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<br />R <br />I <br /> <br />C I <br />U <br /> <br />n <br /> <br />q <br />I <br /> <br />r <br />W <br />I[ <br />II <br />JJ <br />c../ <br />'" <br />~. <br /> <br />II <br />I.. <br />;"=;7/ <br /> <br />. ,,[ <br />~(_ J' <br />iJ <br /> <br />[1 <br />I <br /> <br />Fi <br />,__I <br /> <br />II <br />LJ <br /> <br />J <br />U <br /> <br />001<138 <br /> <br />2.1. BACKGROUND <br /> <br />The native fish community of the San Juan River evolved in a system characterized <br />by dramatic physical environmental changes. Flow varied from almost non-existent <br />to devastating floods. Seasonal and annual environmental changes wrought by <br />widely different flows were not predictable, but over time such variations were <br />certain to occur. Such extremes in abiotic conditions presented aquatic organisms <br />with environmental conditions that changed frequently in availability, quality, and <br />extent. Within the naturally variable system, species evolved strategies to utilize <br />the differentially available habitats and avoid competition for such. The selective <br />pressures of harsh environments contributed to low species richness and diversity <br />of the San Juan River. Although few in number, most native fish species are long- <br />lived. Different life stages (i.e., larva, juvenile, sub-adult, and adult) differentially <br />utilize available resources and thus function as distinct ecological species. Despite <br />a level of niche segregation, the ecological species are interdependent. A <br />surficially simple system belies a complex one of many subtle biotic and abiotic <br />interactions and dependencies. <br /> <br />The imperilment of the native fish community and extirpation of several native <br />species were caused by a variety of human-induced biotic and abiotic modifications <br />of the San Juan River ecosystem. Only four of the verified eight native fish <br />species in the San Juan River system remain comparatively common in the <br />drainage. Dams and diversion structures fragmented ranges, disrupted natural <br />thermal and flow regimes, altered sediment transport dynamics, and diminished <br />availability of seasonally required habitats; agriculture, industry, mineral extraction, <br />and urban development diminished water quantity and quality; artificial channel <br />structuring eliminated important habitats; establishment and encroachment of non- <br />native riparian plant species reduced the naturally varying river channel; watershed <br />management practices, exacerbated by natural climatic change, elevated sediment <br />loading of the river; and introduction and establishment of over 20 non-native fish <br />species and management of non-native sports fishes (including native fish <br />eradication) imposed additional competitive and predaceous pressures. The <br />strategies native fishes evolved to survive in a naturally variable and harsh <br />environment ill-prepared them for persistence in an artificial and human-modified <br />system. <br /> <br />Successful accomplishment of the Long Range Plan is dependent upon acquiring <br />and utilizing information from a variety of disciplines. Considerable effort must be <br />expended to characterize the biological attributes of individual fish species in the <br />San Juan River, intra- and interspecific interactions, relation of various abiotic <br />manipulations to the structure and dynamics of fish communities, and factors <br />which may limit any speCies or native aquatic communities. <br /> <br />4 <br />