Drivers of population dynamics vary across biophysical gradients and after ecosystem regime shifts for a freshwater fish
Long-term, cross-system comparisons can help disentangle global- to local-scale biophysical drivers of fish population dynamics and clarify context-dependent, nonstationary responses to broad-scale ecosystem change. Lake Whitefish ( Coregonus clupeaformis ) is a coldwater, demersal fish species that is socioecologically valuable across its range, including in the Laurentian Great Lakes of North America. Unfortunately, many Great Lakes populations have experienced poor recruitment over recent decades, threatening the sustainability of Lake Whitefish fisheries in the world's largest freshwater ecosystem. In this study, we characterized the suite of important drivers of Lake Whitefish recruitment among six large lakes spanning a broad geographic area and multiple gradients of ecosystem structure and disturbance. We parameterized a suite of regression models by leveraging spatially and temporally extensive data describing Lake Whitefish year-class strength (YCS) and biophysical processes hypothesized to regulate their recruitment. The relative importance of biophysical drivers and their relationships to YCS differed among lakes (i.e., context-dependence) and through time in response to ecosystem shifts (i.e., nonstationarity). Results emphasized the role of a major ecosystem regime shift following establishment of invasive dreissenid mussels—and the varying consequences of this regime shift across biophysical gradients—in explaining recent cross-lake trends in Lake Whitefish YCS. Specifically, benthic food web perturbations and oligotrophication were major broad-scale drivers that appear to have constrained Lake Whitefish recruitment. Climatic changes such as warming temperatures and decreasing ice cover were also important broad-scale drivers with differing consequences across lake morphology and latitude. Our cross-lake analysis provided novel insights into the relative importance of Lake Whitefish recruitment drivers and the underlying mechanisms by which they act to regulate recruitment across biophysical gradients. This research informs stewardship of important populations and fisheries and illustrates how population dynamics spanning biophysical gradients respond non-uniformly to ecosystem change.