The San Francisco Estuary supports several endemic species of fish and the southernmost populations of other species. Many of these native species and populations are imperiled or have experienced recent population declines that indicate a general decline in the estuary's capacity to support pelagic fish species. We studied the distribution and abundance of one of the estuary's native species, longfin smelt Spirinchus thaleichthys, using data from three long-term aquatic sampling programs. Each of the sampling programs we studied revealed a substantial reduction in the abundance of longfin smelt. These trends support the idea that the estuary's capacity to support pelagic fish species has been significantly reduced over the past three decades. Longfin smelt in the estuary displayed consistent patterns in relative abundance and distribution during their life cycle. We also found significant, but weak, spatial autocorrelation among sampling stations. These patterns in distribution reveal differential habitat use and migratory behavior. Managers can use these insights into longfin smelt distribution patterns to improve interpretation of sampling program results.

North American freshwater fishes were studied to determine whether they displayed the same relationships between log (geographical range size) and log (body size) and the same pattern of range shape as found among North American birds and mammals. The forces that produce these patterns were also investigated. The log (geographical range size) : log (body size) relationship was analysed for 121 North American freshwater fish species. Thirty-two imperilled species were compared with 89 non-imperilled species to determine if the overall relationship could result from differential extinction. Range geometries were analysed, within and among habitat guilds, to determine if general patterns could be detected. The log (geographical range size) : log (body size) pattern among freshwater fish species was triangular and qualitatively similar to that found for North American birds and mammals. The results suggest that below a minimum geographical range, the likelihood of extinction increases dramatically for freshwater fishes and that this minimum range size increases with body size. The pattern of fish species' range shapes differs from that found for other North American vertebrate taxa because, on average, fish possess much smaller ranges than terrestrial species and most fish species' geographical ranges extend further on a north-south axis than on an east-west axis. The log (geographical range size) : log (body size) pattern reveals that fish species' geographical ranges are more constrained than those of terrestrial species. The triangular relationship may be caused by differential extinction of species with large bodies and small geographical ranges as well as higher speciation rates of small-bodied fish. The restricted geographical ranges of freshwater fishes gives them much in common with terrestrial species on oceanic islands. Range shape patterns within habitat guilds reflect guild-specific historical and current ecological forces. The overall pattern of range shapes emerges from the combination of ecologically different subunits.

Rapid and extensive genetic introgression has occurred between Pecos pupfish (Cyprinodon pecosensis) and sheepshead minnow (Cyprinodon variegatus) in the wild. We studied both female mate choice and male-male competition for mates among C. pecosensis, C. variegatus, and their F1 hybrids to determine what role these behaviours played in the formation of the hybrid swarm. Female C. pecosensis preferred male C. variegatus to conspecific males, C. variegatus females displayed no significant preference when given a choice between purebred males, and neither C. pecosensis nor C. variegatus females discriminated against F1 hybrid males. We found no evidence for female olfactory recognition of mates. Male F1 hybrids and C. variegatus were more aggressive than C. pecosensis males, achieving greater reproductive success under two different experimentally-induced mating systems. Hybrids were superior to C. variegatus when only two males competed (dominance interactions), but the two types were competitively equivalent in a territorial mating system. Our results indicate that active inter-and intra-sexual selection contributed to the accelerated hybridization between these two species. By including the possibility that some aspects of a hybridization and introgression event may be under positive selection, researchers may better understand the dynamics that lead to hybrid zone stability or the spread of introgressed genetic material.

Many species are jeopardized by hybridization and genetic introgression with closely related species.Unfortunately, the mechanisms that promote or retard gene flow between divergent populations are little studied and poorly understood. Like many imperiled fish species, the Pecos pupfish (Cyprinodon pecosensis) is threatened with replacement by its hybrids with a close congener. We examined swimming performance and growth rate of hybrid pupfish to determine the role of hybrid vigor in the genetic homogenization of C. pecosensis by its hybrids with sheepshead minnow (C. variegatus). The F1 hybrids, backcross hybrids, and purebred C. variegatus displayed greater swimming endurance than purebred C. pecosensis. In addition, F1 hybrids and C. variegatus grew more rapidly than C. pecosensis. The ecological superiority of hybrids probably promoted their rapid spread through and beyond the historic range of C. pecosensis. These results indicate that eradication of hybrids and restoration of C. pecosensis to its native range is unlikely. Extinction of unique species via genetic homogenization can result from human activities that increase gene flow between historically fragmented populations; conservation managers must weigh the potential for such a catastrophe against the presumed benefits of increased interpopulation gene flow. This example illustrates how, after hybridization has occurred, conflict may arise between formerly complementary conservation goals.El Papel de Vigor Híbrido en el Reemplazo de Cyprinodon pecosensis por Sus Híbridos con C. variegatus Resumen: Muchas especies están en peligro por hibridación e introgresión genética con especies cercanamente relacionadas. Desafortunadamente, los mecanismos que promueven o retardan el flujo génico entre poblaciones divergentes son poco estudiados y comprendidos. Como muchas especies de peces en peligro,Cyprinodon pecosensis está amenazada con reemplazo por sus híbridos con un congénere cercano. Examinamos el desempeño natatorio y la tasa de crecimiento de peces híbridos para determinar el papel del vigor híbrido en la homogenización genética de C. pecosensis por sus híbridos con C. variegatus. Los híbridos F1, los híbridos de retrocruza y C. variegatus puros mostraron mayor resistencia natatoria que C. pecosensis puros. Adicionalmente, los híbridos F1 y C. variegatus crecieron más rápidamente que C. pecosensis. La superioridad ecológica de híbridos probablemente promovió su rápida dispersión dentro y más allá del rango histórico de C. pecosensis. Estos resultados indican que la erradicación de híbridos y la restauración de C. pecosensis en su rango nativo es poco probable. La extinción de especiesúnicas por vía de homogenización genética puede ser resultado de actividades humanas que incrementan el flujo génico entre poblaciones históricamente fragmentadas; los gestores de conservación deben sopesar el potencial de tal catástrofe con los supuestos beneficios

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