Flow–recruitment relationships for Shoal Chub and implications for managing environmental flows

Abstract

ObjectiveRegulation of river flow regimes by dams and diversions impacts aquatic biota and ecosystems globally. However, our understanding of the ecological consequences of flow alteration and ecological benefits of flow restoration lags behind our ability to manipulate flows, and there is a need for broader development of flow–ecology relationships. Approaches for establishing flow–ecology relationships have recently shifted away from state‐based methods that analyze snapshots of ecological conditions and towards rate‐based methods focused on mechanisms that link hydrology with dynamics of important ecological components and processes.MethodsWe used a rate‐based approach to validate environmental flow standards developed for the lower Brazos River, Texas, by analyzing the relationship between flow regime components and recruitment strength of imperiled Shoal Chub Macrhybopsis hyostoma, a fluvial specialist and pelagic‐broadcast‐spawning fish. We collected 254 age‐0 Shoal Chub (9–40 mm total length), extracted their otoliths to estimate age in days, and used a generalized additive model to regress the number of captured recruits that hatched on a calendar date against flow regime metrics, such as pulse magnitude, flow rate of change, and pulse timing in relation to environmental flow standards proposed by a science advisory committee (Brazos Basin and Bay Area Expert Science Team).ResultThe model revealed that flow magnitude, rate of change, and timing were all significant predictors that collectively explained 60% of variation in the recruitment strength index. Hindcasting for 1919–2020 showed a general reduction in recruitment strength following commencement of flow regulation in the lower Brazos River and revealed that high recruitment correlated with years in which most or all proposed flow tiers were attained, whereas low recruitment correlated with years when less than half of the targeted tiers were attained.ConclusionOur work represents an effective validation method for environmental flow recommendations and reveals specific flow regimes that benefit an imperiled fish species.

Mesohabitat and macroecological correlates for blue sucker (Cycleptus elongatus) occurrence in regulated rivers

“…However, potential for the conservation of species sensitive to flow regime alteration still exists (Liermann et al, 2012), particularly in locations where fragmentation and flow regime alterations are not coupled. Our data support the use of the blue sucker as a flow‐sensitive species that can be used to guide environmental flow decisions (Freeman et al, 2022), particularly in states such as Texas where ecological validation of flow regime standards is needed (Perkin et al, 2023). Specifically, our work informs management decisions by (1) reinforcing the need to protect riffle mesohabitats that are subject to the effects of flow alteration (Acre et al, 2021), (2) informing ecologically relevant values of habitat fragmentation and flow alteration that should be preserved or perhaps restored in regulated rivers (Wohl, 2017), and (3) providing quantitative descriptions for physical characteristic of rivers where repatriation efforts might be attempted in the future to reverse the pattern of decline for species such as blue sucker (Cathcart, 2021).…”

The duality of drought: Pelagic‐ and benthic‐spawning stream fishes show opposing responses to drought in the southern Great Plains

“…We demonstrated how flow–ecology relationships can be used in predictive context by hindcasting the probability of occurrence across periods with historical flow records but only intermittent ecological data (see also Perkin et al. 2023, this special section). We found that probability of occurrence did not significantly change between 1950 to 1979 and 1980 to 2018, a finding that supports previous designation of the upper Brazos River as a refuge area for endemic Great Plains fishes (Hoagstrom et al.…”

“…For example, Shoal Chub recruitment positively correlated with achievement of recommended flow pulses set by an expert science team in the lower Brazos River (Perkin et al. 2023), and Nguyen et al. (2021) provided similar flow targets for pelagic‐spawning fishes in the upper Brazos River.…”

“…Still, flow rehabilitation, or the partial restoration of natural flow regimes, is known to elicit positive responses among native stream fishes even when entire flow regimes are not restored (e.g., Propst and Gido 2004;Kiernan et al 2012). For example, Shoal Chub recruitment positively correlated with achievement of recommended flow pulses set by an expert science team in the lower Brazos River (Perkin et al 2023), andNguyen et al (2021) provided similar flow targets for pelagicspawning fishes in the upper Brazos River. Our study provides indication of the levels of drought during which flow releases would best benefit pelagic-spawning fishes in the Brazos River and similar southern Great Plains streams.…”

Why are larger fish farther upstream? Testing multiple hypotheses using Silver Chub in two Midwestern United States riverscapes

“…We used total length data from all Silver Chub collected in the field (i.e., those retained for reproductive ecology research and those released) to fit a Bhattacharya model useful for estimating cohorts defined by modes within the length‐frequency histogram. We did not use otoliths to age fish because we were interested in using fish that were retained (otoliths available) and released (otoliths not available) for age estimates, though otoliths provide greater detail when aging (Perkin et al 2023; Wedgeworth et al 2023 [both this special section]). We used the “Bhattacharya” function from the “TropFishR” package to fit the model and develop probability density functions for each cohort detected in the data (Mildenberger et al 2017).…”