Dryland stream fishes are adapted to highly connected habitats with unpredictable hydrologic conditions, including frequent low flows and sometimes extreme drought.The low flow recruitment hypothesis predicts that some fishes spawn in main channel habitats during low flows when water temperatures and prey densities are high.However, extreme low flows during drought periods might be disruptive even among fishes whose life histories otherwise benefit from lower flows. We studied recruitment dynamics of six fishes (family Cyprinidae) at 15 sites in a fragmented Great Plains riverscape in Kansas, USA, during 2 years of extreme drought. We tracked the chronology of gonadal development and age 0 recruitment to test the hypothesis that recruitment by fishes that broadcast spawn planktonic ova would be less successful compared with fishes that spawn demersal or adhesive ova. We found all six taxa were reproductively active but recruitment was evident for only four.The two species for which recruitment was not evident, peppered chub (Macrhybopsis tetranema) and silver chub (Macrhybopsis storeriana), are confirmed or suspected pelagic-broadcast spawning fishes that have declined in other fragmented and dewatered Great Plains riverscapes. Our data highlight the potential for extreme low flows within isolated stream fragments to cause complete or near-complete recruitment failure for pelagic-broadcast spawning fishes, especially those with small population sizes. Failed recruitment during extreme drought may be related to spawning mode, ova characteristics, or both. Our work informs management of fish diversity in drought-prone riverscapes by establishing mechanistic linkages among extreme drought, fish recruitment, and assemblage structure.

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ABSTRACT1. Effective conservation measures for endangered species require basic knowledge of habitat use and critical environmental variables influencing the species' occurrence. However, setting priorities may be difficult when multiple endangered species inhabit the same area and differ in habitat use.2. This study characterized the physical and chemical environment at 30 sites along a 2.5 km stretch of stream associated with the Baños del Azufre hydrogen sulphide spring complex, home to the only population of the widemouth gambusia (Gambusia eurystoma) in existence. Also present in the spring is Poecilia sulphuraria, a narrowly endemic and endangered species known from only a few populations. This study provides the most detailed report to date for physical and chemical drivers of species density in this small, extreme environment.3. Gambusia eurystoma were generally rare throughout the stream, and stream flow and substrate size were the best predictors of G. eurystoma densities. Conversely, P. sulphuraria were considerably more abundant, but no physical or chemical variables predicted their density among sites. Size distributions of adult P. sulphuraria were significantly influenced by stream flow and water chemistry, indicating a cost to living in greater proximity to toxic springs with high hydrogen sulphide concentrations.4. Overall, the results provide the first extensive report of environmental variation and factors associated with G. eurystoma densities, and indicate that conservation measures prioritizing environmental conditions for G. eurystoma will also benefit P. sulphuraria. Fish in sulphide springs provide prime examples of narrowly endemic species in desperate need of conservation, and their habitats face conservation challenges similar to those found in other springs with sympatric, endemic species in the world's arid regions.

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