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Migratory fishes often spawn across broad spatial scales to limit intraspecific interactions among mature and immature individuals, leading to long‐distance movements of both adults and offspring. Thus, offspring are potentially exposed to a range of environmental conditions throughout their development that affect growth, survival, and recruitment. Migratory suckers (family Catostomidae) can be used to test spatial variation in early life stage experiences for iteroparous fishes. We use the Flannelmouth Sucker Catostomus latipinnis, a fish that is endemic to the American Southwest, to illustrate how larval and juvenile stages are affected by the distance of upstream spawning migration in tributaries that vary in flow regime and habitat availability (i.e., perennial versus intermittent streams). Remote detection of PIT‐tagged migrating adults indicated that spawning and larval rearing sites were distributed over 32 km within McElmo Creek, a perennial tributary to the San Juan River, Utah. Physical sampling of an upstream intermittent tributary, the Mancos River, New Mexico showed that spawning adults—and then larvae—were restricted to within 1 km of the main‐stem river. Quantile regression demonstrated recruitment of larvae to the juvenile stage was significantly (i.e., up to 2 weeks) faster in more upstream reaches of the perennial tributary, potentially due to more constant thermal regimes. Spatial differences in animal community structure at sites with larval Flannelmouth Sucker indicated that the potential for predation and competition was greatest in upstream reaches. Small, rare, and even intermittent tributaries concentrate spawning activity that dictates larval sucker experiences. Conserving, maintaining, and enhancing connectivity via flows synchronized with spawning and rearing are critical steps to protect migratory suckers in arid environments.

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Grass Carp Ctenopharyngodon idella stocking in the upper Colorado River basin is statutorily limited to triploid, i.e., functionally sterile, individuals. Systematic fish sampling in the upper Colorado River basin indicates adult Grass Carp remain rare but have been increasing in abundance since 2007. Grass Carp larvae collected in the Colorado River arm of Lake Powell, about 26 km downstream from the inflow, in 2015 and 2016, are evidence of the presence of a spawning population. These larvae were the first documentation of Grass Carp spawning in a river basin of the western United States. While Grass Carp reproductive biology suggests spawning occurred in the Colorado River or its tributaries upstream from Lake Powell, the sample location, age, and number of larvae collected pose the possibility of reservoir spawning. Addition of another, reproductively viable, nonnative, fish species to the already heavily invaded upper Colorado River basin could further impact conservation of the federally endangered native fishes.