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

Abstract

AbstractObjectiveStreamflow is a primary determinant of fish assemblage structure in riverine systems, but alteration of natural flow regimes can result in fish assemblage shifts through the process of environmental filtering. Because natural drought conditions reduce and homogenize streamflow in a manner comparable to projections for climate change in some regions, drought could serve as a proxy for expected future flow conditions.MethodsWe investigated the effects of drought as a temporally dynamic environmental filter of the occurrence of two guilds of fishes, benthic spawners that deposit adhesive ova along the benthic zone of rivers and pelagic spawners that release semibuoyant, nonadhesive ova into the pelagic zone of rivers. We developed species‐specific random forest models to estimate annual probability of occurrence for three benthic‐spawning and four pelagic‐spawning minnow species at three sites in the upper Brazos River, Texas, for the period 1950–2018. We then used a generalized additive mixed‐effects model to assess the relationship between drought intensity and likelihood of occurrence to test whether reproductive modes differed in response to drought (hypothesis 1) and whether response to drought was spatially (hypothesis 2) or temporally (hypothesis 3) variable.ResultWe found support for hypothesis 1 as two of four pelagic‐spawning species (Shoal Chub Macrhybopsis hyostoma, Smalleye Shiner Notropis buccula) declined and two of three benthic‐spawning species increased as drought intensified, support for hypothesis 2 as responses to drought varied by gauge location, and no support for hypothesis 3 as drought response was consistent for periods 1950–1979 and 1980–2018.ConclusionThese findings offer insight into the future of riverine fish assemblages as climate change is expected to exacerbate regional drought conditions. Management of pelagic‐spawning fishes during extreme drought in the southern Great Plains may require strategies such as (1) rescues of fish from drying reaches, (2) captive holding and propagation, and (3) assisted recolonization following subsidence of drought conditions.