As precipitation and temperature patterns change, the resulting alterations in hydrologic conditions may adversely affect some stream fishes. The unique guild of freshwater, pelagic broadcast-spawning minnows found in the western United States appears to be particularly sensitive to low-flow conditions. We examined reproductive and recruitment patterns of one of these species, Rio Grande silvery minnow (Hybognathus amarus), over a three-year period, which included one year of extreme drought. We followed gonadal development from March through September each year, from 2017 to 2019, to determine if Rio Grande silvery minnow were reproductively active during extreme drought. The population structure was also monitored to confirm successful recruitment over the same period. We found that in all years Rio Grande silvery minnow were reproductively active in April through early June, including during extreme drought in 2018. However, almost no recruitment occurred in 2018, and by early 2019 the population was dominated by older, wild age-2 fish and hatchery-reared fishes. Our work supports previous research on pelagic broadcast-spawning minnows and confirms that extreme low-flows results in near-complete recruitment failure of Rio Grande silvery minnow. This work will help inform management and conservation of Rio Grande silvery minnow and other pelagic broadcast-spawning minnows during drought or low flows years.

Antipredator defensive traits are thought to trade-off evolutionarily with traits that facilitate predator avoidance. However, complexity and scale have precluded tests of this prediction in many groups, including fishes. Using a macroevolutionary approach, we test this prediction in butterflyfishes, an iconic group of coral reef inhabitants with diverse social behaviours, foraging strategies and antipredator adaptations. We find that several antipredator traits have evolved adaptively, dependent primarily on foraging strategy. We identify a previously unrecognised axis of diversity in butterflyfishes where species with robust morphological defences have riskier foraging strategies and lack sociality, while species with reduced morphological defences feed in familiar territories, have adaptations for quick escapes and benefit from the vigilance provided by sociality. Furthermore, we find evidence for the constrained evolution of fin spines among species that graze solely on corals, highlighting the importance of corals, as both prey and structural refuge, in shaping fish morphology.

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This study investigated a rapidly vanishing group of fishes in the genus Characodon (including Characodon lateralis and Characodon audax) from the upper Rio Mezquital drainage in Mexico. Using specimens from museum collections, morphological variation was assessed to quantify body shape differentiation among historic (i.e. extirpated and extant) collection sites. In both sexes, body shape (particularly head shape, the proportion of the caudal peduncle and the position and size of dorsal and anal fins) varied significantly among populations and species. Variation among collection sites could at least partially be attributed to geography, as the presence of distinct hydrographic units and a major waterfall coincided with major body shape differences. These results are discussed in the light of previously published molecular genetic analyses, as they have direct implications for taxonomic problems and the need for conservation measures for these endangered fishes.