Seasonal streamflow extremes are key drivers of Brook Trout young‐of‐the‐year abundance

Abstract

To manage ecosystems in the context of climate change, we need to understand the relationship between extreme events and population dynamics. Floods and droughts are projected to occur more frequently, but how aquatic species will respond to these extreme events remains uncertain. Based on counts of Brook Trout (Salvelinus fontinalis) collected over 28 yr at 115 sites in Shenandoah National Park, we developed mixed-effects models to (1) assess how well extreme streamflow, as compared to mean flows and total precipitation, can explain young-of-the-year (YOY) abundance, (2) identify potential nonlinear relationships between seasonal environmental covariates and abundance using nonlinear generalized additive mixed models, and (3) explore likely impacts of expected future weather and streamflow conditions. We found that (1) using covariates of streamflow extremes improved prediction of YOY abundance compared to use of mean seasonal flow values or precipitation as a proxy, (2) warmer maximum daily spring temperatures were associated with increased YOY abundance up to about 1.5 standard deviations, above which abundance declined, and (3) a strong negative effect of extreme winter streamflow, unlikely to be offset by possibly positive effects from other seasons, is expected to have a detrimental impact on Brook Trout populations given predicted increases in winter precipitation. Because YOY abundance is a strong determinant of population dynamics for these short-lived species, extreme events will have the potential to exert a strong influence on population persistence of Brook Trout in a changing climate. Management actions that maximize resiliency of populations in response to extreme events, such as restoration of habitat connectivity, should be prioritized to buffer negative impacts.

Fish life history trends indicate increasing flow stochasticity in an unregulated river

“…Our results are concordant with observed flow trends in the study area as well as the expected effects of land use and climate change on streamflows. Although stream habitat forecasting models often address mean conditions, our results suggest that extreme events (e.g., floods and droughts) will be more important for spawning success and recruitment (Blum et al 2018, Kovach et al 2019. We further suggest that freshwater monitoring programs can increase their sensitivity to land use and climate change by evaluating trends in fish life history strategies dependent on stable, predictable, or stochastic environmental conditions.…”

“…Although stream habitat forecasting models often address mean conditions, our results suggest that extreme events (e.g., floods and droughts) will be more important for spawning success and recruitment (Blum et al. , Kovach et al. ).…”

Quantifying the relative importance of stock level, river temperature and discharge on the abundance of juvenile Atlantic salmon (Salmo salar)

“…Many previous studies have attempted to interpret interannual variability in juvenile salmonid abundance solely in relation to environmental drivers (Blum, Kanno, & Letcher, 2018; Cauvy‐Fraunié et al, 2019; Jensen & Johnsen, 1999; Lobón‐Cerviá, Rasmussen, & Mortensen, 2017). This potentially results in erroneous attribution of processes, or for environmental effects to go undetected due to apparently high levels of noise in recruitment data that is associated with temporally varying stock levels.…”

Age‐0 Smallmouth Bass abundance depends on physicochemical conditions and stream network position

“…Late‐summer abundance of age‐0 Smallmouth Bass was greater in years where July flow EP was greater (i.e., July flows were more benign). Considerable inter‐annual variability in age‐0 stream fish abundance and recruitment is common and often reflects the timing of high flows, with floods that occur during nesting and larval development being particularly detrimental (Smith et al 2005, Blum et al 2018). In streams throughout their range, floods destroy and displace Smallmouth Bass eggs and larvae (Larimore 1975, Winemiller and Taylor 1982, Lukas and Orth 1995).…”