Asynchrony in population dynamics of sockeye salmon in southwest Alaska Lauren A. Rogers and Daniel E. Schindler L. A. Rogers (larogers@u.washington.edu) and D. E. Schindler, School of Aquatic & Fishery Sciences, Univ. of Washington, PO Box 355020, Seattle, WA 98195-5020, USA. Ecologists have examined the synchronization of population dynamics across space as a means to understand how populations respond to climate variation. However, response diversity may reflect important variation among local population dynamics driven by population-specific responses to regional environmental change. We used long-term data on sockeye salmon Oncorhynchus nerka from pristine watersheds of southwestern Alaska to show that populations spawning in close proximity ( B40 km) to one another have a limited degree of synchrony in their dynamics, even after accounting for density-dependent processes. In fact, the dynamics of local populations of stream-spawning sockeye salmon were no more coherent than those of stocks at a much coarser resolution across this region of Alaska. We examined four hypotheses to explain the observed patterns of asynchrony among stream-spawning populations and found that populations spawning in dissimilar habitats, and using different nursery lakes were less synchronized in their productivity. Similarity in the age structure of spawning adults was less correlated with synchrony in productivity. These results emphasize the importance of maintaining diverse spawning and rearing habitat for the conservation of Pacific salmon, and should guide conservation planning for Pacific salmon populations in regions where natural dynamics have been altered by habitat loss, hatchery practices, and over-fishing. Synchronization of population dynamics across space is a widely observed phenomenon (reviewed by Liebhold et al. 2004). Populations may be synchronized over space by exposure to spatially correlated climate fluctuations (Moran 1953), and studies of population synchrony have been useful for identifying common extrinsic drivers of popula- tion size, survival, or growth rate. The general expectation is that populations become more correlated in their dynamics with increasing spatial proximity due to exposure to common environmental conditions (Ranta et al. 1995, Liebhold et al. 2004). This pattern is convenient for monitoring and management because it suggests relatively uniform responses to climate variation among nearby populations. However, asynchrony among proximate, but distinct populations of a species is a potentially important ecological phenomenon that has been largely ignored in the ecological literature (but see Ringsby et al. 2002). In a metapopulation context, asynchrony decreases global extinction risk and increases persistence time (Heino et al. 1997). Causes for asynchrony have not been thoroughly explored but differ- ences in life history traits and habitat conditions encoun- tered by individual populations may influence the way that populations experience environmental conditions, poten- tially leading to a diversity of responses among populations to the same overriding environmental conditions (Ringsby et al. 2002, Hilborn et al. 2003, Crozier and Zabel 2006). Pacific salmon Oncorhynchus spp. have been the focus of a number of studies examining patterns of covariation in survival rates among populations (Peterman et al. 1998, Pyper et al. 2002), as well as the environmental factors that may be driving those patterns (Mueter et al. 2002). At a broad scale, salmon production in the US Pacific Northwest and Alaska has varied inversely with large-scale climate shifts related to the Pacific Decadal Oscillation (Mantua et al. 1997, Hare et al. 1999). But at a regional scale, sockeye salmon O. nerka in Bristol Bay, Alaska have shown patterns in productivity that are only weakly synchronous through time, such that some stocks that were highly productive in the 1960s are only minor contributors to the total Bristol Bay salmon run in the 2000s, and vice versa (Hilborn et al. 2003). It has been hypothesized that the differences in observed dynamics are due to the biocom- plexity of the system, that is, the diversity in spawning and rearing habitats, and the associated diversity in life-history strategies among populations that comprise the stocks and Bristol Bay stock-complex as a whole. This hypothesis remains to be explored in detail. Here we examine patterns of covariation among popula- tions of sockeye salmon O. nerka from a single river system that is essentially undisturbed from watershed development of any type, hydrologic modifications or fish hatcheries. The Wood River lake system, in Bristol Bay, Alaska (Fig. 1), provides spawning grounds for about three million Oikos 117: 15781586, 2008 doi: 10.1111/j.2008.0030-1299.16758.x, # 2008 The Authors. Journal compilation # 2008 Oikos Subject Editor: Christer Bro ¨nmark. Accepted 25 April 2008 1578