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The short larval ontogenetic phase has a large impact on success or failure of fish populations due to naturally high mortality rates. Yet, for the federally endangered fishes Colorado Pikeminnow Ptychocheilus lucius and Razorback Sucker Xyrauchen texanus, information about this sensitive life stage is limited. We determined species-specific age–length functions, spawning periodicity, and environmental factors related to growth and the onset of spawning by these species in the San Juan River of the Colorado River basin. Daily ages were determined from lapillar otoliths, and growth rates were calculated for subsamples of San Juan River larval Colorado Pikeminnows and Razorback Suckers collected from 2009 through 2017 and six candidate age–length functions were fit to evaluate the relationship between age and growth. Spawning periodicity estimates from the best-fit-model and published function were compared to those from observed otolith ages and differences were analyzed. The responses of spawning periodicity and growth to abiotic and temporal conditions were evaluated. For both species, spawning dates produced by otolith ages and best-fit functions were significantly different from those produced by the published functions, but not from each other. Age–length functions determined in this study enable more accurate back-calculation of ages and prediction of spawning periodicity than published functions. The spawning periodicity estimate of San Juan River Colorado Pikeminnow and Razorback Sucker were both impacted by multiple parameterizations of temperature and discharge. For both species, fish age had the biggest influence on growth. Accurate knowledge of spawning periodicity and factors affecting fish growth can improve timing of management activities to maximize benefit to Colorado Pikeminnows and Razorback Suckers in the San Juan River.

SUMMARY 1. Freshwater fish commonly exhibit differences in reproductive seasonality within assemblages. However, the extent of variation in reproductive timing among years is less well understood, particularly in relation to environmental variation. Phenological shifts in response to environmental variation, including climate change, can differ among species and thereby change temporal resource partitioning (e.g. in the use of nursery habitat, dietary items, etc.) and potentially the strength of biotic interactions such as competition. 2. To assess interannual variation in reproductive phenology and temporal resource partitioning in an arid-land river, we collected a data set on young-of-year fish in four years within an overall span of 16 years, from 1995 to 2010. Fish species spawned in a consistent order across years; however, the onset of spawning was between 4 and 28 days earlier in 2008–2010 than in 1995. Spawning onset advanced more for later-spawning species, resulting in reduced temporal partitioning across the assemblage. 3. Based on these observations, we developed four alternate models of assemblage-level responses to alteration of flow regime based on the magnitude of phenological shifts and corresponding temporal partitioning among species. Phenological data are consistent with only one of these models. 4. Convergence and overlap in spawning time among species may decrease resource partitioning at a critical life stage and alter assemblage composition in recruitment-driven systems like arid-land rivers. Understanding factors driving convergence and overlap in spawning time among species is key to predicting effects of ongoing climate change and alteration of flow regimes on fish assemblages.

This is the third compilation of imperiled (i.e., endangered, threatened, vulnerable) plus extinct freshwater and diadromous fishes of North America prepared by the American Fisheries Society'S Endangered Species Committee. Since the last revision in 1989, imperilment of inland fishes has increased substantially. This list includes 700 extant taxa representing 133 genera and 36 families, a 92% increase over the 364 listed in 1989. The increase reflects the addition of distinct populations, previously non-imperiled fishes, and recently described or discovered taxa. Approximately 39% of described fish species of the continent are imperiled. There are 230 vulnerable, 190 thretened, and 280 endangered extant taxa, and 61 taxa presumed extinct or extirpated from nature. Of those that were imperiled in 1989, most (89%) are the same or worse in conservation status; only 6% have improved in status, and 5% were delisted for various reasons. Habitat degradation and nonindigenous species are the main threat...

Several fish species in lotic systems are pelagic broadcast spawners that produce nonadhesive, semibuoyant eggs that drift downstream. This reproductive strategy and egg type appear to be common in Plains stream cyprinids in the west-central United States. Although it is relatively easy to capture semibuoyant eggs, the inability to provide species-specific identification of this life stage has hindered studies on the reproductive ecology and life history of these fishes. While drift nets have been used to collect semibuoyant eggs, the process of separating the reproductive products from other organic drift was time consuming and usually fatal for eggs. We developed a field sampling device, the Moore egg collector, that allowed for the efficient, quantitative, and nondestructive collection of large numbers of semibuoyant fish eggs and that could aid in the study of a variety of organisms that employ drift as a dispersal strategy during a portion of their life history.

Six Rio Grande basin cyprinid taxa compose a reproductive guild that spawns during increases in stream flow and broadcasts semibuoyant (drifting) eggs in the current. Spawning by guild members is naturally initiated by elevated flows during spring snow-melt or summer rainstorms, but reservoir releases now also result in spawning. Location or development of a material that mimics the physical properties of semibuoyant eggs of guild members would enable quantification of the displacement of drifting eggs and assessment of the additive impacts of stream flow, instream barriers, and habitat modifications on populations of these fishes. To initiate this search, the size and terminal velocity of guild members' eggs was determined and their specific gravity (SG=1.00589 ± 0.00011) calculated. Products with specific gravities similar to semibuoyant eggs were obtained, and tests were conducted on their rates of travel and settling times when exposed to current. Materials with specific gravities slightly gr...

Reproductive strategy and egg type of Hybognathus amarus, H. placitus, Macrhybopsis aestivalis, Notropis girardi, N. jemezanus, N. simus pecosensis, and N. stramineus were determined from laboratory experiments conducted between 1991 and 1995. The first six taxa were pelagic-broadcast spawners that produced nonadhesive, semibuoyant eggs, whereas N. stramineus was a broadcast spawner that laid demersaladhesive eggs. High-speed cinematography revealed that a spawning event consisted of a single male wrapping around the female's midsection and fertilizing the eggs upon expulsion. The perivitelline space of recently expelled nonadhesive eggs filled rapidly with water, thereby increasing both egg diameter and buoyancy. Semibuoyant eggs remained in suspension as long as water current was maintained. Discovery of the spawning behavior and egg types of these species allowed for the development of hypotheses to explain extirpations and extinctions of several endemic Rio Grande Basin fishes. We believe the synergistic effects of downstream transport of eggs and larval fishes and dam-related modifications of flow and habitat was probably responsible for the decline and demise of these taxa in the Rio Grande Basin.

growth, and provide further support for a winter breeding season in northwestern Louisiana. Dundee and Rossman (1989) reported that the probable time of hatching of sirens in Louisiana is midwinter. Courtship activity preceding a winter breeding season may explain the higher fall captures reported here. I thank L. M. Hardy for assistance and encouragement throughout the study. S. P. Ly ch provided assistance with the statistical anal ses, and A. C. Raymond prepared Fig. 1.

The Arkansas River shiner Notropis girardi was introduced into the Pecos River, New Mexico, in 1978 and was common in collections made in 1986-1987. From collections made since 1978 in the Pecos River drainage, we analyzed patterns of dispersal and abundance, and compared life history traits with a native population of N. girardi from Revuelto Creek, New Mexico. Notropis girardi dispersed downstream, presumably from a single introduction, colonizing much of the mainstream Pecos River (260 km) by 1981. Since 1981, the distribution and abundance of introduced N. girardi has expanded slightly. Age structure, growth and mortality rates of the Pecos River population are similar to those reported in the literature and to a native population of N. girardi in New Mexico. Timing of reproduction was similar for non-native and native populations, and both may exhibit multiple spawning peaks. Notropis girardi reproduction in the Pecos River coincided with large increases in flow. High discharge presumably initiates spawning and promotes downstream dispersal. Similarity of flow regimes of the Pecos River to those formerly found in lotic systems in the native range of N. girardi (e.g., Cimarron River, Kansas and Oklahoma) may account for the relative success of this introduced population.