Relative importance of salmon body size and arrival time at breeding grounds to reproductive success

The importance of body size to reproductive success has often been emphasized, but timing of arrival to the breeding grounds may also be important. To assess the relative roles of size and timing, we investigated the influence of life history and of behavioral and morphological traits on realized reproductive success in wild pink salmon, Oncorhynchus gorbuscha, during the 1997 and 1998 spawning seasons. Reproductive success was quantified by the number of adult offspring returning to spawn (1999 and 2000; parentage determined via DNA microsatellite genotyping). Males that arrived earlier, lived longer in the stream, and were more often observed in dominant courtship positions had significantly more offspring, although the importance of dominance was overshadowed by timing. Female reproductive success was not closely linked to any measured trait, but stabilizing selection on arrival timing and length is possible, as 1998 fish showing intermediate values of these traits were the most productive. Bear predation accounted for up to 50% of the mortality on the spawning grounds, yet did not have detectable effects on reproductive success. The absence of strong linear relationships between reproductive success and such traits as body size and fecundity may indicate that selection has driven the population to equilibrium values.