Acorn Lake Fishing guide

The nearshore and trap net data were combined with gill net data from a June 2023 survey to describe the fish community and provide a fish-based IBI (FIBI) score. The FIBI uses fish community data to measure a lake's health, and the types of fish species present can help identify any stressors that may be negatively affecting the lake environment. In Minnesota lakes, certain fish species cannot survive without clean water and a healthy habitat (e.g., Blackchin Shiner, Iowa Darter, and Rock Bass), while other species are tolerant of degraded conditions (e.g., Green Sunfish and Common Carp). The FIBI score, composed of several fish community diversity and composition metrics, indicates the overall health of a lake by comparing it to what is expected for a healthy lake. For additional information on the FIBI, search for "lake index of biological integrity" on the mndnr.gov website.

Acorn Lake scored 36 with the IBI 4 model. Results from this survey provide evidence that the fish community in Acorn Lake is adversely impacted by stressors, as indicated by an FIBI score just below the impairment threshold for aquatic life use determined for similar lakes. These results are similar to the previous assessment which determined Acorn Lake was vulnerable to impairment based on the FIBI. These results will be considered when the biological health of the lake is assessed during the Ottertail River Watershed assessment process, which will be completed in coordination with MN Pollution Control Agency.

Temperature and dissolved oxygen profiles were also collected in the deepest basin on Acorn Lake on August 13, 2024, to evaluate the amount of cold, oxygenated water (i.e., oxythermal habitat) available to Cisco, also known as Tullibee, the most sensitive coldwater species present in this lake, however, they have not been detected since 1983. Cisco require a layer of oxythermal habitat that contains water temperatures below 21.2 degrees C (70.2 degrees F) and dissolved oxygen concentrations above 3.0 mg/L. degrees C (67.3 degrees F). Based on the profile from the deepest basin, the water temperature decreased to below 21.2 degrees C (70.2 degrees F) at 12.1 feet and the dissolved oxygen concentration decreased to 3.0 mg/L at 20.6 feet at the time of sampling. The thickness of this layer of water in the water column was 8.5 feet, indicating that oxythermal habitat was present for Cisco.

Further, the temperature at which dissolved oxygen decreased to 3.0 mg/L (i.e., TDO3, a single variable to characterize oxythermal habitat) was 19.6 degrees C (67.3 degrees F). TDO3 values cooler than 21.2 degrees C (70.2 degrees F) indicate that oxythermal conditions were suitable for Cisco at the time of this survey.

This is the first oxythermal habitat survey completed on Acorn Lake during the late summer thermal stress period, however, data collected outside the thermal maximum period from 2008-2023, indicated suitable habitat conditions during those sampling periods. The least favorable oxythermal habitat conditions were recorded in 2013, outside the peak summer stress period, when the suitable habitat layer was 10.0 feet thick. When inadequate oxythermal habitat is available, Cisco are squeezed into unfavorable, warmer water conditions and experience higher levels of stress.

Acorn Lake supports several important game fish species (i.e., Walleye, Northern Pike, Largemouth Bass) as well as a variety of sensitive non-game species (i.e., Banded Killifish, Blackchin shiner, Johnny Darter). In the past, Acorn Lake had other sensitive species such as a population of Cisco (Tullibee) and Rock Bass, however, they have not been detected since 1983 and 1955 respectively. You can help protect Acorn Lake's unique fish community by reducing runoff, maintaining natural shorelines, and preventing the spread of invasive species.