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D. Relation of Food to Larval Fish Survival. Year-class strength of marine fisheries is often related to the Match or Mismatch Principle where environmental conditions such as water temperature as well as the quality and quantity of food organisms must match the hatching of larval fishes to realize high survival (Hjort 1914, 1926). In other words, the density, size, time and duration of availability of zooplankton must "match" the timing of the swimup stage of fish larvae. When the yolk sac absorbed, food organisms of the right size must be available to fish larvae or they will starve. This principle was suggested by Marsh and Langhorst (1988) for razorback sucker larvae in Lake Mohave and documented for the razorback sucker larvae in the laboratory (Papoulias and Minckley 1990). Razorback sucker larvae of about 10 mm total length were maintained in the laboratory at 18 C. Unfed razorback larvae died in 10 to 30 days. Razorback larvae must find food of the right size and density in 8 to 19 days to survive. The "point of no return" when the fish died even though sufficient food of the right size was available was between 19 and 23 days for razorback sucker larvae. Papoulias and Minckley reported that the quantity of food required for survival of the razorback larvae was 20 brine shrimp nauplii per fish or high mortality occurred. E. Food Availability for Larval Endangered Fishes in the Upper Colorado River Basin. The larvae of all endangered Colorado River fishes feed on zooplankton early in life (U.S. Fish and Wildlife Service 1987, 1990a, 1990b, 1991). The first foods of larval razorback suckers in ponds were diatoms, detritus, algae, and rotifers (Papoulias amd Minckley 1992). Soon afterward, the razorback larvae began to select larger organisms such as zooplankton (primarily claderans). The density of zooplankton needed for larval razorback sucker survival (20 organisms per liter of water; Papoulias and Minckley 1990) occurred in flooded bottomland habitats along the Green River but rarely reached that density in backwaters, and never reached it in the river (Mabey and Schiozawa 1993). The decline in razorback suckers and the near extirpation of the bonytail in the upper basin may be linked directly to low or lack of larval survival in these species. Therefore, flooded bottomland habitats are important to some, if not all, of the endangered Colorado River fishes in the upper basin. Zooplankton produced in productive off-channel habitats such as flooded bottomlands also provide food of the proper quantity and size at the right time needed for larval survival of endangered fishes that occupy main channel habitats such as the Colorado squawfish and humpback chub. F. Loss of Critical Low Water Velocity Habitats such as Flooded Bottomlands in the Upper Colorado River Basin. Flooded bottomland habitats occurred along low gradient river reaches with broad valleys. In the upper basin, these reaches-have been channelized by levee development to contain the river near 27 1 I