McLean, Jennifer. 2003. Reproductive success of hatchery and wild steelhead, Oncorhynchus mykiss. Ph.D.
Species and populations evolve through variation in reproductive success (RS) among individuals with different values of heritable traits. Estimates of RS can be obtained from observations of behaviour or data on life history traits, but fitness is affected by many complex factors. Genetic parentage studies, however, can precisely determine the number of offspring attributable to individual parents with known traits. I coupled multi-locus microsatellite genotype-based parentage assignment exclusion testing (8 loci, average HE = 89%) with morphological and behavioural data to test hypotheses concerning the effects of specific traits on fitness of three groups of steelhead trout (Oncorhynchus mykiss): naturally-spawning wild steelhead, naturally-spawning hatchery steelhead, and hatchery steelhead spawned in a hatchery. Hypothesized effects of origin (hatchery or wild), reproductive timing, and adult body size on RS were tested.
Hatchery steelhead spawning in the wild had markedly lower RS than did native wild steelhead. Hatchery females greatly outnumbered wild females on the natural spawning grounds (N=90 and 73 vs. 11 and 10 wild females in the two consecutive years of the study), but produced only 4.4 - 7.0% the number of smolts produced per wild female. The wild steelhead population more than met replacement requirements at the adult stage (approximately 3.7 - 6.7 adult offspring-per-female), but the hatchery steelhead were far below replacement (<0.5 adults-per-female).
With respect to individual RS, the natural mating system was complex; monogamy, polygamy, polyandry, polygyny, and polygynandry were observed. Wild steelhead outperformed hatchery steelhead, and the timing of reproduction had a more significant role in production of smolt and adult offspring than did size, especially for females. Specifically, the return timing distribution of reproductively successful spawners formed a U-shaped pattern; adults breeding at intermediate times during high river flows produced fewer offspring than those earlier and later in the season. Body size did not affect offspring production. The overall production of adults-per-female in the hatchery was on the same order of magnitude as production by naturally spawning wild females. Parent body size and reproductive timing did not affect RS in the hatchery, despite a considerable range in fecundity, which was related to body size.