Lower Trophic Level Response to the Restoration of Native Anadromous Alewife: A Case Study.

Wilson, Karen¹, Sharon L. Mann², ¹University of Southern Maine, Department of Environmental Science and Policy, ²University of Maine, Ecology and Environmental Sciences

The reintroduction of native anadromous alewife to historic spawning lakes in Maine has not been without controversy. We have been collaborating with a lake association concerned that an unusual phytoplankton bloom that occurred seasonally beginning in 2014 was triggered by a large increase in the number of adult alewife spawning in the lake (i.e., lower-level trophic cascades). The bloom showed rapid development and sustained poor water clarity (< 2 m secchi depth) for 3 weeks, followed by sudden improvement in water clarity in mid-August each year until 2018 when blooms began decreasing in duration and severity. In 2018, 2019, 2021, and 2022 we collected monthly juvenile alewife abundance and size data while simultaneously monitoring adult spawner numbers, water quality, and zooplankton communities. Juvenile fish abundance was positively related to adult spawner abundance. In high alewife summers (156 and 247 adults/hectare), zooplankton communities shifted to copepods and rotifers and many fish were stunted. These small fish were not obvious in the lake in 2021 and 2022 when adult alewife numbered approximately 21 -34 adults/hectare, suggesting strong density-dependence. Despite a negative relationship between minimum summer secchi depth and spawning adult numbers (R2=0.40), the lake did not exhibit sustained algal blooms in 2018 - 22. These results suggest that alewife may influence water clarity in this lake through trophic cascades, but that the response of lower trophic levels to alewife predation may be changing over time.