Tuolumne River Report - U.S. Fish and Wildlife Service

TUOLUMNE RIVER TECHNICAL ADVISORY COMMITTEEHABITAT RESTORATION PLAN FOR THE LOWER TUOLUMNE RIVER CORRIDORCHAPTER 2High water temperatures during rearing and smoltemigration are perhaps the most significant damrelatedhabitat alteration (apart from flow reductionand sediment blockage) in the Tuolumne River. Forthe period 1966 to 1984 (CDFG 1987), flows in Mayduring smolt emigration were less than 5,000 cfs atLa Grange in 13 of 19 years, and 8 of these 13 yearshad median water temperatures at Vernalis (on theSan Joaquin River) exceeding 67.6 ºF (19.7 ºC).These temperatures were characterized by Rich(1987) as resulting in high chronic thermal stress.A study by Brett (1952) used chinook salmon fromthe Dungeness hatchery in Washington, anddetermined a range of upper incipient lethaltemperatures of 73 ºF (22.8 ºC) to 77 ºF(25.0 ºC) ,and 12-hour median lethal temperatures rangingfrom 71.2 ºF (21.8 ºC) to 80.6 ºF (27 ºC), dependingon the acclimation temperatures. Watertemperatures as high as 86 ºF (30 ºC) have beenrecorded near La Grange Dam in summermonths. These high water temperatures on theTuolumne River correspond to periods when thereare few salmon in the river, but likely was thecause for the decline in “yearling” or oversummeringsalmon. Comparable temperaturestudies using San Joaquin River basin chinookhave not been conducted, but results could differfrom those obtained using northern races.Not only are the effects of high water temperaturedirect (e.g., thermal stress, mortality), but hightemperatures may also contribute indirectly toother limiting factors such as bass predation,smolt survival during emigration, spawningdistribution, and incubation success. The firstyear of the fry emergence study (EA 1992)showed much lower egg survival (1-2%) thanpredicted based on gravel quality, and wasattributed to mortality caused by high incubationtemperatures. High ocean and Bay/Delta watertemperatures have also been shown to cause eggmortality prior to the salmon entering their natalstreams. In addition, beyond the seasonal affectsof high temperatures on specific life historyphases, high daily fluctuations in water temperature(ranging from 12 to 14 ºF daily) are knownto occur at low flows.High water temperatures are also most likelyresponsible for limiting habitat of yearlingchinook salmon. Low summer flows and resultanthigh water temperatures can be lethal to summerrearing. The revised FERC flow schedulesprovide better conditions for summer rearingduring wet years, particularly in the upper reachesnear La Grange Dam. According to the SNTEMPmodel, 300 cfs in the summertime would provide15 miles of habitat with suitable summer watertemperatures for summer rearing.2.4.2.7. Current status of chinookpopulationThe San Joaquin River basin and Tuolumne Riverfall-run chinook salmon populations havefluctuated widely during the period in which runestimates are available (1940 to present), and aregenerally characterized by a series of years withhigh escapements alternating with years ofextremely low escapements (Figure 2-35). Therecent drought (1987-1992) resulted in populationlevels as low as 100 returning adults for threeconsecutive years. More recently, a succession ofwet years has improved flow conditions. Changeshave also been implemented in Delta exportoperations and ocean harvest that have probablybenefited the salmon runs. Adult escapementshave rebounded to 3,300, 7,200, and 8,800 adultspawners in 1996, 1997, and 1998, respectively.Preliminary estimates indicate that the 1999escapement will approximate 1998.While population levels appear strongly related tostreamflows, improved hydrologic conditionsalone may not restore populations to levelstargeted by AFRP and CALFED. Recent improvementsin escapement levels may suggest thatwetter years alone have the potential to restoreand maintain a viable salmon population at lowescapement levels between 1,000 and 10,000. Butto sustain escapements of 30,000 or more salmonwill probably require a combination of improvedstreamflows and physical habitat. The AFRP hasset preliminary production targets for theTuolumne River fall-run chinook salmon populationat 38,000 adults (harvest and escapement).These target production levels are based on twicethe average escapement levels attained during theperiod 1967-1991 and a reduced harvest rate. TheCALFED program has not set specific productiontargets, but their explicit goal of restoringsustainable populations of species by improvingecological conditions, and their collaboration withthe AFRP program, suggests similar objectives.72