The maintenance of diverse life history strategies within and among species remains a fundamental question in ecology and evolutionary biology. By using a near-complete 16-year pedigree of 12,579 winter-run steelhead (Oncorhynchus mykiss) from the Hood River, Oregon, we examined the continued maintenance of two life history traits: 1. the number of lifetime spawning events (semelparous vs. iteroparous) and 2. age at first spawning (2-5 years).
Results/Conclusions
We found that repeat-spawning fish had more than 2.5 times the lifetime reproductive success of single spawning fish. However, first-time repeat spawning fish had significantly lower reproductive success than single spawning fish of the same age, suggesting that repeat spawning fish forego early reproduction to devote additional energy to continued survival. For single-spawning fish, we also found evidence for a fitness trade-off for age at spawning; older, larger males had higher reproductive success than younger, smaller males. For females, on the other hand, we found that three-year-old fish had the highest mean lifetime reproductive success despite the observation that four and five-year-old fish were both longer and heavier. This phenomenon was explained by negative frequency dependent selection; as four and five-year-old fish decreased in frequency on the spawning grounds, their lifetime reproductive success became greater than that of the three-year-old fish. Using a combination of mathematical and individual-based models parameterized with our empirical estimates, we demonstrate that both fitness trade-offs and negative frequency dependent selection observed in the empirical data can theoretically maintain the diverse life history strategies found in populations.