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

Abstract

Observational data from the past century have highlighted the importance of interdecadal modes of variability in fish population dynamics, but how these patterns of variation fit into a broader temporal and spatial context remains largely unknown. We analyzed time series of stable nitrogen isotopes from the sediments of 20 sockeye salmon nursery lakes across western Alaska to characterize temporal and spatial patterns in salmon abundance over the past ∼500 y. Although some stocks varied on interdecadal time scales (30-to 80-y cycles), centennial-scale variation, undetectable in modern-day catch records and survey data, has dominated salmon population dynamics over the past 500 y. Before 1900, variation in abundance was clearly not synchronous among stocks, and the only temporal signal common to lake sediment records from this region was the onset of commercial fishing in the late 1800s. Thus, historical changes in climate did not synchronize stock dynamics over centennial time scales, emphasizing that ecosystem complexity can produce a diversity of ecological responses to regional climate forcing. Our results show that marine fish populations may alternate between naturally driven periods of high and low abundance over time scales of decades to centuries and suggest that management models that assume time-invariant productivity or carrying capacity parameters may be poor representations of the biological reality in these systems.Oncorhynchus nerka | nitrogen stable isotopes | fisheries | paleolimnology L arge fluctuations in abundance are a hallmark of fish stocks (1, 2); fish abundance can fluctuate substantially over interannual to centennial time scales (3-5). Recent short-term variation in stock abundance can be characterized by fisheries catch records and scientific survey data, but to characterize low-frequency variability requires novel ecological approaches (6). Knowing how fish stocks have varied over long time scales can provide an important context for recently observed shifts in abundance, provide insight into the potential effects of climate change, and inform management frameworks (7). In this study, we describe and analyze time series of nitrogen (N) stable isotopes in sediments from 20 lakes as a proxy for the abundance of anadromous sockeye salmon, Oncorhynchus nerka, to quantify how salmon stocks have varied in abundance during the past ∼500 y. This analysis provides a comprehensive synthesis of the natural patterns of variability in salmon abundance before the onset of commercial fishing and encompasses a study region that currently produces over 70% of global sockeye salmon catches (8).Low-frequency variation in fish abundance is often attributed to climatic forcing (2, 9) or harvesting (10, 11). 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 ...

Sockeye salmon population dynamics over the past 4000 years in Upper Russian Lake, south-central Alaska

“…The d 15 N values averaged 4.7% throughout the dated portion of the core, which is substantially higher than in salmon-free control lakes and generally consistent with expected values for similar spawning densities, suggesting that variation in d 15 N was related to MDN input (Finney et al 2000;Rogers et al 2013). For the first 1900 years (* 2000 BCE to 100 BCE) of the * 4000-year time series represented in the core, d 15 N averaged 4.9% (Figs.…”

“…2, 4). Over the period from * 100 BCE to 550 CE, d 15 N abruptly declined to an average of 2.7%, which is closer to values * 2.3% observed in sediments of salmon-free reference lakes elsewhere in Alaska (Finney et al 2002;Rogers et al 2013) and suggests a significant, multi-centennial Upper Russian Lake Age Model (year and core depth) 2015 Fig. 3 Age at depth for the Upper Russian Lake master core, constructed from the 4 radiocarbon-dated macrofossils and 1 dateable tephra and assuming constant sedimentation rates between successive dated layers decline of Upper Russian Lake sockeye salmon abundance.…”

“…These productivity regimes were often asynchronous among lakes, presumably related to differential responses of habitats and salmon populations to regional climatic conditions, resulting in a ''portfolio effect'' that stabilizes regional salmon returns over time (Rogers et al 2013). When viewed across longer time scales, however, there is evidence for regionally coherent climate-related shifts in abundance that may synchronize the dynamics of different populations.…”

“…Paleolimnological analyses have led to reconstructions of sockeye salmon (Oncorhynchus nerka) abundance spanning hundreds or thousands of years in many nursery lakes (Finney 1998(Finney , 2000(Finney , 2002Gregory-Eaves et al 2003;Rogers et al 2013). These insights are possible because adult sockeye salmon, after gaining nearly all of their body mass at sea, return to natal river systems where they spawn and die, primarily in lakes and associated feeder streams.…”

“…A synthesis of paleolimnological data spanning * 500 years from 20 nursery lakes in southwestern and south-central Alaska, the center of wild sockeye salmon abundance, reveals substantial shifts in salmon productivity within individual lakes that often lasted for decades or centuries (Rogers et al 2013). These productivity regimes were often asynchronous among lakes, presumably related to differential responses of habitats and salmon populations to regional climatic conditions, resulting in a ''portfolio effect'' that stabilizes regional salmon returns over time (Rogers et al 2013).…”