Molecular Ecology (2010) 19, 2562–2573 doi: 10.1111/j.1365-294X.2010.04652.x Selection on breeding date and body size in colonizing coho salmon, Oncorhynchus kisutch J. H. ANDERSON,* P. L. FAULDS,† W. I. ATLAS,‡ G. R. PESS§ and T. P. QUINN* *School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195, USA, †Seattle Public Utilities, 700 5th Avenue, Suite 4900, Seattle, WA, 98124, USA, ‡Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6, §National Marine Fisheries Service, Northwest Fisheries Science Center, 2725 Montlake Boulevard East, Seattle, WA 98195, USA Abstract Selection during the colonization of new habitat is critical to the process of local adaptation, but has rarely been studied. We measured the form, direction, and strength of selection on body size and date of arrival to the breeding grounds over the first three cohorts (2003–2005) of a coho salmon (Oncorhynchus kisutch) population colonizing 33 km of habitat made accessible by modification of Landsburg Diversion Dam, on the Cedar River, Washington, USA. Salmon were sampled as they bypassed the dam, parentage was assigned based on genotypes from 10 microsatellite loci, and standardized selection gradients were calculated using the number of returning adult offspring as the fitness metric. Larger fish in both sexes produced more adult offspring, and the magnitude of the effect increased in subsequent years for males, suggesting that low densities attenuated traditional size-biased intrasexual competition. For both sexes, directional selection favoured early breeders in 2003, but stabilizing selection on breeding date was observed in 2004 and 2005. Adults that arrived, and presumably bred, early produced stream-rearing juvenile offspring that were larger at a common date than offspring from later parents, providing a possible mechanism linking breeding date to offspring viability. Comparison to studies employing similar methodology indicated selection during colonization was strong, particularly with respect to reproductive timing. Finally, female mean reproductive success exceeded that needed for replacement in all years so the population expanded in the first generation, demonstrating that salmon can proficiently exploit vacant habitat. Keywords: behaviour, conservation, dams, natural selection, reintroduction, sexual selection Received 4 February 2010; revision received 25 March 2010; accepted 1 April 2010 important traits influencing production and survival of Introduction offspring. Sexual selection tends to favour traits in males that Large individuals of both sexes enjoy reproductive increase their access to receptive mates and traits in advantages. Large males can dominate competitors in females that aid in competition for breeding resources contests for mates or breeding territories, and females required for nest sites or parental care (Andersson 1994; may prefer large males (Andersson 1994). However, Clutton-Brock 2009). However, reproductive success is small males can sometimes be successful, especially if determined not only by the number of offspring that an they gain fertilizations via sneaking or other alternative individual produces but also how many of them sur- behaviours (Taborsky 1994). Large females can produce vive to reproductive maturity (Clutton-Brock 1988). more numerous and larger offspring (Stearns 1992; Ei- Body size and breeding date are among the most num et al. 2004). In addition, large size may provide advantages in competition for nest sites (van den Ber- Correspondence: J. H. Anderson, Fax: +1 206 685 7471; E-mail: ghe & Gross 1989) or other resources necessary for [email protected] parental care provided by females (Andersson 1994). Ó 2010 Blackwell Publishing Ltd SELECTION IN COLONIZING COHO SALMON 2563 Breeding date is also linked to sexual selection and selection on traits such as body size. Thus there are con- offspring survival. Breeders arriving early may have a flicting predictions regarding the strength of selection prior residence advantage in competition for mates or during colonization, but its role in adaptive evolution reproductive territories (Foote 1990; Haley 1994). In has most often been inferred well after population birds, both clutch size and offspring survival often establishment and not directly measured in the very decline over the course of the breeding season (Perrins first generations. 1970; Svensson 1997). Early emergence of young pro- Here we present the results of a study using DNA- vides increased growth opportunities, resulting in based parent–offspring assignments to characterize higher survival rates when mortality is size selective selection on breeding date and body size in a popula- (Landa 1992; Einum & Fleming 2000; Feder et al. 2008). tion of coho salmon expanding above a former migra- However, juveniles hatching too early may face tion barrier. Our primary goal was to quantify the increased predation risk (Bra¨nna¨s 1995; O’Donoghue & form, direction and magnitude of selection on these Boutin 1995) or unfavourable habitat conditions and traits. To identify a potential mechanism for selection food availability (Crecco & Savoy 1985). on breeding date, we measured the body size of age-0 Coho salmon (Oncorhynchus kisutch) are ideal subjects juvenile coho salmon sampled from the stream and pre- for studies of competition in sexual and natural selec- dicted that early breeding parents would produce off- tion. Females compete for, construct, and guard nest spring that were larger at the end of the summer than sites, and the largest individuals were estimated to those produced by later breeding parents. We also com- enjoy a 23-fold advantage over the smallest due to egg pared selection on body size with a simple length-based production, territory quality, and nest defence (van den egg production model to evaluate the importance of Berghe & Gross 1989). Males compete for access to breeding competition amongst females. Finally, to test receptive females and large males typically win (Flem- the hypothesis that selection is strong during coloniza- ing & Gross 1994) but small males maturing at a youn- tion, we compared our selection gradients with others ger age (‘jacks’) can sneak fertilizations and may in a comprehensive review of selection in natural popu- represent a evolutionarily stable strategy (Gross 1985). lations (Kingsolver et al. 2001) and similar salmonid Other than a brief (c. 1–2 weeks) period of nest defence studies (Seamons et al. 2007; Ford et al. 2008). by the female following egg deposition, coho salmon provide no parental care and invariably die after breed- Methods ing (Sandercock 1991). Thus females determine the off- spring’s environment almost exclusively by spawning Sampling site and date, and parents cannot compensate for adverse conditions by feeding or protecting their off- This study was conducted in the Cedar River, Washing- spring from predators. Adults breed in late fall or win- ton, USA, where Landsburg Diversion Dam blocked ter, embryos incubate in streambed gravels for several fish migration from 1901 to 2003. During the past cen- months before emerging in the spring, and juveniles tury, coho salmon persisted in the Cedar River and its commonly grow in freshwater for 1 year before migrat- tributaries below the dam (located at river 35.1 km), ing to sea (Sandercock 1991). although precise abundance estimates were unavailable. Pacific salmon have repeatedly colonized new habi- In fall 2003, the dam was modified with fish passage tats throughout their range, and such episodes provide structures, and management adopted a policy of natural special opportunities for phenotypic evolution and local recolonization rather than transplantation. Salmon adaptation. Much of the native habitat currently occu- accessed the new habitat on their own volition and pied by Pacific salmon was colonized following glacial spawned primarily in the lower reaches of the Cedar recession (c. 10 000–15 000 years, McPhail & Lindsey River immediately above the dam (Anderson & Quinn 1986), and indeed this is an ongoing process (e.g. Gla- 2007). Thus, expansion of an existing population into cier Bay, Alaska, Milner et al. 2000). In general, intro- newly accessible habitat, as opposed to the founding of duced or invading populations often experience rapid a population in an isolated new area, best describes the rates of adaptive evolution due to the opportunity for colonization demographics in this case. population growth (Reznick & Ghalambor 2001), and Adult salmon were sampled as they ascended the fish selection may be primarily responsible for phenotypic ladder, including a measurement of body length (tip of evolution during population expansion (Clegg et al. snout to fork of tail), determination of sex based on 2002; Koskinen et al. 2002; Yeh 2004). Although rapid external examination, and excision of a small piece of evolution would imply strong selection during coloniza- tissue for subsequent DNA analysis. The ladder was tion, low densities typical of colonization might attenu- operated such that all salmon accessing the new habitat ate sexual selection (Kokko & Rankin 2006), reducing were handled from well before the first coho salmon Ó 2010 Blackwell Publishing Ltd 2564 J. H. ANDERSON ET AL. arrived (approximately 1 month) until the run dwin- age-1 fish about to migrate downstream. Juveniles were dled to 1–2 salmon per week (late January in most collected over a range of dates so we adjusted observed