Recurrent evolution of life history ecotypes in sockeye salmon: implications for conservation and future evolution

Abstract

We examine the evolutionary history and speculate about the evolutionary future of three basic life history ecotypes that contribute to the biocomplexity of sockeye salmon (Oncorhynchus nerka). The ‘recurrent evolution’ (RE) hypothesis claims that the sea/river ecotype is ancestral, a ‘straying’ form with poorly differentiated (meta)population structure, and that highly structured populations of lake-type sockeye and kokanee have evolved repeatedly in parallel adaptive radiations between recurrent glaciations of the Pleistocene Epoch. Basic premises of this hypothesis are consistent with new, independent evidence from recent surveys of genetic variation in mitochondrial and microsatellite DNA: (1) sockeye salmon are most closely related to pink (O. gorbuscha) and chum (O. keta) salmon with sea-type life histories; (2) the sockeye life history ecotypes exist as polyphyletic lineages within large drainages and geographic regions; (3) the sea/river ecotype exhibits less genetic differentiation among populations than the lake or kokanee ecotypes both within and among drainages; and (4) genetic diversity is typically higher in the sea/river ecotype than in the lake and kokanee ecotypes. Anthropogenic modification of estuarine habitat and intensive coastal fisheries have likely reduced and fragmented historic metapopulations of the sea/river ecotype, particularly in southern areas. In contrast, the kokanee ecotype appears to be favoured by marine fisheries and predicted changes in climate.

Sulfur isotopes in otoliths allow discrimination of anadromous and non-anadromous ecotypes of sockeye salmon (Oncorhynchus nerka)

“…For example, Oncorhynchus nerka commonly occurs both as anadromous sockeye salmon (hereafter called "sockeye") which spend most of their life in the ocean, typically rearing in fresh water for less than 2 years, and as non-anadromous "kokanee" which live in fresh water for their entire lives. Although these ecotypes are typically genetically distinct, genetic analyses are sometimes inadequate to discriminate them when differentiation is limited or when hybridization occurs (Foote et al 1989;Taylor et al 1996;Wood et al 2008).…”

Introduction to a Special Section: Genetic Adaptation of Natural Salmonid Populations

“…Genetically divergent but geographically overlapping Chinook salmon populations are described in the interior Columbia River basin with multiple marker types (allozymes: Waples et al 2004; singlenucleotide polymorphisms [SNPs] and microsatellites: Smith et al 2007;Narum et al 2008). Presumed contemporary or recent gene flow among basins of sea/river-type sockeye salmon O. nerka contrasts with minimal within-basin gene flow among lake-type sockeye salmon populations (inferred from respective low and high F ST values; Wood 1995;Wood et al 2008). Applications of allelic data to problems in fishery management and research range from studies of families within small streams to specieswide oceanic mixtures; such applications have expanded from a visionary luxury to a necessity.…”

Landscape heterogeneity and local adaptation define the spatial genetic structure of Pacific salmon in a pristine environment

“…The potential adaptive differences observed between the population below Klutina Lake and the five populations that spawn in tributary habitats above Klutina Lake are primarily due to allele frequency differences observed at One_MHC2_190 and One_MHC2_251. Interestingly, the Klutina Lake outlet population may be of the sea/river ecotype as defined by Wood et al (2008) whereas the five populations spawning above Klutina Lake are presumably of the lake ecotype (Ackerman et al 2011). The lake ecotype is the typical form of sockeye salmon that spends about half its life in a nursery lake before emigrating to the marine environment to mature, whereas the sea/river ecotype is a rarer form that rears in the freshwater environment for a much shorter and more variable period of time.…”

“…The high fidelity of Pacific salmon to their natal spawning and rearing environments results in genetic variation among discrete populations (Taylor 1991;Dittman and Quinn 1996;Quinn 2005). Juvenile sockeye salmon primarily spend a year or more rearing in freshwater before emigrating to the marine environment; although a 'seatype' occurs in which juveniles may migrate to the marine environment in their first year (Quinn 2005;Wood et al 2008). Adult sockeye salmon usually mature and return to spawn in their natal freshwater environment at 4-5 years of age.…”