| Summary: | For a sexually reproducing species, the two major decisions facing all individuals are when and with whom to reproduce. When scaled to the population level, the outcome from all individual decisions determines reproductive variance, and age-class contribution to population growth rate. Both of these attributes determine a population's effective size (Ne), which is directly correlated with its fitness, persistence probability, and adaptability. The questions of when and with whom to reproduce, and their subsequent effects on Ne and age-at-maturity were assessed for wild brook trout (Salvelinus fontinalis) populations. Mating pairs were significantly size-assortative, with individual length accounting for 37% of the variation. This pattern of size assortative mate choice resulted in a reproductive strategy closer to monogamy than polygamy. Of all reproducing adults (n=157), 80% (n=126) produced only one full-sibling family, and only 6% (n=9) contributed to more than two full-sibling families. The number of families and offspring contributed increased with length for both males and females. Comparison of the effective population size estimate to the adult census size (Nc) estimate returned an Ne:Nc ratio of 0.49 averaged over both populations. This value is nearly five times greater than the average reported across 165 (0.14) and 102 (0.10) different species. Age-at-maturity ranged from 0 to 2 years, with the proportion of age-0 and age-1 individuals maturing in a given year dependent upon growth opportunities determined primarily by environmental conditions. Mature fish were significantly larger than immature fish within an age-class, however, survival rates of mature and immature fish were similar. Furthermore, parental length did not influence offspring survival. These data suggest that the cost of early maturation is instead manifested through a reduction in egg number for females, and a reduced ability to acquire mates for males, both determined by an individual's size. Indeed, fecundity predicted by mean length of immature and mature fish within an age-class would result in mature fish producing an average of 38% (age-0) and 33% (age-1) more eggs than immature fish. These findings are discussed in the context of population persistence given the trend of increasing habitat fragmentation and looming climate change.
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