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8 known-age larval series obtained from Willow Beach National Fish Hatchery and published descriptions (Snyder, 1981). Humpback chub. At this time, it is not possible to reliably separate larval/juvenile stages of Gila either in the field or laboratory and this problem is addressed in objective three. This project is designed to evaluate three critical research areas relative to the humpback chub: 1. To evaluate the validity of standard morphomeristic techniques for the differentiation of known (hatchery-reared) YOY/juvenile roundtails, humpbacks, and bonytails. 2. To employ the results of (1) above for the identification of field collected immature chubs. 3. To determine and describe any variation in physical habitat requirements and other biotic associations for sympatric Gila spp. In order to meet the first research need (above), known-age larval series of humpbacks and bonytails have been obtained from Willow Beach National Fish Hatchery (Arizona) and Dexter National Fish Hatchery (New Mexico). Additionally, developmental series of six hybrid crosses have been provided and will be evaluated i.e. humpback x humpback - bonytail bonytail x roundtail - bonytail roundtail x bonytail roundtail x roundtail - bonytail humpback x bonytail bonytail x humpback - bonytail The original humpback chub brood stock was captured at Black Rocks. Bonytail brood stock was from Lake Mojave, California. Unfortunately, roundtail brood stock was captured at Black Rocks and was therefore sympatric with humpback material. Therefore, we are attempting to produce a developmental series from the Juniper Springs area of the Yampa River and will also utilize a series from the White River (Rio Blanco County). Since no historical humpback records exist for these latter localities, we assume a high degree of genetic purity. Standard morphomeristic techniques are described in Appendix B. Data for each known developmental series will be statistically evaluated to determine if key identification characters can be meaningfully determined. Multivariate statistical testing will involve both principal components and discriminant function analysis, following procedures employed by Daly and Balling (1978), Davis and Baker (1974) and Smith et al. (1979)• A differentiation of unknown field-collected Gila larvae into humpback, roundtail, "intergrade" and/or "hybrid" will follow the laboratory phase above and will be a function of the success of the laboratory research and the degree of morphomeristic divergence of these closely-related species. Similarly, the recognition of variation in habitat requirements will also _ relate to laboratory findings.