Conservation of stream fishes requires an understanding of how their reproduction and recruitment are affected by environmental conditions. Flow regulation and habitat loss in North American Great Plains streams threaten many native fishes, including pelagic-broadcast spawning cyprinids. Pelagic-broadcast spawning by flathead chub (Platygobio gracilis), a species declining throughout much of its extensive range, began after temperatures exceeded 15 °C in Fountain Creek, Colorado (USA), despite different annual flows. Alternatively, streamflow patterns controlled recruitment of juveniles because high-magnitude summer flows (spikes) reduced abundance of larvae hatched just prior to or during such events. Juvenile recruitment was highest for larvae hatched just after summer flow spikes occurred or during stable base flow periods. Understanding environmental conditions suitable for reproduction as well as subsequent recruitment should inform conservation actions, especially in flow-altered streams, and improve prospects for persistence of imperiled Great Plains cyprinids, including flathead chub.

We studied age‐0 Flathead Chub Platygobio gracilis through two reproductive seasons in Fountain Creek, Colorado, to better understand effects of hatching date, water temperature, and streamflow on daily growth rates. Analysis of laboratory‐reared Flathead Chub of known age confirmed that daily growth increments were deposited in otoliths beginning at hatching and at a rate of one per day thereafter. That information enabled relatively accurate and unbiased estimation of age and daily growth rates of age‐0 fish hatched from May to August in 2012 and 2013. Flathead Chub growth was substantially higher in 2012 than 2013, and in both years, daily growth rate was faster for individuals captured earlier in the year and in warmer water temperatures. Growth was faster during stable low‐flow periods following short‐term flow spikes caused by runoff from thunderstorms but was slower when flows fluctuated over longer periods. Insights into the reproductive ecology and early life history of Flathead Chub, a species imperiled in portions of its range, demonstrated implications of hydrologic alteration on growth of fishes of North American Great Plains streams and may inform strategies to assist in conservation of remaining populations. Received August 24, 2015; accepted February 18, 2016 Published online July 7, 2016

Early life history ecology, in particular timing of hatch, is important in understanding population dynamics and community structure of stream salmonids. Larval stages are vulnerable to predation and physical disturbances, and their production is often regulated by annual variation in timing and magnitude of high flow events during egg incubation and post-hatch periods (Kovach et al., 2016). Timing of hatch influences variation in body size in early life stages (Yamamoto et al., 1997), which then mediates intra-and inter-specific competitive interactions (Blanchet et al., 2008;Peterson et al., 2004). Direct sampling of newly hatched fish is challenging in the subarctic region

Plains stream fishes in North America, including flathead chub Platygobio gracilis, are negatively affected by stream-flow alterations and fragmentation, and limited information on egg type and reproductive strategy hinders their conservation. On the basis of several lines of evidence, including laboratory culture, observations of reproduction in captivity, and capture and rearing of eggs from Fountain Creek, Colorado, we report that flathead chub produce nonadhesive eggs. Flathead chub eggs are relatively small at 2.3 mm mean diameter, have a greater yolk-to-egg volume ratio and thus sink faster, and take longer to hatch, compared with nonadhesive eggs from pelagic spawning species. Flathead chub are also longer lived compared with pelagic spawning species and the wider variety of habitat types they occupy may influence upstream egg retention. Although spawning mode (e.g., pelagic, lithopelagic, other) is incompletely known for flathead chub, habitat needs in terms of flows and reach lengths suitable for reproduction and recruitment may vary with habitat type but may be similar to that for other pelagic spawning species. Accommodating specialized reproductive life histories of fishes, including egg type and transport characteristics, in stream conservation planning may assist with maintaining or enhancing populations of all Great Plains cyprinids, including increasingly rare flathead chub.

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