Acorn

A targeted survey of nearshore fish species in Acorn Lake was conducted on August 12-14, 2024, by Index of Biological Integrity (IBI) Program staff. Sampling sites were evenly spaced around the lake, and each was sampled by backpack electrofishing and seining with a 50-foot or 15-foot seine, where possible. Backpack electrofishing was completed at ten sampling stations, one station was sampled from the boat due to soft sediment and dense aquatic vegetation. A 50-foot seine was used to sample four stations, and a 15-foot seine was used at five stations, and site conditions prevented the use of seines at one station. Nearshore sampling captured 14 species of fish including two species that are intolerant of disturbance (i.e., Banded Killifish and Blackchin Shiner) and one that is tolerant of disturbance (i.e., Green Sunfish). The most common species caught in the nearshore survey was Bluntnose Minnow. A concurrent summer trap net survey was also conducted by IBI Program staff. Nine trap nets were set along the shoreline in locations that encompassed multiple habitat types. Trap net sampling captured ten species of fish; no species captured in trap nets were intolerant of disturbance. Bluegill comprised most of the catch by number and biomass. Bluegill showed a moderate size structure, their average size was around 6.5 inches with 8% of individuals being larger than 8 inches and the largest Bluegill caught was 9.25 inches. 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.

Acorn is a 154-acre lake located two miles northwest of Frazee, MN. With a couple of exceptions, shoreline dwellings are limited to the southern half of the lake due to the presence of steep banks along the railroad tracks on the northeast shore and along the U.S. 10 roadway on the northwest shore. Several cold-water springs enter the lake along the west shore and the outlet typically flows year-round. Acorn Lake is moderately productive with floating-leaved vegetation growing throughout much of the lake's nearshore area. On 06/20/2023 a survey was completed in Acorn Lake to assess the fish community. Northern Pike abundance in 2023 was above average at seven fish per gill net. Lengths of sampled fish ranged from 15 to 31 inches and averaged 25 inches. Sixty-six percent of the Northern Pike that were measured in 2023 were 24 inches or longer. This is almost double the historical average of thirty-four percent. Gill nets captured an average of four Walleye per net in 2023, which is near the historical average for Acorn Lake. Walleyes averaged 17 inches in length and ranged from 10 to 26 inches. Forty-two percent of the Walleye measured in gill nets were longer than 20 inches. Walleye grew rather slow, reaching an average length of 14 inches in four years. Gill net catches of Yellow Perch in Acorn Lake have historically been high, averaging 140 fish per net from 10 surveys dating back to 1955. In 2023, gill nets captured an average of 115 perch per net. Perch averaged seven inches in length and ranged in size from 6 to 10 inches. Just over ten percent of the gill net sampled Yellow Perch were 8.0 inches or longer. Bluegill abundance was below average in 2023, with trap nets catching 28 fish per net. The average length of sampled Bluegills was six inches and Bluegills up to 9 inches were observed. The percent of Bluegills over eight inches in length was only five percent. For both trap and gill nets, 11 Largemouth Bass, and 23 Black Crappie were captured. The length of Largemouth Bass ranged from six to 18 inches and averaged 12 inches. Black Crappie had an average length of 10 inches with fish up to 13 inches being measured. Other fish species captured were Pumpkinseed Sunfish, Green Sunfish, Bowfin, White Suckers, and Brown and Yellow Bullheads. Cisco have not been sampled since 1983.

A targeted survey of nearshore fish species in Acorn Lake was conducted on June 12-13, 2022, by Index of Biological Integrity (IBI) Program Staff. Sampling sites were evenly spaced around the lake and each was sampled by backpack electrofishing and seining with a 50-foot seine, where possible. Backpack electrofishing was completed at all ten sampling stations. Similarly, a 50-foot seine was used to sample nine stations and site conditions prevented the use of seines at one station. Nearshore sampling captured 17 species of fish including two species that are intolerant of disturbance (i.e., Banded Killifish and Blackchin Shiner) and three species that are tolerant of disturbance (i.e., Black Bullhead, Fathead Minnow, and Green Sunfish). The nearshore data were combined with gill net and trap 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). 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. Data from this survey indicates 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. Data from this survey will contribute biological information about the health of the fish community to the Ottertail River Watershed assessment process in coordination with MN Pollution Control Agency.

Based on Minnesota DNR survey data, anglers target Walleye, Northern Pike, Black Crappie, Yellow Perch, and Bluegill in Acorn. See the full species list above for catch rates, size structure, and quality scores.

Minnesota DNR records list public access for Acorn. Check the DNR LakeFinder page for the launch type and directions before you go.

Acorn has a maximum depth of 55 feet and a mean depth of 19.8 feet, per Minnesota DNR data.

The most recent Minnesota DNR survey data for the main sport fish in Acorn is from 2024.

No invasive species are on record for Acorn in Minnesota DNR data. Always clean, drain, and dry your equipment to help keep it that way.