8033

• UCREFRP Catalog Number: 8033
• Author: Lentsch, L. D., et al.
• Title: Endangered Fish Interim Management Objectives for the Upper Colorado River Basin Recovery And Implementation Program -Final Report.
• USFW Year: 1998.
• USFW - Doc Type: 55,
• Copyright Material: NO

1995). However, additional evidence on the mutation rate of quantitative traits (L6pez and
L6pez-Fanjul 1993a, 1993b) suggest a rate much lower (10% of the original) than that
incorporated into Franklin (1980) and Soule's (1980) estimate. Indeed, if the L6pez (1993) and
L6pez-Fanjul (1993) results are taken into consideration then Ne should be increased by a factor
of ten to N. =5000 (Lande 1995). For the maintenance of adaptive variation of quantitative traits,
Lynch (1996) argued that for long-term planning, an acceptable Ne is on the order of 1000. This
value of N. is more likely to prevent loss of adaptive traits, especially those associated with life-
history characters whereas a number greater than 1000 is not expected to enhance the amount of
genetic variance maintained in a population.
The effective population size K) will deviate from the census population size due to fluctuations
in population size, variation in reproductive success and deviation from a 1:1 sex ratio (Wright
1931). Fluctuations in population size and variation in reproductive success are difficult to
quantify directly and empirical data are limited. However, data are available to suggest a
deviation from a 1:1 sex ratio in breeding populations of Colorado River fish despite the fact that
the entire population sex ratio may be unity.
It is not surprising that the population of breeding adults or the "operational sex ratio" deviates
from 1:1. In some species (e.g. Rana catesbeiana, bullfrogs), females breed asynchronously
whereas males are capable of fertilizing eggs during the entire breeding period. This difference in
synchrony results in male-biased operational sex ratios. Additionally, the mating system itself can
alter the operational sex ratio. In polygamous or promiscuous species, for example, males have a
much higher potential for reproductive rate that females (Krebs and Davies 1987) and alternative
male mating strategies can result in a male-biased operational sex ratio as has been observed in
both sunfish (Gross and Chamov 1980) and salmon (Gross 1985). Additional explanations for
biased sex ratio of breeding adults include differential mortality rates between sexes and limited
resources for either of the two sexes (Pitcher 1993).
There are.several empirical examples of biased sex ratios in fish. A 4.4:1 (males:females) ratio
was demonstrated in Arctic char (Sigujonsdottir and Gunnarsson 1989), and an estimate of 4:1 in
two separate studies of northern pike (Kozmin 1980, Harrison and Hadley 1983). Additionally,
several studies have quantified sex ratio of adult Colorado and northern squawfish and almost all
have found a biased sex ratio toward a greater number of males (Table 2).

t

1

Estimates of census population size are more readily available than are estimates of effective
population size (N j. However, rarely is it observed that all individuals in a population contribute
equally to the next generation. The number of adults in the population should not be assumed to
equal Ne. Indeed, the ratio of Ne N from a wide variety of natural and captive organisms range
from as little as 0.01 to as much as 0.89 (Table 3). Nonetheless, to obtain a target Ne for
management, it is necessary to determine some "acceptable" ratio of Ne/N.
Variation in lifetime reproductive success is largely due to the mating system and longevity.
Nunney (1995) argues that NJN ranges from approximately 0.25 - 1.0 and suggests that in