7786 - Laserfiche WebLink

Home Browse Search

~, ~. 468 EMLEN ET AL. numbered in the hundreds of thousands. The most recent large run, in 1969, was speculated to contain about 100,000 fish. Adults originating from the last major run before 1969, the one in 1950, are now few, contributing only slightly to recruitment. From 1970 to 1979 there was only modest re- cruitment from some river access in high-water years and from spawning in the Pyramid Lake fishway. In the 8 years from 1980 through 1987, there were seven runs; with the exception of the run in 1986, these were very small (average, 12,000 fish; range, 5,000-36,300). The 1986 run, occur- ring in a 100-year-flood year, produced about 200 million larvae and, with some contribution from other smaller runs, led to an approximately three- fold increase in adult population from about 100,000 adults in 1983 to about 300,000 in 1990. Since 1987, however, there have been no runs at all, the result of drought and human water use (diversion and storage). The apparent dramatic decline in the cui-ui pop- ulation led the federal government to institute pro- tective measures in 1967 and eventually to list the species as endangered. Because upriver passage is frequently blocked by a delta (Scoppettone et at. 1986), the Marble Bluff Fish Facility, including a dam and a fish way, were constructed in 1975 to facilitate migration. However, only a tiny fraction of the potential spawners use this fish passage in low-water years. Upriver passage still depends on high lake levels and high flows to inundate the delta. It is not only access to the lower Truckee River that requires critical amounts of water. The pre- spawning aggregate forms only if there has been sufficient "attractant" flow in the river for several weeks before passage is to occur (Scoppettone et al. 1986). In addition, variation in reproduction and larvae survival appears to be overwhelmingly controlled by hydrology (Scoppettone et al. 1981). Finally, viability of cui-ui eggs is temperature de- pendent (Coleman et at. 1987) and river temper- ature is strongly related to flow. The plight of the cui-ui thus comes into focus when we consider that for almost a century this species has had to com- pete for water with humans, whose agricultural and other demands have cut the supply to Pyramid Lake by an average of 250,000 acre-feet per year, approximately 50% of the historical runoff. This decrease in river flow has brought reductions in the frequency and size of the spawning migration and, via ~sed temperatures, higher egg mortality. Because cui-ui live to about 40 years, a 40-year sequence of dry years, or years in which human water use is such to preclude spawning, might ex- terminate the population. There is no record of drought lasting this long, and no reason to believe that anthropogenic effects will produce such a long run of poor years for the fish. On the other hand, a series of poor years could lead to cumulative damage that would eventually result in extinction. Thus, there is no assurance the population would disappear after 40 sterile years and, similarly, no guarantee it would persist after a few much shorter sequences of poor years. Extinction is largely a function of the probability that sterile-year se- quences of whatever duration will threaten per- sistence of the cui-ui. This probability is related to population size. The chance in any given year that at least one male and one female fish of re- productive age are present, and that at least one such pair will spawn, increases with the number of individuals in a population. Thus the likelihood of extinction rises markedly with decreasing pop- ulation size of spawners. Finally, total population size invariably drops following years without re- production, and is likely to drop even if repro- duction occurs when low water level limits the number of spawners or survival of the eggs. It follows that even when poor conditions occur in sets of much less than 40 years, a disconnected but sufficiently proximate string of such short se- quences easily could lead to a collapse of the cui- ui population. Clearly, recovery depends on the restoration, or at least partial restoration, of the local historical hydrology. But will supplemental water result in cui-ui persistence and recovery? To answer this question it is necessary to predict future cui-ui populations as a function of the amount of water restored to the lower Truckee River. The following presentation is offered as an exposition and illus- tration of a general approach to population pro- jection that may prove useful in population as- sessments and recovery planning. Methods Definition of Terms What constitutes recovery? An endangered spe- cies can be said to be capable of recovery when a management plan that predicts indefinite persis- tence with some acceptable level of probability, and shows convincing evidence that the plan will perform as indicated, is implemented. Two ad- ditional terms need definition. Indefinite persistence implies continued exis- tence in perpetuity. However, this definition is not