After a several-year absence, rainbow trout may soon return to Square Lake, but not everyone is happy about it.
Residents who live on the lake are concerned the non-native species has an impact on the lake’s food chain — raising the amount of algae and decreasing the water clarity. That’s significant, because Square Lake is known for some of the clearest water in the state, and boasts a popular swimming beach in Washington County’s Square Lake Park.
Representatives from the homeowners association, the Minnesota Department of Natural Resources and lake ecology scientists discussed future trout stocking March 8, during a board meeting of the Carnelian-Marine-St. Croix Watershed District.
Meghan Funke, a limnologist from the environmental consulting firm EOR, presented an overview of the studies that have been completed on Square Lake related to water clarity and the stocking of rainbow trout. The studies have shown that the water clarity has decreased in Square Lake due to an increase in algae, Funke said.
“There was no change in the amount of phosphorus or nutrients in the lake, so we looked to see if there was a change in the lake’s food web,” Funke said.
The Carnelian-Marine-St. Croix Watershed District hired Leif Hembre of Hamline University to study the effects of the rainbow trout on the food web of Square Lake. Between 2013 and 2015, the DNR agreed to put a moratorium on the stocking of rainbow trout to see if there was a change in the lake’s water clarity.
Starting in the 1970s, the DNR began stocking rainbow trout in Square Lake in a pattern of about 3,000 in the spring and 2,000 in the fall. In his report, Hembre found that the majority of the trouts’ diet in Square Lake consists of small organisms called zooplankton, specifically daphnia.
Between April and June, these daphnia have large increase in population and graze on the spring algae. In turn, the water clarity in Square Lake increases due to the consumption of the algae. With the stocking of the trout, the daphnia are consumed by the fish and there are fewer daphnia to eat the algae.
Funke explained that during the fall and winter, the daphnia populations decrease and the trout have fewer sources of food. As the trout eat the daphnia over the winter, there are fewer left to reproduce in the spring, compoundingthe problem and allowing more algae to grow.
“Comparisons between pre-moratorium and moratorium years during different time periods in the open-water season show that Square Lake had significantly lower surface water algae biomass, and significantly clearer water during moratorium years compared to years when the lake was stocked with trout,” Hembre said in his report.
After a review of the study, TJ DeBates, east metro area fisheries supervisor, said the DNR recognizes that “the science is good,” and agrees the rainbow trout are one of several factors that affect the water clarity of Square Lake. However, DeBates proposed compromise that would limit the number of rainbow trout stocked and would stock the fish in the spring — after the daphnia reproduce and are plentiful. Over the next three years, the DNR would continue researching the relationships among the fish, daphnia and algae.
“We would stock 3,000 trout in the spring and stock none in the fall,” DeBates said. “We could put the trout in the lake as late as a few days before fishing opener.”
Unlike past years when the rainbow trout were stocked, DeBates said there would be no regulation on the trout if stocked this year.
“We don’t want them in the lake over winter,” DeBates said. “We want to put them in and have the anglers take them out.”
DeBates said the DNR receives many calls and emails from anglers in the metro area looking for trout fishing opportunities. Cenaiko Lake in Anoka County and Holland Lake in Dakota County are currently the closest lakes to Washington County that are stocked with rainbow trout.
Rich Cummings, a representative of the Square Lake Homeowners Association, said during the meeting that homeowners on the lake oppose the DNR proposal to resume trout stocking — pointing out that the watershed district and the homeowners have spent significant time and money to research and care for the lake.
“The science is clear and compelling,” Cummings said. “The presence of rainbow trout has a direct detrimental impact on Square Lake water quality.”
Jim Shaver, administrator of the Carnelian-Marine St. Croix Watershed District, said the decision to stock rainbow trout is based on a combination of “science and policy,” and that politics is a factor in how decisions are made.
DeBates said the DNR has not made its decision yet as to whether rainbow trout will be stocked in Square Lake for the May fishing opener. The watershed board scheduled a workshop for March 22 to continue discussing the proposed stocking plan and whether the board would support or oppose the plan.
Contact Alicia Lebens at [email protected]