347 Reports Ecology, 86(2), 2005, pp. 347–352  2005 by the Ecological Society of America RELATIVE IMPORTANCE OF SALMON BODY SIZE AND ARRIVAL TIME AT BREEDING GROUNDS TO REPRODUCTIVE SUCCESS B. R. DICKERSON, 1,6 K. W. BRINCK, 2 M. F. WILLSON, 3 P. BENTZEN, 4 AND T. P. QUINN 5 1 University of Washington, School of Aquatic and Fishery Sciences, P.O. Box 355020, Seattle, Washington 98195 USA 2 University of Washington, Quantitative Ecology and Resource Management, P.O. Box 355020, Seattle, Washington 98195 USA 3 5230 Terrace Place, Juneau, Alaska 99801 USA 4 Dalhousie University, Department of Biology, Halifax, Nova Scotia B3H 4J1, Canada 5 University of Washington, School of Aquatic and Fishery Sciences, P.O. Box 355020, Seattle, Washington 98195 USA Abstract. The importance of body size to reproductive success has often been em- phasized, 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 suc- cess 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 inter- mediate 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 suc- cess and such traits as body size and fecundity may indicate that selection has driven the population to equilibrium values. Key words: adult progeny; body size; breeding timing; DNA microsatellites; Oncorhynchus gorbuscha; parentage analysis; reproductive success; southeast Alaska; spawning grounds; wild pink salmon. INTRODUCTION Understanding the factors affecting variation in re- productive success among individuals is key to under- standing the selective pressures driving adaptation and evolution (Stearns 1992). Reproduction has been well studied in salmonid fishes (Fleming 1998); their breed- ing system, especially that of Pacific salmon, Oncor- hynchus spp., makes them ideal for examining repro- ductive success. These anadromous, semelparous fishes achieve most of their adult body size while feeding at sea, do not feed or grow during the breeding season, home to their natal stream to breed at very discrete times of the year, and die upon completion of spawning, allowing an individual’s lifetime reproductive output to be estimated in one season (reviewed by several authors in Groot and Margolis [1991]). Most research on reproductive success in salmonids has emphasized the importance of adult body size. Fe- males can benefit from large size by producing larger Manuscript received 15 September 2003; revised 11 August 2004; accepted 10 September 2004. Corresponding Editor: J. R. Bence. 6 Present address: NOAA/NMFS, National Marine Mam- mal Laboratory, 7600 Sand Point Way NE, Seattle, Wash- ington 98115 USA. E-mail: Bobette.Dickerson@noaa.gov and more numerous eggs (Beacham and Murray 1993, Dickerson et al. 2002), obtaining and defending high- quality breeding sites (Foote 1990), and burying their eggs deeper (hence safer; Steen and Quinn 1999). Large males may have advantages in intrasexual competition and mate choice (Quinn and Foote 1994, Fleming 1998, Dickerson et al. 2002). However, the timing of arrival on the spawning grounds may have at least as much influence on reproductive success as body size. Early, arriving males are more likely to obtain mates because later in the season the females are nest guarding and no longer reproductively active (Quinn et al. 1996, Dickerson et al. 2002). Earlier-arriving fish also typi- cally, though not always (Smoker et al. 2000), live longer than later arrivals, allowing males more time to participate in spawning events and females a longer nest-guarding period (McPhee and Quinn 1998, Dick- erson et al. 2002). Countering these advantages, large size and early arrival may increase vulnerability to bear predation (Quinn and Buck 2001, Quinn et al. 2003). Reproductive success in salmon has typically been estimated by using surrogate traits such as observations of courtship, spawning events, or body size or shape (e.g., Fleming and Gross 1994). However, several males may participate in each spawning event, females