The evolution of alternative migratory tactics: lessons from anadromous salmonids

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1 Not to be cited without prior reference to the author The evolution of alternative migratory tactics: lessons from anadromous salmonids Julian Dodson 1, Nadia Aubin-Horth 2, Véronique Thériault 3 and David Paez 1 1 Département de biologie, Pavillon Vachon, 1045, avenue de la Médecine, Université Laval, Québec (Québec) G1V 0A6, Canada 2 Institut de Biologie Intégrative et des Systèmes (IBIS), Pavillon Charles-Eugène- Marchand, 1030, avenue de la Médecine, Université Laval, Québec (Québec) G1V 0A6 Canada 3 Hatfield Marine Science Center, Marine Fisheries Genetics Program, Oregon State University, 2030 SE Marine Science Drive, Newport, OR 97365, USA Abstract: Alternative life-history tactics are common in fishes and many cases involve resident and migratory life styles. The resident tactic may involve both sexes or be restricted to males. Disruptive selection favouring alternative tactics may arise from two different evolutionary mechanisms. On the one hand, alternative migratory tactics may be the result of sexual selection for alternative reproductive phenotypes that impact other aspects of life-history, including resource exploitation. On the other hand, alternative migratory tactics may be caused by natural selection favouring divergence to exploit multiple niches which in turn impacts reproductive tactics. Frequency dependent selection plays a central role in maintaining alternative reproductive tactics and their 1

2 associated migratory phenotypes in natural populations. However, in cases where individuals exploit multiple niches, natural selection favours multiple specialized phenotypes and frequency dependent selection is not necessarily expected to be important. Based on anadromous salmonids, we summarize evidence that (1) alternative migratory tactics co-exist within single gene pools and that individuals may potentially adopt one or the other alternative phenotype. The expression of different migratory phenotypes is a function of sex and condition (growth or some correlated trait) and is under both environmental and genetic control. Non-linear reaction norms that underlie alternative life-history phenotypes vary among species and among different ages of the same species. (2) The suppression of frequency-dependent selection may underlie the existence of two common gradients observed anadromous salmonids: increasing residency with altitude or distance upstream, and increasing anadromy at higher latitudes. Anadromy may also be subjected to the forces driving the evolution of dispersal within metapopulations, with selection favouring increased dispersal in more northerly populations characterised by greater environmental stochasticity. The interplay between condition-dependant alternative migratory tactics and condition-dependent dispersal in metapopulations determines the degree of population genetic structure through its influence on reproductive isolation and gene flow. Introduction A variety of migratory patterns co-exist within populations of most salmonid fishes. Individual fish comprising a population may complete their entire lifecycle in freshwater streams (residency) whereas others may migrate to sea (anadromy) or to lakes (e.g. Olsson and Greenburg 2004) before returning to freshwater streams to spawn. Originally referred to as partial migration (Jonnson and Jonnson 1993), these different migratory tactics are generally associated with important differences in body sizes in adult reproductive fish. As variation in body size has an important impact on reproductive 2

3 success, both sexual and natural selection must be considered to understand the evolution of alternative migratory tactics (AMTs). Size polymorphisms may be the result of sexual selection for alternative reproductive tactics (ARTs) (Brockman and Taborsky 2008). Intense intra-sexual selection is a characteristic of such mating systems and involves high variance in reproductive success among individuals of the same sex (Schuster and Wade 2003). Sexual selection can be important in producing the ART most commonly observed in fishes. Under a 1:1 sex ratio, if some males monopolize and mate with several females, other males will necessarily fail to reproduce. This intra-sexual competition strongly favours alternative mating phenotypes (Shuster and Wade 2003). In salmonids, these are generally represented by bourgeois males that monopolize mates using their large body size and secondary sexual characteristics, and small parasitic males that exploit bourgeois males by sneaking fertilisations (Taborsky 2008). It is more difficult to identify ARTs related to intense intra-sexual competition among females. The female equivalent of male parasitism would be intraspecific brood parasitism (placing eggs in another females nest, for example). However, given the predominance of external fertilisation in fishes, brood parasitism is expected to be rare (Taborsky 2008). Cooperative breeding is another example; smaller, subordinate mature females may participate in reproduction. But given the almost complete absence of parental care in salmonid fishes, there seems to be little opportunity for cooperative breeding. A third possibility is that disruptive selection on body size in females is a result of female competition for oviposition sites (Fleming and Gross 1994). In other cases, non-sexual selection on non-reproductive traits may provide the conditions required for the evolution of AMTs and lead to the evolution of ARTs as a consequence. Any morphological or metabolic variation that improves foraging success in one habitat may lead to the development of alternative foraging tactics. If such tactics lead to important size differences, the stage is set for the development of ARTs. In such cases, non-sexual selection favours size dimorphism. 3

4 We must thus consider the simultaneous action of two evolutionary mechanisms to understand the development and maintenance of AMTs. They are influenced to some extent by sexual selection for alternative reproductive tactics that impact other aspects of life-history, particularly foraging tactics that directly influence metabolism and growth. Simultaneously, alternative migratory tactics are influenced by non-sexual selection favouring ecological divergence across multiple niches which in turn impacts reproductive tactics. The two sources of selection are inseparable in any discussion of alternative migratory tactics. Our purpose in this presentation is to review the evidence that alternative migratory tactics are maintained under a threshold-trait model (Roff 1996). We first consider the evidence that alternative migratory tactics co-exist within a single gene pool and that individuals may potentially adopt one or the other alternative phenotype. We then look at the evidence that the expression of different migratory phenotypes is a function of some environmental cue. We also consider the evidence that a genetic component plays a role in the development of the alternative migratory phenotypes. Second, we explore the basis of alternative migratory and reproductive tactics among female salmonids whose mating success may be relatively free of frequency-dependant selection. Thirdly, we aim to apply this knowledge to understanding the biogeography of AMTs, in particular the observation that residency tends to increase within rivers upstream and at higher altitudes and that anadromy is favoured at higher latitudes. I - The importance of condition-dependant selection in the maintenance of alternative migratory tactics. For an alternative phenotype to invade a population and to persist, the average fitness of the alternative phenotype when it is rare must be greater than the average fitness of the conventional phenotype and, to persist in a population at a stable frequency, the average fitness of the alternative phenotype at equilibrium must equal the average fitness of the conventional phenotype (Schuster and Wade 2003). Together, these conditions define 4

5 negative frequency-dependant selection which is associated with crossing fitness curves of alternative phenotypes (Brockman and Taborsky 2008, Fig. 1). Figure 1. Negative frequency dependant selection acting on hypothetical migrant and resident male phenotypes. Migrants with large body sizes adopt a bourgeois mating tactic whereas small-bodied residents adopt a sneaker mating tactic. The success of each morph depends on its frequency in the population, each doing better when rare. The relative frequency of each phenotype stabilizes at a point where the average fitness of each phenotype is equal (the evolutionary stable state, ESS). (modified from Brockman and Taborsky 2008). Developmental and behavioural strategies, which are commonly associated with salmonid alternative mating strategies (Gross 1985, Hutchings and Myers 1994) result from the interaction between the environment and an individual s genome, coupled with a genetically determined threshold of sensitivity. Such developmental trajectories may be considered as threshold traits whose expression is controlled by many loci of small effects rather than in a Mendelian fashion (Roff 1996). Each genotype possesses its own threshold and the expressed phenotype is a function of the genotype and the environment. Individuals with phenotypes that exceed the threshold tend to display one reproductive tactic, while individuals with phenotypes falling below the threshold display the alternative reproductive tactic (Hutchings and Myers 1994, Figure 2). 5

6 Figure 2. Threshold traits and discrete developmental phenotypes. (A) Threshold reaction norms are non-linear functions defining the probability of different genotypes to express a dimorphic trait (eg. migrate, stay) as a function of a switch point and environmental variation. For a given genotype, individuals with phenotypes below the switchpoint express a default tactic (X); phenotypes above the threshold express a modified tactic (Y). (B) There is a normal distribution of switchpoints (F(sp)) in the population. At any specific value of the environmental clue, the environmentally determined threshold (e*) defines the relative frequency of the two tactics in the population. (modified from Tomkins and Hazel 2007). The condition dependent selection hypothesis suggests that the environment will select for a mean threshold value and that this value may thus vary among populations facing different environments. Using common garden experiments, Piché et al. (2008) demonstrated genetic differentiation among populations of Atlantic salmon for thresholds of precocious maturity. When the fitness consequences of alternative behavioural phenotypes depend on the relative competitive ability of interacting individuals (i.e. their relative status), then 6

7 selection is said to be status dependent (Gross 1996). Status may be considered as a threshold trait and the status-dependent selection model may thus be considered as being equivalent to the condition-dependant model if individuals have genetic variation for switchpoints and populations have different mean threshold values (Tomkins and Hazel 2007). Figure 3. Status-dependant selection. (A) The distribution of condition (status) in the population and the fitness (F) functions of the resident and migrant phenotype in a population. Fitness functions cross over and are thus frequency dependent. The intersection of the two fitness functions defines the evolutionary stable switch point where the fitness of each phenotype is equal. Low status fish maximize their fitness by developing as migratory fish that delay reproduction and adopt the bourgeois behavioural repertoire while high status fish maximize their fitness by suspending migration, maturing early and adopting the sneaker behavioural repertoire. (B) If the average fitness of the resident phenotype declines due to density-dependent competition for example (eg. Fleming and Gross 1994), the switch point shifts towards higher status thus reducing the frequency of residents in the population and re-establishing the evolutionary stable state (modified from Tomkins and Hazel 2007). In salmonids, many different patterns of alternative migratory strategies exist. In predominantly anadromous populations of Atlantic salmon (Salmo salar) for example, all females are anadromous and only the males exhibit the residency/anadromy dichotomy (Hutchings and Myers 1994). In Pacific salmonids (Oncorhynchus sp.), both sexes are anadromous, but some anadromous males may spend only several months at sea before 7

8 returning to freshwater to spawn (jacks) relative to others that may stay at sea for several years (eg Gross 1991). Strictly freshwater resident males occur but are rare (residual sockeye salmon, for example (Ricker 1938)). In other species of salmonids, both males and females may exhibit the residency/migratory dichotomy, exploiting streams, rivers, estuaries and the open ocean for feeding (eg. brook charr Salvelinus fontinalis, Theriault and Dodson 2003, Lenormand et al. 2003; arctic charr Salvelinus alpinus, Nordeng 1983; rainbow trout Oncorhynchus mykiss, Pascual et al. 2001; brown trout Salmo trutta; Jonsson, 1985) such that both sexes may exhibit considerable variance in body size at the moment of reproduction. Does the hypothesis of condition-dependant selection apply to alternative migratory phenotypes? Firstly, we must establish that alternative migratory tactics co-exist within a single gene pool and that individuals may potentially adopt one or the other phenotype. Much empirical evidence exists to support the proposition that the sympatric occurrence of alternative migratory phenotypes in salmonids is not accompanied by genetic differentiation when the alternative phenotypes occupy the same spawning grounds at the same time (Arctic charr (anadromous, small and large freshwater residents) (Nordeng 1989), anadromous and freshwater brown trout (Hinder et al. 1991, Charles et al. 2005, Charles et al. 2006), anadromous steelhead and resident O. mykiss (Docker & Heath 2003, Olsen et al. 2006, McPhee et al. 2007), resident and anadromous brook charr (Thériault et al (2007)). They may therefore be considered as conditional tactics expressed within common gene pools. In cases where residency is restricted to males, as in predominantly anadromous populations of Atlantic salmon, no such differentiation can occur as early maturing parr must necessarily spawn with anadromous females. Second, we must establish if the expression of different migratory phenotypes is a function of some environmental cue. Assuming that individual growth performance integrates environmental variation and is the most reliable indicator of condition, we may then ask if the expression of different migratory phenotypes is related to growth (or body size-energy reserves). Several studies have supported the hypothesis that faster growing individuals migrate to more productive feeding areas than slower-growing individuals 8

9 because they maintain higher metabolic rates and are energetically constrained at a younger age (or smaller size) than slow-growers (brown trout, Forseth et al. 1999, brook charr, Morinville and Rasmussen 2003). Thus fast growth leads to migration and the exploitation of richer feeding grounds. However, other studies have shown that fast early growth is associated with freshwater residency and, in some cases, early male maturity (brown trout Olsson et al. (2006), Atlantic salmon (Rowe and Thorpe 1990, Aubin-Horth and Dodson 2004). Third, we must also establish that genetic variation for the threshold level exists among individuals. Several studies have shown a significant heritable component in the development of alternative migratory tactics (eg. chinook salmon (Oncorhynchus tshawytscha) Heath et al. 2002, Atlantic salmon, Garant et al. 2003, brook charr, Thériault et al. 2007). However, this does not necessarily mean that the male offspring of one phenotype will be composed mainly of that phenotype because of the role of environmental variation. For example, resident Atlantic salmon fathers may produce more anadromous offspring if the offspring experience poor growth conditions. These anadromous fish in turn will father resident male offspring if growth conditions improve. Complicating further this relationship, growth shows high heritability in salmonids (Garcia De Leaniz et al. 2007). It is plausible then that the relationship found between the tactic of the father and that of the sons in a given environment could be explained largely by the heritability of growth rate or size, rather than the heritability of the threshold size, leading to the expression of the same tactic as the father. Some support for this hypothesis was provided by Garant et al. (2002) who found that young Atlantic salmon fathered by precocious males grew faster than those fathered by anadromous males during the time from hatching to yolk sac absorption. In contrast, Morrasse et al. (2008) working with the same Atlantic salmon population as Garant et al. (2002) demonstrated in the laboratory that weight, length, and protein content of the progeny of anadromous and sneaker males did not differ. Experiments were conducted to determine if the higher incidence of Atlantic salmon sneaker males in the upstream sites of the Ste-Marquerite R. documented by Aubin-Horth et al. 2006, is due to selection acting on growth thresholds, or whether it is due to varying 9

10 growth opportunities particular to the different sites. Wild anadromous and sneaker progenitors from upstream and downstream sites were used in a factorial mating design. Families were reared in captivity, and individual growth of PIT-tagged age-1 fish was monitored for an additional 10 months, at which time all male progeny were either smolts or sexually mature. No difference in the incidence of smolt and sneaker male progeny was observed between upstream and downstream sites or between the two paternal reproductive tactics. However, 42% of the total variation was explained by the variation in incidence among sire groups, providing preliminary evidence of genetic variation in the incidence of alternative migratory and reproductive tactics. Second, the predicted growth asymptote differed between offspring that matured as sneakers and offspring that smolted, with smolt attaining larger body sizes than mature parr as previously documented (Fig. 4). Figure 4. Differences in asymptotic length for smolt (left) and sneaker parr (right) offspring originating from different paternal tactics (F-fighter, S-sneaker) in upstream (U) and downstream (D) sites. Significant differences were seen only in the smolts (stars). Error bars are 95% CI. Please note different y-axis scales. Third, even though no effects from the paternal tactic and its population of origin were observed for sneaker parr, significant growth variation among mature parr offspring between sire groups was observed (LRT = 227.3, P-value < 0.001) (Fig. 5). Because 10

11 common environmental effects, repeated measurements made on each individual and maternal effects are accounted for, the variation among sire groups is evidence of genetic variation for growth rates. Figure 5. Fig. 5. Variation in asymptotic size among mature parr offspring of fighter (F) and sneaker (S) sires. Error bars are 95% confidence intervals Evidence exist that population reaction norms associating growth or size with migratory strategies are highly plastic, varying as a function of age, sex and possibly parental tactic. In brook charr, migration first occurs at the age of 1+ and outmigrants are among the bigger, most metabolically active juvenile fish. However, outmigration also occurs at the age of 2+ and outmigrants are among the smallest juvenile fish. The largest juveniles remain in freshwater and begin spawning the following year (Thériault and Dodson 2003, Thériault et al. 2008). Furthermore, the sex ratio of outmigrants is male-biased among 1+ fish and female biased among 2+ fish, suggesting somewhat different selective pressures on male and female migration thresholds. Rikardsen et al. (1997) also found that more male than female Arctic charr migrated at a younger age. Ricker (1938) formulated a 2- step scenario while studying anadromous and residual (freshwater resident phenotype) sockeye salmon in Canada. Fast-growers matured in freshwater, medium growers migrated at 1+ and slow growers decided upon migration or residency the following year. Among these, the fastest growing fish remained resident and matured in fresh water whereas the slowest-growing fish migrated at

12 The frequency dependent nature of alternative reproductive tactics in males suggests that alternative migratory tactics must also be influenced by frequency dependent selection acting during breeding. However, there is little empirical evidence either for or against this hypothesis. We may speculate that a reduction in the proportion of migratory fish, particularly among males, will cause an increase in the proportion of resident males. As resident fish experience less mortality than do migrants (eg brook charr, Thériault et al. 2008) we would expect the overall density of males on the spawning grounds to also increase, thus altering the operational sex ratio and the number of males competing for females at any point in time. Experiments that artificially manipulate overall density and the relative proportions of resident and migratory phenotypes are needed to evaluate how the mating success of different migratory phenotypes changes as a consequence. We would expect that foraging decisions and associated migratory tactics may be modulated and constrained by differential mating success on the spawning grounds. In conclusion, the hypothesis of condition-dependant selection applies reasonably well to alternative migratory tactics in salmonids. Most, but not all, cases of AMTs co-exist within common gene pools. Secondly, the expression of alternative phenotypes appears to be a function of individual condition (expressed as growth rate, body size or some correlated measure). In some cases, faster growing individuals choose to migrate to more productive habitats whereas in other cases, slower growers migrate. Genetic differentiation among populations for thresholds of precocious maturity has been demonstrated supporting the threshold-trait, condition dependant model. The significant variation in the nature of the reaction norms associating growth (or some surrogate measure of condition) and migratory/reproductive phenotype observed among paternal types, populations and species suggests that phenotypic plasticity plays a predominant role in the origin and maintenance of AMTs. II - Alternative female migratory strategies 12

13 Given the absence of brood parasitism and cooperative breeding in most female salmonids, disruptive selection acting upon females to produce different migratory phenotypes may be associated with competition for breeding sites. There is strong competition among females for the best spawning sites and body size is a major factor in determining the winner of such contests (van den Berghe and Gross 1989, Fleming and Gross 1994). There is evidence that females of different sizes exploit streams and spawning grounds of different dimensions (Fig.5) such that the maintenance of different body sizes among females may reduce competition among females for oviposition sites. Smaller females should do better in smaller streams with smaller substrate sizes and more spatially restrained spawning sites than bigger females. Such an advantage may counterbalance the loss of fecundity associated with smaller size. Under such a scenario, alternative female phenotypes will not be under frequency-dependant selection as the mating success of one phenotype will not detract from the mating success of the alternative. Rather, their relative abundance will depend upon the relative abundance of the spawning habitats and not on the frequency of the alternative phenotype. Large anadromous males can displace small males in a variety of salmonids species, (eg. anadromous coho salmon, Fleming and Gross 1994). We speculate that in spatially constrained spawning habitats, small males may be restricted to breeding with small females. In such spatially constrained habitat, both males and females could be released from frequency-dependent selection favouring the loss of the anadromous phenotype from both sexes. This may lead to the development of isolated populations of small fish inhabiting small streams within the drainage basin. This could happen in the absence of geographical isolation and may represent a case of isolation through adaptation (Nosil and Funk 2008) rather than isolation by distance or physical barrier. 13

14 Figure 5. The size distribution of anadromous male and female brook charr in the main stem and anadromous and resident brook charr in a tributary stream of the St. Marguerite River, Québec Canada. Anadromous fish found in the spawning tributary stream are the smallest of the anadromous fish, both sexes combined. III - The biogeography of alternative migratory tactics Two major gradients in the incidence of alternative migratory strategies have been documented in salmonids fishes: increasing residency with altitude or distance upstream, and increasing anadromy at higher latitudes. A number of studies have demonstrated that the incidence of anadromy in partially migratory salmonid populations tends to diminish with altitude and/or distance upstream and that anadromous individuals preferentially occupy the lower reaches of their nursery rivers. In two separate studies, male parr of Atlantic salmon of a given age and size were more likely to be sexually mature if located further upstream/at higher altitudes (Baum et al. 2004, Aubin-Horth et al. 2006). Both studies concluded that the gradient reflected the increasing costs of migration. In brown trout, the recruitment of anadromous populations declined with altitude relative to that of resident populations illustrating a cost of migration positively correlated with altitude (Bohlin et al. 2001). Several studies have demonstrated that resident fish are found 14

15 further upstream than anadromous fish of the same population (brook charr (Curry 2005), Dolly Varden (Salvelinus malma) (Armstrong and Morrow 1980), rainbow trout (Riva- Rossi et al. 2007)). These observations are consistent with the hypothesis that increased migratory costs favour residency over anadromy as distances to spawning grounds increase. Migratory costs may include significant energy expenditures during the upstream migration (Dodson 1997, Jonsson and Jonsson 2006) as well as an increasing probability of losing access to upstream or higher elevation spawning grounds because of low discharge and/or high summer temperatures (Aubin-Horth et al. 2006). If the average fitness of the migrant male phenotype declines due to mortality selection (Endler 1986) or an increased probability of exclusion from the spawning grounds, the reproductive success of residents may exceed that of migrants independent of variations in condition (body size or growth). If the fitness curves of the two tactics no longer intersect (Fig. 4), negative frequency dependent selection would no longer play a significant role in determining the incidence of alternative reproductive tactics among males and the resident tactic would be favoured. In the case of females, fecundity selection and competition among females favours body size (Fleming and Gross 1994) such that the anadromous tactic should always garner the greatest fitness benefits. A resident phenotype would only be expected to develop in females in cases of extreme mortality selection against the anadromous phenotype. The many cases that document the development of land-locked populations above impassable barriers to migration, whether they are natural (e.g. rainbow trout, Kostow 2003) or artificial (eg. white-spotted charr, Yamamoto et al. 2004), represents an extreme situation whereby downstream migrants cannot return to spawning sites and the migratory phenotype is culled from both males and females. Considerable evidence exists to demonstrate that anadromous phenotypes of many salmonid species predominate at higher latitudes (Dolly Varden (Maekawa and Nakano 2002), white-spotted charr (Salvelinus leucomaenis), (Yamamoto et al 1999), cherry trout (Oncorhynchus masou) (Kato 1991), brook charr (Castric et al. 2003), rainbow trout (McPhee et al. 2007)). The latitudinal gradient in anadromy is to some extent the result of 15

16 the altitudinal gradient previously mentioned. As downstream areas at the southerly limits of distribution are less and less hospitable for juvenile rearing because of temperature constraints (among other habitat variables), anadromous fish must migrate greater distances upstream and to higher latitudes to insure that their young are associated with appropriate rearing habitat. Thus, the cost of migration increases and the average reproductive success of anadromous phenotypes declines to the point where fitness curves may no longer intercept. In such a case, frequency dependent selection no longer plays a role and natural selection culls the anadromous strategy from the most southerly populations. This mechanism however does not explain why the frequency of the resident phenotype should decline at higher latitudes. Several mechanisms may be involved. The food availability hypothesis proposes that anadromy evolves in situations where marine productivity is greater than productivity in freshwater (Gross et al. 1988). The importance of food intake for body growth and the contribution of growth to fitness through increased fecundity and improved male and female breeding success have been documented (Gross et al. 1988). This hypothesis was originally formulated to explain the difference in the latitudinal distribution of anadromy and catadromy. It was based on a species level analysis ranging from 0 to 75 degrees of latitude. The original food availability hypothesis has been subsequently appropriated by many authors to explain intraspecific differences in the incidence of anadromy of salmonids exhibiting alternative migratory tactics across rather limited latitudinal ranges. This particular interpretation of the original hypothesis is incorrect for several reasons. Estuaries and adjacent coastal waters are among the most productive habitats on the planet and growth in such habitats will always be better relative to that achieved in upstream fresh waters. If anadromy is uniquely the result of selection for increased body size, migration to estuarine habitat in salmonids would always be favoured. Secondly, body size is not simply the result of ecosystem productivity. Size at age is also influenced extrinsically by the length of growing season and intrinsically by the balance between rates of anabolism and catabolism that determines growth rate. Some studies have demonstrated that fish from high-latitudes have a higher capacity for growth than their low-latitude conspecifics at the 16

17 same temperature (Conover and Present 1990). As such, the reaction norms of growth as a function of temperature are greater at high latitudes. This may involve two types of trade-offs (Angilletta et al. 2003); allocation trade-offs between different functions and acquisition trade-offs, whereby an increase in foraging results in an increase in mortality. If higher mortality is a result of increased foraging at high latitudes (eg. Langford et al. 2001), switching to more productive habitats could increase the average fitness of the anadromous phenotypes relative to the resident phenotype at high latitudes. As such, the benefits of foraging in more productive marine environments is realised only when foraging in freshwater involves acquisition trade-offs. One benefit of the migratory phenotype not yet considered is that migration provides the opportunity to disperse and reproduce elsewhere. Alternative migratory phenotypes are thus also subjected to the forces driving the evolution of dispersal (Castric and Bernatchez 2003). In the context of metapopulation theory, factors acting at the betweenpopulation level favour dispersal. In particular, variation in population size caused by habitat disturbance, ecological succession and demographic stochasticity may all favour dispersal, while a costly dispersal form should decline in frequency in more stable habitats and populations (Olivieri et al. 1995). Recent theoretical work has shown selection for increased dispersal in metapopulations with high environmental stochasticity and low dispersal costs (Bonte and de la Pena 2009). Along the eastern coast of North America, anadromous salmonids have dispersed northwards from southern refugia over the past years since the last glacial maximum. Populations at northern latitudes have thus been more recently founded than those found further south (eg. brook charr, Castric and Bernatchez 2003). If environmental stochasticity increases with latitude, we may expect greater levels of dispersal and hence a greater incidence of the migratory tactic in the more recently founded and more environmentally stochastic populations found at higher latitudes. At present, the interface between conditiondependant alternative migratory tactics and condition-dependent dispersal in metapopulations has yet to be explored. 17

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