Selection on Increased Intrinsic Growth Rates in Coho Salmon, Oncorhynchus Kisutch

Abstract

Substantial evidence from the animal kingdom shows that there is a trade-off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast-growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild-genotype fry. In seminatural streams, fry of the fast-growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast-growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild-genotype fry were more likely to migrate than fry of the fast-growing genotypes. After being returned to the experimental streams after migration, fast-growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast-growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast-growing variants to survive under natural conditions. The large-scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.

Quantitative trait locus analysis of hatch timing, weight, length and growth rate in coho salmon, Oncorhynchus kisutch

“…The QTL analysis offers no evidence to suggest that there would be a molecular basis to any phenotypic correlation with this trait. This result was unexpected in light of previous ecological studies that suggest that hatch timing, as a measure of early development, is correlated with growth as alevin and fry (Beacham et al, 1988;Einum and Fleming, 2000;Sundstrom et al, 2005). In an inbred line of rainbow trout, one major QTL for hatch timing was associated with length at the start of exogenous feeding, suggesting a possible genetic correlation between development rate and later growth (Robison et al, 2001).…”

“…Growth rates may be correlated with other life-history traits including development rate and size at age (Cheverud et al, 1983;Mangel and Stamps, 2001). Whether such traits are positively or negatively correlated may be strongly influenced by environmental conditions (Lankford et al, 2001;Sundstrom et al, 2005;Gregersen et al, 2008). For example, both timing of development and large body size are often crucial for exploiting ephemeral resources or avoiding predation (Sundstrom et al, 2005;Biro et al, 2006).…”

“…Whether such traits are positively or negatively correlated may be strongly influenced by environmental conditions (Lankford et al, 2001;Sundstrom et al, 2005;Gregersen et al, 2008). For example, both timing of development and large body size are often crucial for exploiting ephemeral resources or avoiding predation (Sundstrom et al, 2005;Biro et al, 2006). However, depending on resource availability, some species may adopt a strategy of allocating energy toward rapid development rather than toward achieving large body size.…”

“…Salmon are anadromous, rearing in freshwater for varying lengths of time (depending on species) before migrating to salt water where resources are more abundant and growth rates are high (Quinn, 2005). Large size and rapid growth are thought to be important during the freshwater phase, because salmon must compete with conspecifics for resources and escape from predation (Sundstrom et al, 2005). Salmon eggs are deposited in gravel of lakes and streambeds.…”

“…Greater size at the fry stage is positively correlated with freshwater overwintering survival (Quinn and Peterson, 1996). However, when predation is density dependent, early development and corresponding high growth rates as fry may increase an individual's exposure to predation risk and result in lower fitness (Sundstrom et al, 2005). In chinook salmon, juvenile growth rates are also positively correlated with the age and timing of smoltification, the physiological process of adapting to salt water (Beckman and Larsen, 1998), and with increased marine survival during the first winter at sea in coho salmon (Beamish et al, 2004).…”