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k' or other means and served by surface inflows and outflows in addition to groundwater (fig- ure 5a, 5b). Other options include using more distant intakes (figure 5b, 5c), positioning habi- tats between canals, or integrating them with river-drain or canal-drain features (figure 5c, 5d). Single inlets and outlets for multiple habi- tats reduce construction costs. Floodplain position is critical. A complex behind a levee is as well protected from invasion by nonnatives during floods as the structures and uses the levee was originally designed to protect are. If inside the floodplain, however, a complex must at least be protected from high flows. Multiple complexes need to be spaced along the river and other protection imple- mented to ensure against loss of all the com- plexes from a single catastrophe. Severe, un- controlled flooding may destroy both protected and unprotected complexes, but the presence of adults in the river channel plus connectives precludes loss of whole species. Indeed, big- river fishes of the Colorado persisted through millennia that featured floods of greater volume than the controlled river can produce today. Drying of off-channel habitats during low flow or water outage is a concern that can be circumvented by ensuring that the bottom is below the water table. Arranging complexes from upstream to downstream will also benefit management by re- ducing transport distances and assuring a diversity of habi- tats for adult fish. Land ownership and topography (fewer sites exist in canyons) also control placement, number of habitats per complex (e.g., figure 5), and number of complexes. Areas al- ready reserved for state and federal use, either undeveloped or within existing wildlife refuges, are obvious choices. Deeded and Native American property may also be leased or pur- chased. Because large numbers of fish are required, we recom- mend a habitat configuration avoiding as many problems in harvest as possible. Decisions must be made a priori on methods, extent, and season of harvest; access, holding, and transport of fish; and agency responsibility, gear limitations, and manpower. Other major questions include how much area per habitat, how many habitats per complex, and how many complexes are needed. Habitat size should be a function of ease of control of nonnatives, which are certain to appear. Thus a complex would be better if it had a number of small units rather than a single large one. We recommend no fixed size, but 1 to 2 hectares (ha), or 2.5 to 5 acres, per habitat seems optimal. We judge that less than a hectare would be too small to provide the diversity and productivity that is required to simultane- ously accommodate adults, larvae, and fast-growing juve- niles of one species, or all life stages of multispecies popula- tions, if such are developed. Habitats tens of hectares in size or larger are too large to harvest and too large for efficient water exchange (local conditions become lentic and thus more susceptible to problems of high temperature, oxygen de- pletion, and other physicochemical extremes) or for manip- ulations such as complete renovation (fish removal). We fa- vor elongate, narrow shapes to promote uniform water passage, and a depth that inhibits rooted aquatic plants. Habitat heterogeneity (e.g., lotic [near intakes, outlets, or both] to lentic; shallow to deep; gravel to silt or sand [natural] substrate) may be spatially or temporally manipulated as de- sired. Ten complexes seem a reasonable goal. The number of off-channel habitats per complex depends on availability of land, security, and other factors and can vary from one to many. Answering the question of how many are needed de- pends on the numbers of fish desired. Population goals We do not quantify the numbers of fishes required to satisfy level III or above of the conceptual plan for managing lower Colorado basin native fishes (i.e., population stabilization, ex- pansion, and recovery; figure 3, USFWS 1996), but we pro- vide examples to support our proposal. We are convinced that large populations and high genetic diversity are the only sound biological options for all four species, and these are fea- sible and sustainable through dedicated management Even a modest effort using off-channel habitats will yield popula- tions far exceeding the meager 700 to 5800 individuals pro- posed by USFWS (2002x, 2002b, 2002c, 2002d). We advocate and describe means of producing and rearing recruits in iso- lated habitats for introduction en masse into the channel plus connectives to establish and maintain a large, genetically diverse, panmictic population that closes the circle by supplying brood fish for ongoing production in isolation (figure 6). 228 BioSdence • March 2003 / Vol. 53 No. 3 Figure 5. Some potential arrangements far hypothetical lower Colorado Fiver off-channel habitats.