for visual assessment; however, these estimates are removed from subsequent analysis

FIGURE 5

Effect sizes (±95% confidence limits) for eight explanatory variables on cumulative juvenile salmon survival during 2011–2017. Effects sizes estimates are the model coefficients from generalized linear mixed effects models. Black points and confidence bars are statistically significant, whereas grey coloring indicates non‐significant variables (p > .05)

FIGURE 6

Partial dependence from generalized linear mixed models of juvenile salmon survival. Model estimates (solid lines) and 95% confidence intervals (shading) for days of disconnection (a), cropland area (b), and minimum pool volume (c). Tick marks on the x‐ and y‐axes indicate values of data observations