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, Recovery Program and the San Juan Basin Recovery Implementation Program. The only remaining population of (partially) wild razor- back sucker inhabits the lower through middle Green River and lower Yampa River, but despite evidence of successful reproduction through the annual capture of larvae and supplementation with hatchery-reared fish, it continues to decline (Bestgen et al. 2002). Elsewhere in the UCRB, wild fish have not been collected since 1981 in the lower Gunnison River, 1995 in the Colorado River near and downstream of its confluence with the Gunnison River, and 1988 in the middle and lower San Juan River (McAda 2003, Platania et al. 1991), but small populations have been maintained or reintroduced by stocking in those reaches (Ryden 1997, Burdick 2003). Continued presence in the lower ends of other tributaries to Lake Powell (Bestgen 1990) is unknown. Monitoring of larval production has documented recent razorback sucker reproduc- tion, presumably by stocked fish, in both the lower Gunnison River (Osmundson 2002) and the middle and lower San Juan River (Bran- denburg et al. 2003). Flannelmouth, bluehead, and white suckers are the most widely distributed catostomids in the UCRB. Flannelmouth sucker and bluehead sucker remain common in the main-stem rivers and larger tributaries below Flaming Gorge Reservoir, but some populations are declining and both species are of special concern in Utah and Wyoming (Bezzerides and Bestgen 2002). White sucker is common in the Colorado, Gunnison, Yampa, and middle and upper Green Rivers, especially in upstream reaches. It also has been reported in the Duchesne River and in and below Navajo Reservoir at the upper end of the San Juan River. The status and distribution ofthe remaining UCRB catostomids are poorly documented and less certain. Mountain sucker, a Colorado spe- cies of special concern, is mostly restricted to headwater tributaries throughout much of the Green River Subbasin. Although rarely found in main-stem rivers, individual specimens of mountain sucker had been reported in the Green River near the confluence with the Yampa River and in the White River near and above the confluence with Piceance Creek. In the Colo- rado River Subbasin, it has been reported in headwaters of Dirty Devil River (Fremont River) in Utah and the Colorado River in or below Lake Granby, Colorado, but its historical or continued presence at the latter location remains unconfirmed. Utah sucker is restricted largely to portions of the Duchesne River drain- age and upper reach of the Fremont River, with incidental occurrences reported in the Green River in or below the lower end of Dinosaur National Monument. Longnose sucker is report- ed or presumed present in most middle and upstream portions of the Gunnison River Basin and is especially common in reservoirs of the Aspinall (Curecanti) Unit, but it has been col- lected as far downstream as River Kilometers 48 to 67 (Burdick 1995). It also has been reported in headwaters of the Colorado River in and above Lake Granby and probably is present in the river and tributaries for some distance below the lake. Longnose sucker no longer appears to be present in the upper reaches or tributaries of the Green River above Flaming Gorge Reservoir as historically reported. The distribution and ecology of catostomid larvae and young-of-the-year juveniles in the UCRB have not yet been summarized, except for razorback sucker in the Green River by Muth et al. (1998). However, selected information can be found in various publications and reports by regional researchers (e.g., McAda 1977, Carlson et al. 1979, Miller et al. 1982a, Haynes et al. 1985, Carter et al. 1986, Tyus et al. 1987, Gutermuth et al. 1994, Burdick 1995, Muth and Snyder 1995, Modde 1996, Bestgen et al. 2002, Bezzerides and Bestgen 2002, Osmundson 2002, Brandenburg et al. 2003). A Combined Developmental Interval Terminology It is often convenient and desirable to divide the ontogeny of fish into specifically defined intervals. If the intervals selected are used by many biologists as a frame of reference, such division can facilitate communication and comparison of independent results. The largest intervals, periods (e.g., embryonic, larval, juvenile, and adult), are often subdivided into phases and sometimes into steps (Balon 1975b and 1984); the word "stage," although com- 5