Great Lakes Fishery Commission

Understanding the factors that regulate recruitment across spatiotemporal scales and biophysical gradients is critical for predicting how fish communities will respond to ecosystem change. Disentangling the drivers of declining, poor, and sporadic recruitment is a key knowledge gap for stewardship of Lake Whitefish (Coregonus clupeaformis) and Cisco (C. artedi) populations and fisheries across the Laurentian Great Lakes region. In this project, we characterized the recruitment dynamics and drivers of Lake Whitefish and Cisco across each of the Great Lakes and Lake Simcoe (Ontario). Central to this research was drawing inference from cross-species, cross-population comparisons, which together show strong promise for identifying when recruitment bottlenecks occur, discerning the contexts under which various processes are important for regulating recruitment, and predicting how populations may respond to ongoing climatic and ecosystem change. First, we synthesized understanding of which biophysical processes are most important for driving contemporary recruitment of Lake Whitefish and Cisco in each of the Great Lakes, along with the mechanisms by which those drivers are hypothesized to regulate recruitment at key life stages (Brown et al. 2024). Climatic conditions during early life were understood to be among the most important drivers across the basin; other drivers were only deemed influential in certain lakes, highlighting context-dependent recruitment dynamics. Several drivers were hypothesized to similarly impact both species during embryonic and larval life stages, indicating that interventions targeting early life bottlenecks could improve recruitment for both species. Next, we reconstructed long-term (1956–2015) recruitment dynamics of Lake Whitefish and Cisco across each of the Great Lakes and Lake Simcoe (Brown et al. 2025). We estimated year-class strength (YCS), an index of recruitment to the adult population, using catch and age data from 38 long-term surveys and subsequently quantified synchrony in YCS between species, among lakes, and through time. Results demonstrate that these species exhibit fundamentally different recruitment dynamics (i.e., periodicity, synchrony) despite sharing similar early life histories. Lake Whitefish recruitment was above average in all six lakes for nearly two decades during the 1980–90s, but lake-specific trajectories diverged by the turn of the 21st century. In contrast, Cisco YCS was sporadic (“boom-and-bust”), not synchronous among lakes, and highly variable around the long-term mean for each lake. Lastly, we evaluated the suite of important biophysical drivers for recruitment of Lake Whitefish (Brown et al., in press) and Cisco (Brown et al., in revision), respectively, among the Great Lakes and Lake Simcoe. Important driver-response relationships between biophysical processes and YCS were dissimilar between species, highlighting the relative importance of pelagic versus benthic ecosystem change and when recruitment bottlenecks may occur. Notably, oligotrophication was associated with stronger recruitment for pelagic Cisco but weaker recruitment for demersal Lake Whitefish. For Lake Whitefish, results suggest that benthic food web perturbations resulting from dreissenid mussels and oligotrophication have reduced juvenile growth and survival across multiple lakes. For Cisco, climatic processes had high explanatory power but dissimilar relationships among lakes, suggesting that regional climatic forcing does not consistently regulate recruitment to the adult population across lakes. Lastly, important driver-response relationships varied across lakes spanning ecosystem gradients; for example, climatic processes such as warming temperatures and decreasing ice cover were important regional drivers, but with differing consequences across lake morphology and latitude. Our cross-lake, cross-species analyses provided novel insights into important drivers of recruitment, the underlying mechanisms by which drivers act to regulate recruitment across environmental gradients, and how the relative importance of drivers has changed through time. Collectively, this research advanced understanding of how Lake Whitefish and Cisco recruitment responded to past perturbations and how populations may respond to future climatic and ecosystem change.

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