Hamline News

A Fish Stocking Story About Square Lake

A recently published multi-year research study by Hamline University Biology Professor Leif Hembre, Ph.D. showed that Minnesota Department of Natural Resources (MN DNR) moratorium on stocking a local lake with rainbow trout improved water clarity and quality.

Rainbow trout are a popular fish to catch and the MN DNR began stocking them in Square Lake in Washington County in 1981. Since the 1980s, the water in this popular recreational lake has become more cloudy due to increased levels of algae. However, the phosphorus levels in the lake, the usual cause for increased algae levels, have not changed. This suggested that “top-down” changes in the lake’s food web caused by the annual stocking of rainbow trout, rather than nutrient pollution, could be responsible for the cloudier water in Square Lake.

The Carnelian-Marine-St. Croix Watershed District reached out to the MN DNR with concerns about the possible negative impact of trout stocking on the lake’s water quality, and Hembre was called on to investigate this hypothesis.

Hembre had been tracking the population of Daphnia — crustaceous zooplankton otherwise known as water fleas— and measuring aspects of the lake’s water quality at Square Lake since 2003.

Given the evidence from previous research studies in the lake, the thinking was that predation by the rainbow trout stocked by the MN DNR might be depleting the lake of Daphnia, which eats lake algae and keeps the water clear. Hembre’s research included years when the DNR had stocked the lake with trout, so halting the stocking practice for a few years and continuing zooplankton and water quality data collection provided a way to test the idea that rainbow predation was responsible for the degradation in the lake’s water quality over the years.

“It was a perfect opportunity for a natural pre- and post-test in the field,” said Hembre. “I feel fortunate to have been able to conduct this type of research and provide insight into the effect of fisheries management practices on water quality.”

The MN DNR stopped stocking Square Lake in the fall of 2012. During the open water seasons of 2013-2015, Hembre and Hamline students researchers sampled zooplankton from various lake depths and monitored the lake’s water quality for clarity, algae levels, oxygen levels, and nutrients. Back at the campus biology lab, the team analyzed the zooplankton samples to assess the Daphnia population, and evaluated connections between Daphnia abundances and water quality measurements such as water clarity, algae and phosphorus levels, and levels of oxygen in the water These results were compared to similar data collected between 2010 and 2012 while the DNR was still stocking the lake with trout.

“We found that during the moratorium years, there was a larger population of algae-eating Daphnia, and as expected, the water was more clear and more oxygen persisted in deep water than when the lake was being stocked,” said Hembre.

The Hamline team did find one unexpected result of the stocking moratorium.

“We were surprised to see how much the phosphorus levels in the surface water of the lake decreased as the population of Daphnia rose,” said Hembre. The researchers suspect that the decrease in phosphorus was caused by the higher biomass levels of Daphnia, which sequester phosphorus in their bodies when they consume algae.

The MN DNR has no immediate plans to resume stocking Square Lake with rainbow trout.

July 16, 2019
Written by staff

Photo provided by Professor Hembre.