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~ ~ - 2 - Task lC. Assess reprodu~iYe success and distribution of young humpback chub. Dip nets and small-mesh seines will be used to sample the l6-km-long river reach between the lower end of Black Rocks and the Westwater Ranger Station to estimate the relative abundance and distribution of larval humpback chub. Samples will be taken weekly at 3-km intervals for four weeks. In addition, more intensive larval sampling will be done within Black Rocks. Beginning in July, the above river reach will be sampled with l/S-inch-mesh seines on a monthly basis to estimate the relative abundance and distribution of post-larval humpback chub. Based upon preliminary information from the first year of study in both Black Rocks and Westwater Canyon, drift-net samples may be taken in future years. Task lD. A..e.. recruitaent to the adult population. The trammel netting and electrofishing data accumulated during the spring sampling will be used in conjunction with the fall monitoring data (ISMP) to aSSeSS the relative strsngth of the various size classes of the chub population. As data are accumulated, we can evaluate the relative proportion of young (based on size) and old chubs in the humpback chub population. We will also be able to determine whether more chubs are recruited into the adult population in some years than in others. This information, in conjunction with the estimate of reproductive success determined in task 5C, can then be used to determine what environmental conditions (e.g. flow, temperature or other variables) are most important to successful recruitment of humpback chub. Continued PIT tagging of all humpback chub collected may allow us to make an estimate of the population size in Black Rocks. In addition, the tagging and continual estimate of recruitment should allow us to determine whether the population is declining, increasing, or stable. Task lEI A.....m.nt of compo.ition of Black Rocks fi.h community. The regular netting conducted as part of this study and the monitoring conducted during ISMP will allow us to document any changes that may occur in the fish community in Black Rocks. Data collected by Yaldez et al. (19S2) and Kaeding et al. (1990) will be compared with data from this study to determine if any eignificant shifts have occurred. Changes in the ratio of humpback chub to roundtail chub collected from Black Rocks could be important in evaluating the stability of the humpback chub population. Further, significant changes in relative abundance of other native and non-native fish in the area could also be important to this evaluation. Task 2: Examine Preserved Specimens to Asss.. Recruitaent. The LFL will examine specimens of humpback chub from past collections to determine year class strength for the Black Rocks populations. Samples were collected in these areas in routine manner that will allow calculations of catch rates. Following the capture of a number of adult humpback chub in Yampa Canyon by Fish and Wildlife Service biologists, and the capture of reproductively mature humpback chub in the Little Snake River, the existence of a remnant population in the Yampa River appears likely. Assuming that this adult population originated in the Yampa River and has existed for some time, the objective of this project is to determine the past reproductive success of humpback chub in the Yampa River system by analyzing past collections of post-larval and early juvenile Gila. Similarly, the humpback chub population existing in the Colorado River at Black Rocks is currently being monitored, but the reproductive success of this population has only been assumed. This project also proposes to analyze past collections of early life stage Gila collected at Black Rocks.