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Genetic basis of the timing of smolt outmigration

The Wood River system in southwestern Alaska provides a pristine, well-studied system in which to examine fine-scale population structure and its influences on juvenile life histories. Sockeye spawn in small tributary streams, deep-water lake beaches, and large outlet rivers and the fry from all populations rear for at least 1 year in freshwater before emigrating. The variability in spawning and rearing habitat has resulted in populations that exhibit a wide range of phenotypes, spawn timing, and other life history strategies. We wanted to know: are there any patterns to the timing of smolt outmigration? Do smolts from all populations leave at the same time, or is there perhaps structure related to habitat (and the genetic adaptation to that habitat)?

Smolts were collected using a fyke net at the outlet of Lake Aleknagik, the downstream most lake in the system, every other night from June-September in 2008 and 2009. [We’ve continued these collections and now have data and samples from 2008-2020]. Up to 200 fish per sampling event were weighed, measured, and a fin clip was taken.  After laboratory and statistical analyses were completed, we were able to determine that there is a pattern to the timing of smolt outmigration. Fish that were spawned in small tributaries and rivers dominated catches in early June, while fish that were spawned on beaches or were genetically assigned to Lake Kulik (primarily beach habitat) were more prevalent from late June to early September.

McGlauflin et al. 2011. Spawning Habitat and Geography Influence Population Structure and Juvenile Migration Timing of Sockeye Salmon in the Wood River Lakes, Alaska Transactions of the American Fisheries Society 140(3):763-782. doi:10.1080/00028487.2011.584495.

A and C Creeks, small population dynamics

We utilize data from two small stream populations of sockeye salmon, located on Little Togiak Lake. These stream, A and C Creeks, are relatively unique: they are located only 1km apart, are only 250-350m long, and there are very few other stream-spawning populations nearby. These characteristics make A and C Creeks an ideal model system for studying eco-evolutionary dynamics. Over the past 15+ years, the Alaska Salmon Program has intensively sampled these populations. Tissue samples are taken from every returning individual, and data are recorded on individual size, sex, daily location, cause of death, and age. We extract DNA, sequence, and genotype each individual and use this genetic data to reconstruct multi-generation pedigrees (family trees) of each stream. Pedigrees provide valuable information on the reproductive success (fitness) of each individual and allow for the identification of dispersers originating in other populations.

Recently, our molecular-based pedigree studies have revealed the prevalence of not just spatial, but also temporal assortative mating systems. This assortative mating, both within and among subpopulations, suggests that dispersal away from an individual’s natal location and parents’ spawn time may have significant effects on fitness and recruitment. These assortative mating systems may drive patterns of adaptation-by-time or adaptation-by-distance, which may serve to maintain variation within populations. At the meta-population scale, we have demonstrated how local adaptation results in a reduced fitness of dispersers. However, by investigating multi-generation fitness effects, we are finding that dispersal may still contribute positively to recruitment due to an increased fitness of the hybrid offspring of dispersers. This work highlights the importance of considering multi-generational effects and the value of long-term sampling programs in studies of eco-evolutionary dynamics.

PhD student Sam May is currently investigating how limited gene flow within and among populations of sockeye salmon may influence recruitment and resilience in natural populations. Some questions I am addressing are: How accurately do individual sockeye home to their natal location in space and in time? How might homing result in assortative mating systems? To what extent do assortative mating systems affect rates of adaptation and gene flow within and among populations? How is genetic and phenotypic diversity maintained within populations? Does gene flow contribute to population productivity? What is the scale of local adaptation? And, how do fine-scale processes affect population resilience in the face of anthropogenic influences? To address these questions, I am utilizing multi-generation, pedigree-based estimates of relatedness and fitness combined with individual-level metadata on phenotypic traits from A & C Creeks. We are finding that sockeye may return to the same location within the creek and at the same time as their parents, resulting in fine-scale population structure in this system. This assortative mating regime may indicate adaptation-by-distance and adaptation-by-time and serve to maintain local phenotypic diversity. In addition, dispersal of individuals within and among populations may serve to increase population productivity over the course of multiple generations – shedding light on the mechanisms by which gene flow and local adaptation influence recruitment and resilience.