Resistance and resilience of pelagic and littoral fishes to drought in the San Francisco Estuary

Abstract

Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial changes in the past several decades, largely due to multiple interacting stressors that are often of anthropogenic origin. Few are more impactful than droughts, which are predicted to increase in both frequency and severity with climate change. In this study, we examined over five decades of fish monitoring data from the San Francisco Estuary, California, U.S.A, to evaluate the resistance and resilience of fish communities to disturbance from prolonged drought events. High resistance was defined by the lack of decline in species occurrence from a wet to a subsequent drought period, while high resilience was defined by the increase in species occurrence from a drought to a subsequent wet period. We found some unifying themes connecting the multiple drought events over the fifty-year period. Pelagic fishes consistently declined during droughts (low resistance), but exhibit a considerable amount of resiliency and often rebound in the subsequent wet years. However, full recovery does not occur in all wet years following droughts, leading to permanently lower baseline numbers for some pelagic fishes over time. In contrast, littoral fishes seem to be more resistant to drought and may even increase in occurrence during dry years. Based on the consistent detrimental effects of drought on pelagic fishes within the San Francisco Estuary and the inability of these fish populations to recover in some years, we conclude that freshwater flow remains a crucial but not sufficient management tool for the conservation of estuarine biodiversity.

Seasonally variable relationships between surface water temperature and inflow in the upper San Francisco Estuary

“…Our results indicate that flow‐related changes in water temperature may be a contributing mechanism that underlies a number of these flow‐abundance relationships. For example, Longfin Smelt ( Spirinchus thaleichthys ), a fish species listed under the California Endangered Species Act, exhibit a strong positive relationship with flow (Nobriga and Rosenfield 2016; Mahardja et al 2021), but the mechanism behind this relationship is not entirely clear (but see Grimaldo et al 2020). Larval and juvenile Longfin Smelt demonstrate sensitivity to high temperatures; thus warmer springtime water temperatures may partially explain their decline during droughts (Jeffries et al 2016; Mahardja et al 2017; Peterson and Barajas 2018).…”

“…Model 3: Incorporating long-term temperature change Water temperature in the upper San Francisco Estuary has risen over the past five decades (Bashevkin et al 2021), which may confound our results because dry years were more frequent in the latter half of our time series (Mahardja et al 2021). To assess whether long-term underlying temperature changes could affect our results, we fit model 3, which was identical to model 1 but with an additional component to estimate the long-term water temperature trend (i.e., climate change).…”

Temperature and salinity preferences of endangered Delta Smelt (Hypomesus transpacificus, Actinopterygii, Osmeridae)

“…The recent drought from 2012 to 2016 was followed by one of the wettest years on record for California (2017), which provided hope that a natural recovery would occur, similar to that observed in 2011. However, numbers of Delta Smelt did not increase substantially despite high outflow 10 , perhaps due to relatively high water temperatures in 2017 11 . Recent abundance trends strongly suggest that Delta Smelt are nearly extinct in the wild, and most recent surveys to locate Delta Smelt have failed to detect the species 12 , 13 .…”

Relative Bias in Catch Among Long-Term Fish Monitoring Surveys Within the San Francisco Estuary

“…Trawling gear is used by most fish surveys in the estuary (Stompe et al 2020). Data from these surveys have been used to understand the patterns and environmental drivers of individual species abundance to patterns of the entire fish community (Sommer et al 2007;Mac Nally et al 2010;Feyrer et al 2015;Colombano et al 2020;Mahardja et al 2021). However, differences in catchability can influence trends based on count data from these surveys.…”