Individual variation in Atlantic salmon fertilization success: Implications for effective population size

Mating structure can influence the variance in individual reproductive success, which in turn has important implications for a population's effective size. Atlantic salmon (Salmo salad males are characterized by alternative reproductive strategies, maturing as small parr in fresh water and/or as considerably larger anadromous males following a migration to sea. The potential for significant variation in individual reproductive success in both sexes is high. We established two experimental crosses each involving four anadromous females, four anadromous males, and 20 mature male parr in a seminatural spawning environment. We used hyper-variable microsatellite loci to identify the parents of 755 embryos and to quantify the variance in individual fertilization success. Anadromous males generally dominated fertilization success, with a mean individual fertilization success of 15.7% and 19.3% in replicates A and B, respectively, as compared with 1.9% and 1.2% for the mature male parr. There was no relationship between individual mature male parr size and individual fertilization success in any individual nest, nor was there any relationship between anadromous male size and individual fertilization success in either replicate. There was some evidence of size being an important determinant of both the frequency of spawning and the overall individual parr fertilization success among those parr identified as having spawned, although these relationships were not always significant. Both anadromous males and females were identified as having spawned with multiple partners, although the frequency of multiple anadromous males spawning simultaneously with a female was low. Fertilization of eggs by parr can significantly increase the effective number of males on a nest-by-nest basis; however, the variance in individual anadromous male fertilization success appears to have the greatest overall influence on effective population size.