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In the proximate sense, populations of razorback suckers have declined because recruitment has been insufficient to maintain population numbers. Some reduction in recruitment is attributable to alterations to the physical habitat, or reduced access to suitable habitat. But even with major modifications to physical conditions, razorback suckers continue to spawn in riverine and lacustrine habitats. A number of investigators have collected viable embryos and/or larvae in areas where razorback suckers have been observed spawning (Bozek et al. 1984, Medel-Ulmer 1983, Marsh and Langhorst 1988, Tyus 1987, Mueller 1989), but few have collected larvae larger than 14 mm. Many small larvae are collected in certain areas (e.g. Lake Mojave), but the small number of larger larvae and juveniles suggests that recruitment is curtailed at this point. The failure of adequate recruitment is largely attributable to predation by nonnative fishes (Minckley et al. 1991, Johnson et al. 1993, Tyus and Saunders 1996). Razorback suckers now exist in areas where the populations are geographically isolated. With the possible exception of the population in Lake Mojave, all razorback sucker populations are small. Small, isolated populations (isolates) are very susceptible to "faunal collapse," a phenomenon that has been observed when such isolates are invaded by nonnative species (Wilcox 1980, Frankel and Soule 1981). Increasing fragmentation of the Colorado River system presents problems that are presumably very similar to those observed in isolates elsewhere, where declining habitat diversity and introduction of new predators are main reasons for declines of native species (e.g., see Frankel and Soule 1981). Thus, the impact of nonnative fishes is considered a significant detriment to isolates of a once larger Colorado River native fish community (reviewed by Tyus and Saunders, 1996). Conservation and Recovery Recovery Planning Recovery plans, written under the authority of Section 4 of the Endangered Species Act, guide recovery activities. A recovery plan promotes conservation and provides the steps required for delisting species. In addition, a recovery plan provides guidance for implementing recovery actions and establishes priorities for those actions. The Act incorporates several measures to promote conservation of listed species, making the conservation of endangered species a high priority of Federal agencies (Section 2). The act also aids recovery by identifying the status of a species by listing and identification of critical habitat (Section 4), providing Federal grants to States (Section 6), requiring Federal agencies to engage in conservation activities (Section 7), prohibiting the unauthorized take of listed species (Section 9), requiring permits to enhance survival of listed species (Section 10), and other means (e.g., research, land acquisition, etc.). All of these measures are brought into action by recovery planning, and the completion of recovery plans. 24