Cross-basin analysis of long-term trends in the growth of lake whitefish in the Laurentian Great Lakes

We conducted a basin-wide analysis of trends in the growth rate of lake whitefish (Coregonus clupeaformis), a commercially harvested species in the Laurentian Great Lakes. Juvenile growth (measured as the growth between ages 1 and 2 years) was back-calculated from agency archived scale collections going back as far as the 1950s. We examined trends for 11 locations within the Great Lakes, and investigated the role of multiple explanatory factors (dreissenid mussel establishment; lake whitefish relative abundance; growing degree days) in contributing to the variation observed. Juvenile growth rates declined in all but one location where dreissenid mussels have had widespread establishment. Growth of juvenile lake whitefish from Lake Ontario showed the largest decline following dreissenid establishment, decreasing by 32%. In several locations, lake whitefish growth rates declined or had breakpoints prior to dreissenid establishment and have stabilized or increased in recent years, thus indicating the contribution of other factors. One location in Lake Superior (Apostle Islands) also showed a marked decline and subsequent increase in growth, whereas the other two Lake Superior locations showed no obvious trends. Changes in relative abundance of lake whitefish and growing degree days contributed to growth patterns among locations, but the effect was inconsistent and in most cases weaker than that from the timing of dreissenid establishment. Although our study cannot identify a specific mechanism involved, the suite of changes at the base of the food web that coincided with the timeline of dreissenid establishment appear to have had a broad-scale impact on lake whitefish.