Scale and the detection of climatic influences on the productivity of salmon populations

Abstract

Ecological studies relating population parameters to climate conditions are limited by a lack of experimental control systems and rely instead on correlative evidence to draw inferences about how populations respond to environmental forcing. Consequently, some correlations turn out to be spurious and not ecologically meaningful. To strengthen inferences, multiple populations may be examined simultaneously to confirm whether relationships can be generalized across multiple systems; however, this assumes that populations respond similarly to climate drivers, ignoring the potential for ecological complexity. Using data on eight sockeye salmon populations from southwestern Alaska, we constructed a series of models based on ecological hypotheses, relating salmon population productivity to climate factors experienced at different life stages. We modeled populations at a range of organizational scales, from distinct populations, to populations grouped by common nursery lake, to all populations within a watershed, and determined the relative statistical support for climate drivers at each scale. In general, warmer lake and sea surface temperatures in the summer coincided with increased productivity of these populations, but the most sensitive life-stage for climate effects varied among populations, particularly among nursery lakes. The best model when considering all populations together, despite strong statistical support, failed to represent the complexity which became evident when populations were modeled by common nursery lake, or independently. These results emphasize that the most appropriate organizational scale to model salmon stocks will depend on specific management, scientific, or conservation goals.

Foraging and growth responses of stream‐dwelling fishes to inter‐annual variation in a pulsed resource subsidy

“…First, though it is not surprising that a decrease in salmon resources significantly depressed the ration sizes and growth rates of resident fishes, streams with higher in situ productivity are less dependent on salmon-derived resources for supporting growth of consumers. Because the abundance of salmon at the scale of an individual stream is highly variable among years even in relatively pristine ecosystems (Schindler et al 2010, Rogers andSchindler 2011), the availability of alternative resources will determine the relative effects of low resource pulse magnitudes on resident consumers. Therefore, maintaining access to diverse foraging opportunities through multiple food web pathways (e.g., Nakano andMurakami 2001, Wipfli andBaxter 2010) will buffer consumers from the substantial fluctuation in the magnitude of resource pulses that are an inherent characteristic of natural populations.…”

“…A subsample of rainbow trout and Arctic grayling was also monitored over the 11 years of our study using uniquely identifiable tags to estimate individual growth rates ( Sockeye salmon were enumerated visually by wading the entire main stem of each stream at approximately the same time each year during the peak of spawning activities from 1956-2012 Rogers 1998, Rogers andSchindler 2011). Although our study region has experienced substantial warming over the last half century (Schindler et al 2005), there has been little change in the peak spawn date.…”

Centennial-scale fluctuations and regional complexity characterize Pacific salmon population dynamics over the past five centuries

“…At a broad scale, persistent multidecadal shifts in North Pacific salmon production throughout the 20th century have been linked to the Pacific Decadal Oscillation (PDO), with total salmon production in Alaska high when the PDO is in a warm phase and vice versa for cold phases (12). At finer spatial and temporal scales, however, salmon populations have varied in their responses to climatic variation, exhibiting asynchronous dynamics even within close proximity, presumably due to differences in habitats, life histories, and other local drivers of population dynamics (13,14). Although the mechanisms that produce this response diversity remain elusive, these spatial differences in productivity patterns, which have been observed on interannual to interdecadal time scales, can stabilize fisheries and ecosystem processes by reducing the variation in aggregate salmon returns over time (13,15).…”

Four decades of foraging history: stock-specific variation in the carbon and nitrogen stable isotope signatures of Alaskan sockeye salmon

“…This biocomplexity within the stock complex has stabilized salmon fishery production despite environmental variation resulting from changing climate conditions (Hilborn et al 2003, Schindler et al 2010. In some climate regimes, specific salmon stocks flourish, whereas other stocks increase in productivity during other climate regimes (Rogers & Schindler 2011). Such weak covariation among the components of the stock complex of Bristol Bay sockeye salmon stabilizes the production from the overall ecosystem.…”