Variation in developmental biology and microsatellite DNA in reproductive ecotypes of kokanee, Oncorhynchus nerka: Implications for declining populations in a large British Columbia lake

Abstract

Kokanee are the nonanadromous (freshwater resident) form of sockeyesalmon (Oncorhynchus nerka) found in lake ecosystems throughoutthe North Pacific region. Kokanee commonly exhibit two reproductiveecotypes; `stream-spawners' that reproduce in streams tributary tolakes, and `beach-spawners' that reproduce on submerged lakeshore gravelbeaches. Okanagan Lake, in the southcentral interior of BritishColumbia, Canada, contains beach- and stream-spawning kokanee and bothecotypes have declined dramatically in abundance over the last 20 years.We examined developmental biology (developmental rate to hatching andemergence) and genetic divergence at eight microsatellite loci toinvestigate phenotypic and genetic differentiation between ecotypes tounderstand selective and demographic factors that might influence therecovery of depressed populations. Beach-spawning female kokanee weresmaller and produced smaller eggs than females from stream-spawningpopulations. There was no striking difference in time to 50%hatching between ecotypes, but beach-spawning kokanee developed fasterfrom hatching to emergence. Microsatellite loci were highly polymorphicin kokanee (between 5 and 23 alleles per locus) and showed significantdifferentiation among populations (average θ = 0.018). There was,however, no significant variation attributable to spawning ecotype afteraccounting for variation within ecotypes. Simulated population-mixtureanalyses indicated good potential for genetic classification of kokaneeas beach- or stream-spawners; estimated mixture proportions were within11% of actual proportions averaged over 50 replications. Our datasuggest that Okanagan Lake kokanee constitute at least two managementunits within a single watershed; the ecotypes appear adapted to distinctthermal reproductive environments and show modest moleculardifferentiation from one another. Persistence of kokanee within OkanaganLake may depend, in part, on management plans that recognize thedistinctions between the sympatric reproductive ecotypes.