Fitness and Iteroparity in Winter-Run Steelhead: Reproductive Success and the Egg Size/Fecundity Tradeoff

Parity (spawning history) of salmonids exists as a continuum from the obligate semelparity of some Pacific salmon species to the iteroparity of trout and charr.  The life history of steelhead, the anadromous form of Oncorhynchus mykiss, falls between these extremes with some individuals spawning more than once and many that die after spawning just one time making steelhead useful subjects of evolutionary studies of iteroparity.  Using scale pattern analysis and genetically inferred pedigree data we estimated age and spawning history for individual steelhead.  We then examined two different aspects of individual fitness and their relationship to iteroparity in two different populations of winter steelhead.  First, we examined the egg size/fecundity trade-off in a population of hatchery steelhead by comparing the relationship between egg size and fecundity of repeat spawning females to that of same-aged first-time spawning females (total age 4) and to that of females whose first time spawning was the same year as the first year of repeat spawning females (one year younger, total age 3).  Second, in a wild population, we examined patterns of reproductive success of adults that, in their lifetime, spawned twice or only once.  Our results provide insights into evolutionary trade-offs associated with iteroparity and the effect of reproductive life history on fitness traits and demography of steelhead populations.