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