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....a ....~;! 1 drifting, regardless of sample time. The native species (flannelmouth sucker, bluehead sucker, roundtail chub, and speckled dace) were captured at the highest frequency in drift during their mesolarva1 phase of development (Fig. 5). A relative increase in late mesolarvae and metalarvae in shoreline seine hauls was accompanied by a concurrent decrease in these forms in drift samples. The introduced white sucker, also a riverine species, showed the same propensity to drift as mesolarvae. However, the fathead minnow, an introduced nonriverine species, exhibited no distinct pattern of drifting by length (Fig. 6). The life history or phenology of the native species may help to explain the observed leng~h-specific drift phenomenon. As newly-hatched proto1arvae, these fishes have en1argened yolk sacs and little mobility since they lack distinct fin rays or spines. Having been hatched from eggs deposited in gravel bars, these proto1arvae lie in the relatively quiet interstitial spaces of the spawning area. As mesolarvae, the fish still lack fully-developed fins, fin rays, and musculature for full mobil ity. But at this phase of development, the yolk sac is nearly absorbed and the mouth and gut are developed for active feeding so that the fish emerge from the interstitial spaces. It is apparently at this developmental phase that many of the young fishes become exposed to the river currents and are transported downstream. Some are probabl y swept immed ia te 1 y to shore whil e others are transported varyi ng distances. This drift phenomenon may also serve as an effective feeding mechanism. Even with their apparent limited mobility, diel patterns indicate that these mesolarvae are able to escape surface drift by either descending to the benthic zone or by moving to shoreline habitats. Their low abundance in shoreline habitats suggests that many fish descend into the hard-to-sample benthic zone. In metalarva1 and juvenile phases, the fish have a nearly full complement of fins and fin rays as well as musculature to provide for full mobil ity. It is at these phases that the fishes concurrently dwindle from the drift and increase in abundance along the shoreline. This is particularly apparent for f1annelmouth suckers and speckled dace (Fig. 5). It is hypothesized that the fish are sufficiently mobile to actively escape drift and find their way to the sheltered shoreline habitats. RESULTS OF PALISADE STUDY Specles Composition And Relative Abundance Of the 14 fish species encountered at the Palisade Site, only 8 were found in drift samples (Fig. 2). In addition to these 8, largemouth bass (Micro terus salmoides), carp (Cyprinus carpio), red shiner (Notropis lutrensis , channel catfish (Icta1urus melas), brown trout (Sa1mo trutta), and rainbow trout (Salmo gairdneri) were captured with other gear types, but not as drifting larvae. These 6 species are non-native to the Upper Colorado River Basin. The five native species (bluehead sucker, 179