ABSTRACT
The short-term dynamics of salmonid populations are directly related to the mean sizes of individual families. The amount of genetic variation maintained in the population is directly related to the variance in sizes of individual families. Both the mean and variance of individual family sizes have important implications for conservation actions and sustainable levels of harvest of salmonid fishes. We develop a context for examining variation in family size, and we provide estimates of mean and variance of family size from five groups of marked pink salmon released into the north Pacific Ocean. We then present two important results: (1) a statistically detectable genetic component of marine survival exists in groups with high marine survival and (2) ratios of variance-to-mean family size are linearly related to mean family size over the interval that we observed. These results imply that short-term population increases come from a small fraction of the population’s families, that salmon encounter a fluctuating marine environment, and that the most favored phenotype changes from generation to generation. These results also support the widely held view that protecting genetic variation in recovering or exploited salmon populations has important economic benefits.
QUOTES FROM TEXT:
The prevailing view in fisheries science is that the most important population parameter is abundance. Fisheries management and harvest decisions are based on abundance as are models used by conservation biologists in the analysis of risk of extinction of populations. For family size defined as the number of returning adult breeders per number of parents, abundance is just the product of family size and the number of breeders in the previous generation. In other words, abundance is equivalent to average family size for fixed breeding numbers. Little attention has been directed toward how family size varies in fish populations or how this variation indexes the amount of genetic variation that can be carried by a population. Although this variability has not been used explicitly in models of population dynamics or risk assessment, its importance has been acknowledged in conceptual models.
The important biological conclusion is that a relatively small part of the breeding population is the most productive and that this productive segment is both changing and unpredictable. Thus, population dynamics models that only incorporate abundance will fail to predict decreases in stock productivity as a consequence of actions that maintain abundance in the short term, but decreases genetic variability.