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II..' 1 'f~ , I :i, ! ,; I! I I ; ! 1,..--'.'.. i i ! I ; l' j 394 Recovery of Long-lived Species collectively maximized in field studies con- flicted with the model's prediction. Subse- quent evaluations revealed that aerial photo- graphic mapping was a more appropriate means of evaluating these habitats. Flow recommendations must also consider temperature effects on larvae, because in- creased flows of colder water also lowered av- erage temperatures in the Green River up- stream from Desolation Canyon. We noted a temperature differential of 100C at the junc- tion of the Green and Yampa rivers in 1983, whereas in better recruitment years the differ- ential was usually less than 20C (1979 = oOC, 1980 = I.S oC, 1981 = I.SoC; Tyus et a1. 1987). Berry (1988) demonstrated that 100- 150C cold shock adversely affected larval Col- orado squawfish (fourteen days old, 9.0 :t 0.3 mm TL). No such effects were noted for larger, forty-day-old fish (24.4 :t 0.4 mm TL). Cold temperatures during larval drift could be im- plicated in the partial loss of the 1983 year class. However, temperatures in 1984 (3.00- 60C) did not approach this differential and thus do not explain loss of that year class. Also, similar losses occurred in the Green River downstream of the Gray Canyon spawning area, a reach that was not affected by low temperatures from the dam, but was affected by high discharges as in the upper river. Temperatures in the few backwaters ob- served during USFWS autumn sampling in 1984 averaged 22.80C; main-channel temper- atures were 19.5oC (Tyus et a!. 1987). Thus, field data supported the concept of a loss of recruitment due to high discharges that flooded ephemeral backwater habitats. Low temperature can have an influence, but it was not the primary factor limiting larval produc- tion in 1983 and 1984. Yampa River Habitat use and stream-flow needs of rare and endangered fishes in the Yampa River were presented by Tyus and Karp (1989), who evaluated requirements of fishes by interpret- ing empirical biological and discharge data, and then recommended flow regimens. Protec- tion of flow and temperature regimens for ini- tiation of spring spawning migrations and flows, temperatures, and sediment transport conditions during spawning were empha- sized. Based on habitat use and behavior, they demonstrated that high spring discharges in concert with increasing water temperatures were predictably associated with initiation of spawning migration. Decreasing discharges in early summer to midsummer were associated with successful spawning and downstream drift of larvae to nursery habitats. Existing winter base flows appeared adequate for main- tenance of winter habitat conditions (Wick and Hawkins 1989). Tyus and Karp (1989) also reviewed out- puts of IFIM/PHABSIM modeling of Colorado squawfish requirements developed in 1984 for staging and spawning (Archer and Tyus 1984; Rose 1984), and in 1987 for spawning, for possible use in developing discharge recom- mendations. Recommended discharges at which 90% simulated spawning habitat was obtained varied between 9.9 and 31.1 m3 S-1 (HO-IIOO ft3 S-1) for staging and 8.5-42.5 m3 S-1 (300-1500 ft3 S-1) for egg deposition, using habitat utilization curves developed in 1984. However, curves produced in 1987 pro- duced different results. Adult habitat was op- timized as flows approached zero, and spawn- ing habitat was optimized at 42.S m3 S-1 (IS00 ft3 S-1). Furthermore, some discharges presumably optimizing habitat for other en- dangered species conflicted with those pre- ferred by squawfish. The model outputs were rejected in deference to interpretations of em- pirical results. Many constraints have been placed on the use of IFlM habitat-suitability curves (Valdez et a1. 1987), and our findings indicated that a simple three-variable model (depth, velocity, and substrate) did not accu- rately or adequately describe relations be-