The Minnesota Department of Natural Resources (MnDNR) and the Carnelian-Marine-St. Croix Watershed District (CMSCWD) agreed to stop stocking rainbow trout in Square Lake in 2012. The terms of the moratorium were spelled out in a Memorandum of Understanding (MOU) that allowed the District and its consultants three years to test an hypothesis that linked rainbow trout to a decrease in water clarity in the lake. Simply stated, the theory was that rainbow trout fed on daphnia, a zooplankton that in turn grazed on the algae in the lake; no trout would lead to greater daphnia populations, less algae, and therefore greater water clarity.
The District has been looking for the causes of a declining water quality trend in Square Lake since 1999 when it received funding through the Minnesota Pollution Control Agency for a Clean Water Partnership Grant. It worked with local residents, units of government, and state agencies including the MnDNR, to study possible causes for decreasing water quality. A second round of funding in 2009 led to implementation of improvement projects including the trout research project.
The final report by District Consultant Dr. Leif Hembre was submitted to the MnDNR in June of 2016 and reviewed by a third party and Agency scientists. The consensus among all of the reviewers is that the District’s hypothesis has been supported by the evidence in the report and that the presence of rainbow trout does lead to a decrease in water clarity.