Estimating required habitat size for fish conservation in streams

1. A procedure is described to estimate viable population size and the area of habitat needed to support an endangered stream fish population. Monte Carlo simulations were used to evaluate population fates with stochastic demography and random variation simulated to cause age class 0 failures in some years. Viable population was defined in this paper to be large enough to have less than a 10% chance of extinction in 100 years and to have a long-term effective size of at least 500 breeding adults, although the method could be applied for any assumed extinction rate and effective size. 2. Using data for Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis) in an age class model, it was inferred from simulations that minimum viable population size was 2750 fish, which would require 2.2 ha of habitat at median density of the subspecies in New Mexico streams. 3. Minimum viable population size occurred at the highest survival rate of young of year and no population-wide year class failures. Viable population size, and hence required habitat size, increased as the failure rate for age class 0 increased or when the survival rate from age 0 to 1 declined. This suggested that managers should avoid managing for smallest possible viable population size and instead plan for much larger population sizes to accommodate temporal variation in demography and habitat quality. 4. Decreased survival rate of young of year caused the stable age class distribution to be skewed toward the age class 0, which profoundly reduced effective population size. This suggested that habitat restoration that improves survival of young of year would be a good strategy to increase effective size over the long term. 5. Estimating required habitat size could improve introductions of fish to create new populations. Although requiring time-consuming simulations, the procedure can be used to estimate required habitat size for any species for which population and demographic data are available. Copyright (c) 2007 John Wiley & Sons, Ltd.