The evolutionary stability of partial migration

Natural selection, in its most basic form, is described as a process in which traitsincrease or decrease in frequency depending on their fitness, and only the trait withthe highest fitness will remain in the population. Yet, populations rarely have asingle `optimal' trait. The way natural selection maintains this observed variationwithin populations has been a keen focus of evolutionary biologists. In the followingchapters, I focus on how natural selection maintains a form of phenotypicvariation referred to as `partial migration'. Partial migration is the coexistence ofmigratory and non-migratory phenotypes, and is found in a wide variety of taxa. Ifound that some, but not all forms of density-dependent competition can lead to theevolution and maintenance of partial migration (i.e., partial migration as an evolutionarilystable strategy (ESS) and convergent stable strategy (CSS)). Whetherdensity-dependent competition allows for partial migration as an ESS and a CSSdepends on how it influences the relative fitnesses of the phenotypes. If competitionchanges the relative fitnesses in opposing directions, then it will allow forpartial migration. If it affects the relative fitnesses in the same direction, it willnot. I then apply these results to a fish species of conservation and commercialconcern: Oncorhynchus mykiss, or steelhead and rainbow trout. I demonstratehow female steelhead and rainbow trout competing separately for spawning habitatcan still be subject to frequency-dependent selection and how this allows forpartial migration. The frequency-dependent selection also results in strong feedbacksbetween survival and reproduction, which produces a non-linear response inthe migration propensity ESS and CSS. In practical terms, this means that conservationor management actions may not affect the population as expected, andmeasuring the propensity for migration in wild populations is notoriously difficult.To address this difficulty, I develop a method to measure the propensity for migrationin wild populations that can be used to test the predicts I generated in thetwo previous chapters. The method is called sex-ratio balancing and it relies on afundamental relationship between sex ratios and the propensity for migration. Sexratios are much easier to measure than the propensity for migration and the easeof measurement makes this method valuable for studying many different partiallymigratory taxa.Keywords: competition, density dependence, rainbow trout, frequency dependent selection, sex-ratio balancing, evolutionarily stable strategy, steelhead, partial migration, sex rati

Influence of sex, migration distance, and latitude on life history expression in steelhead and rainbow trout (Oncorhynchus mykiss)

In partially migratory species, such as Oncorhynchus mykiss, the emergence of life history phenotypes is often attributed to fitness trade-offs associated with growth and survival. Fitness trade-offs can be linked to reproductive tactics that vary between the sexes, as well as the influence of environmental conditions. We found that O. mykiss outmigrants are more likely to be female in nine populations throughout western North America (grand mean 65% female), in support of the hypothesis that anadromy is more likely to benefit females. This bias was not related to migration distance or freshwater productivity, as indicated by latitude. Within one O. mykiss population we also measured the resident sex ratio and did not observe a male bias, despite a high female bias among outmigrants in that system. We provide a simulation to demonstrate the relationship between sex ratios and the proportion of anadromy and show how sex ratios could be a valuable tool for predicting the prevalence of life history types in a population

Ecological and evolutionary patterns of freshwater maturation in Pacific and Atlantic salmonines

Reproductive tactics and migratory strategies in Pacific and Atlantic salmonines are inextricably linked through the effects of migration (or lack thereof) on age and size at maturity. In this review, we focus on the ecological and evolutionary patterns of freshwater maturation in salmonines, a key process resulting in the diversification of their life histories. We demonstrate that the energetics of maturation and reproduction provides a unifying theme for understanding both the proximate and ultimate causes of variation in reproductive schedules among species, populations, and the sexes. We use probabilistic maturation reaction norms to illustrate how variation in individual condition, in terms of body size, growth rate, and lipid storage, influences the timing of maturation. This useful framework integrates both genetic and environmental contributions to conditional strategies for maturation and, in doing so, demonstrates how flexible life histories can be both heritable and subject to strong environmental influences. We review evidence that the propensity for freshwater maturation in partially anadromous species is predictable across environmental gradients at geographic and local spatial scales. We note that growth is commonly associated with the propensity for freshwater maturation, but that life-history responses to changes in growth caused by temperature may be strikingly different than changes caused by differences in food availability. We conclude by exploring how contemporary management actions can constrain or promote the diversity of maturation phenotypes in Pacific and Atlantic salmonines and caution against underestimating the role of freshwater maturing forms in maintaining the resiliency of these iconic species.Keywords: Maturation, Salmon, Diversity, Salmo, Oncorhynchus, Oncorhynchus Resilience, Life history, Conservatio

Quantifying partial migration with sex-ratio balancing

Partial migration, the phenomenon in which animal populations are composed of both migratory and non-migratory individuals, is widespread among migrating animals. The proportion of migrants in these populations has direct influences on population genetics and dynamics, ecosystem dynamics, mating systems, evolution, and responses to environmental change, yet there are very few studies that measure the proportion of migrants. This is because existing methods to estimate the proportion of migrants are time-consuming and expensive. In this paper, we demonstrate a new method for estimating the proportion of migrants in a population, based on sex-ratio measurements. Many partially migratory taxa exhibit sex-biased migration or residency, and in these cases, the sex ratios of migrants and non-migrants are fundamentally related to the proportion of migrants in the population. We define this relationship quantitatively and show how it can be used to infer the proportion of migrants in a population through a process we term â sex-ratio balancing.â We obtain Bayesian estimates of proportion of migrants, and quantify the uncertainty in these estimates with highest posterior density intervals. Lastly, we validate the sex-ratio balancing approach with a Chinook salmon (Oncorhynchus tshawytscha Walbaum, 1792) data set. Sex-ratio balancing holds promise as a tool for quantifying partial migration and filling a key data gap about partially migratory taxa.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author