Fig. 3.

For single-spawning females (A) and males (D), older fish that spawned later in life were larger than younger fish. Of all of the ages at which steelhead return to spawn, 3-y-old females had the highest RRS (B). This observation is explained by a combination of fitness trade-offs for 2- vs. 3-y-olds and negative frequency-dependent selection for 4- and 5-y-olds vs. 3-y-olds (C). Each point represents a single run-year, the frequency of that particular age group at spawning, and its lifetime reproductive success relative to 3-y-old females. For males, in contrast, older, larger individuals had greater lifetime reproductive success across all age classes found on the spawning grounds (E), and there was no relationship between frequency of age class and its fitness (F).