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Evaluating the effect of stage-specific survivorship on the Ne/N ratio


  • Robin S. Waples

    Corresponding author
    1. National Marine Fisheries Service, Northwest Fisheries Science Center, 2725 Montlake Boulevard East, Seattle, WA 98112 USA
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Robin S. Waples. Fax: (206) 860–3335; E-mail:


Evaluating effective population size (Ne) and the effective size to census size ratio (Ne/N) in species with Type III survivorship curves is complicated when key demographic parameters [mean (k̄) and variance (Vk) of family size] are measured during early life stages. The method of Crow & Morton (1955) for scaling demographic data collected at a juvenile stage to expected values at adulthood is extended to consider sequential episodes of random and family correlated survival. Results show the following: (i) The order in which the episodes of random and family-correlated survival occur does not affect Ne or Ne/N; (ii) If a population experiences an episode of family-correlated survival, Ne/N scaled to its expected value in a population of constant size (k̄ = 2) is simply the survival rate during the family-correlated stage. If multiple such stages occur, scaled Ne/N is the product of the survivals during all family-correlated life stages; (iii) Under the assumption of random post-enumeration survival, adjusting the variance effective size to its expected value at k̄ = 2 is equivalent to computing the inbreeding effective size at the earlier life stage. Application to experimental data for hatchery populations of Pacific salmon (Oncorhynchus spp.) indicates that nonrandom survival during the marine phase led to estimated reductions in effective size of 0–62% (mean 19%) in 12 different cohorts. This approach can provide insights into Ne/N in highly fecund species, including some marine species in which Ne has been estimated to be several orders of magnitude less than N.