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Pages 94-110

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From page 94... ... MARINE MAMMALS AND SALMONIDS The effect of predation by marine mammals on salmonids has been controversial since at least the 19th century (Merriam, 19011. In 1899, the president of the California Board of Fish Commissioners proposed to kill " 10,000 of the 30,000 [California sea lions, Zalophus californianus, and Steller's sea lions, Eumatopias stelleri] Read the entire page →
From page 95... ... Lake Washington steelhead are winter-run fish, returning to the fresh water to spawn from December to April. As the fish enter the ship canal below the locks, some of them are captured and eaten by California sea lions. Read the entire page →
From page 96... ... habitat for conserving endangered species becomes more constricted. The greatest potential for conflicts in protecting species and for management of individual species under current policies will arise in situations in which habitat reductions especially extreme reductions- themselves are the causes of endangerment and the habitats of listed species are largely overlapping. Read the entire page →
From page 97... ... 1994. California Sea Lions and Steelheac! Read the entire page →
From page 98... ... from Predation by California Sea Lions in the Lake Washington Ship Canal, Seattle, Washington. National Marine Fisheries Service Northwest Regional Office, Seattle; and Washington Department of Fish and Wildlife, Olympia. Read the entire page →
From page 99... ... the vulnerability of small populations to extinction. The material focuses on random changes in population sizes and in their structure, changes in genetic variability, environmental fluctuations, and habitat fragmentation. Read the entire page →
From page 100... ... Several assumptions must be macle about the ways in which populations grow, in particular, about the way population growth rates respond to density. From the standpoint of an endangered species, the simplest conceivable mode} assumes that the population has been pushed to its limits resources (habitat and food availability) Read the entire page →
From page 101... ... density dependence the influence of population density on a specific phenomenon, e.g., density-dependent growth. effective population size-the number of breeding adults that would give rise to the rate of inbreeding observed in a population if mating were at random and the sexes were equal in number. Read the entire page →
From page 102... ... Almost no data exist on this, except for Drosophila, although a fair amount of empirical work is now going on to fill this gap in our knowledge. The general principle of all experiments to estimate ,u is the same start with a genetically uniform stock; create sublines; maintain the sublines in isolation from each other with a minimum possible population size (to minimize the efficiency of natural selection against new mutations) Read the entire page →
From page 103... ... Richter-Dyn and Goel (1972) developed a general solution for the mean extinction time starting from an arbitrary density, again assuming that random fluctuations in birth and death rates are the only source of 2 The derivation of this relationship is as follows: the probability that an individual is male is 0.5, and the probability of all individuals being male in a sample of N individuals is 0.5N. Read the entire page →
From page 104... ... Variation in K alone cannot cause extinction, unless the carrying capacity actually declines below zero. However, K puts a ceiling on the attainable population size and bottlenecks in K can magnify the effects of demographic stochasticity by enhancing the variation in the population growth rate due to the smaller sample of reproductive adults. Read the entire page →
From page 105... ... the long-run growth rate is positive, the mean extinction time increases exponentially with the carrying capacity under this model, with the rate of scaling increasing with the frequency of occurrence anct magnitude of catastrophes. Assuming catastrophes act locally, spatial subdivision of a species provicies a simple means of protection against extinction cause(i by clevastating events. Read the entire page →
From page 106... ... Typically, because of high variance in family size, the effective population size is a third to a tenth of the actual number of breeding adults (Heywood, 1986; Briscoe et al., 19924. Thus, as a first approximation, if the number of breeding adults is less than 21s, natural selection will be essentially incapable of eliminating a deleterious gene its future frequency will be governed bY chance. Read the entire page →
From page 107... ... However, for populations with effective sizes of a few hundred or fewer inclivicluals, the expected amount of variation for a typical quantitative character is nearly inclepenclent of the strength of selection and proportional to the product of the effective population size and the rate of mutational input of variation (Burger et al., 1989; Foley, 1992~. This implies that for populations containing hundreds or fewer individuals, the rate of environmental change that can be sustained for a prolonged period of time is directly proportional to the effective population size. Read the entire page →
From page 108... ... models are idealize~i in that they envision a florid consisting of two kinds of habitat patches hospitable an(l inhospitable, all of equal size. The real worIcl, of course, is more complex. Read the entire page →
From page 109... ... Furthermore, an overly protective captive breeding program may simply result in a relaxation of natural selection and the gradual accumulation of cleleterious genes. For hatchery salmoni(ls, egg-to-smolt survivorship is typically 50% or greater, as compared with 10% or less in natural populations (Waples, 1991; NRC, 19951. Read the entire page →
From page 110... ... endangered species can be part of a comprehensive strategy that also addresses the problems affecting species in the wild (Foose, 1989; Povilitis, 1990; Ballou, 1992~. For example, captive breeding and reintroduction enabled the peregrine falcon (Falco peregrinus) Read the entire page →

From page 94...

... MARINE MAMMALS AND SALMONIDS The effect of predation by marine mammals on salmonids has been controversial since at least the 19th century (Merriam, 19011. In 1899, the president of the California Board of Fish Commissioners proposed to kill " 10,000 of the 30,000 [California sea lions, Zalophus californianus, and Steller's sea lions, Eumatopias stelleri]