|
<br />474
<br />
<br />EMLEN ET AL.
<br />
<br />existing adults can be calculated from the esti-
<br />mated (adjusted Peterson method) 1983 adult
<br />population of 146,000 and the contributions of
<br />larvae from year-classes 1976-1980 (correspond-
<br />ing to adults entering the population between 1984
<br />and 1988). Using larvae estimates provided in Bu-
<br />chanan and Strekal (1988) this becomes
<br />
<br />(146,000)(0.85)7
<br />+ (33,000 + 747,000)(0.001)(0.75)
<br />x (0.444)(0.85)6
<br />+ (1,835,000)(0.00 I )(0. 7 5)(0.444)(0.85)5
<br />+ (57,000 + 5,663,000)(0.001){0.75)
<br />x (0.444)(0.85)4
<br />+ (87,000 + 1,000)(0.001)(0.75)(0.444)(0.85)3
<br />+ (1,507,000 + 317,000)(0.001)(0.75)
<br />x (0.444)(0.85)2
<br />= 48,144.
<br />
<br />Based on a larval survival rate of 0.001, then, the
<br />projected 1990 adult population is 7,416 + 48,144
<br />= 55,560. This is short of the estimated value of
<br />300,000 by a factor of5.4. Hence we must consider
<br />0.00 I to be a minimal estimate oflarval survival.
<br />Because the survival estimate of 0.00 I seemed
<br />high in regards to general ecological considera-
<br />tions, a revised value five times that seems incon-
<br />ceivable. In addition, it is entirely possible that
<br />either the 1983 or the 1990 adult population es-
<br />timates could be in error. Accordingly, 0.003 was
<br />chosen as a compromise upper limit on estimated
<br />survival. On the presumption that the real value
<br />should lie somewhere between the minimal and
<br />maximal estimates, 0.002 was picked as the "best
<br />guess" value.
<br />
<br />Predictions of Population Size
<br />
<br />If the level of Pyramid Lake were to exceed
<br />3,848 feet above mean sea level, the elevation of
<br />the Marble Bluff Dam spillway, water would in-
<br />
<br />~ m@rnowrn 00
<br />MAY 41994
<br /> FWRS
<br />
<br />undate the delta, thereby enhancing fish passage
<br />to the dam and, once the spillway was topped,
<br />hugely facilitating transport upstream of the dam.
<br />To reflect the corresponding benefits, number of
<br />spawners was permitted to double under this sce-
<br />nano.
<br />Density dependence acts, at least in part, via
<br />limits on the number of potentially spawning fe-
<br />males that can migrate through the restricted pas-
<br />sages leading upriver. Based on observations of
<br />migrating females and the nature of the fishway
<br />and the Marble Bluff Fish Facility river trap through
<br />which the fish must pass, the maximum possible
<br />run was set at 100,000 females. Other density ef-
<br />fects probably occur, but information is not avail-
<br />able on their mode of action or their intensity. It
<br />was necessary, therefore, to ignore other density
<br />effects in the model and to consider model output
<br />to err on the side of overestimation offemale num-
<br />bers.
<br />No information exists on year-to-year variabil-
<br />ity in demographic parameters. This being so, and
<br />inasmuch as existing variation is almost certainly
<br />not independent of hydrological events, it seemed
<br />foolhardy and almost certainly misleading to mod-
<br />el birth and death as stochastic processes. Quali-
<br />tatively, fecundity and (probably) survival vary
<br />positively with water flow. Therefore, as the vari-
<br />ance in these parameters increases, the severity of
<br />drought effects is magnified and population growth
<br />is curtailed. By utilizing fixed values, the model
<br />again errs on the side of overestimating numbers.
<br />A population of two individuals cannot repro-
<br />duce, even under ideal conditions, ifboth fish are
<br />of the same sex. Under such circumstances, and
<br />occasionally under even less stringent conditions,
<br />a real population would die out. The model, by
<br />ignoring this sort of statistical variation in fecun-
<br />dity, ignores also the corresponding possibility for
<br />extinction. In principle, this, yet again, means the
<br />model might err on the optimistic side. In fact,
<br />however, very few of the simulated populations,
<br />even under conditions of no supplemental water,
<br />ever fell below about 50 individuals without even-
<br />tually going, and being recorded as, extinct.
<br />
<br />
|