Grindstone

A targeted survey of the nearshore fish community was conducted by Hinckley fisheries staff on Grindstone Lake on August 4, 2025. Nearshore sampling captured 19 species of fish: Black Crappie, Bluegill, Bluntnose Minnow, Common Shiner, Creek Chub, Fathead Minnow, Golden Redhorse, Greater Redhorse, Iowa Darter, Johnny Darter, Largemouth Bass, Pumpkinseed, Rock Bass, Shorthead Redhorse, Smallmouth Bass, Spottail Shiner, White Sucker, Yellow Bullhead, and Yellow Perch. The nearshore survey data is combined with standard gill and trap netting survey data from previous Grindstone Lake surveys to calculate a Fish IBI (FIBI) score and helps assess aquatic life in Grindstone Lake. The FIBI score for Grindstone Lake based on this survey is above the impairment threshold for aquatic life use determined for similar lakes. This survey is contributing to a watershed assessment process in coordination with the MN Pollution Control Agency. For additional information on the FIBI, search for "lake index of biological integrity" on the mndnr.gov website.

Grindstone Lake is a 526-acre lake located in west-central Pine County eight miles northwest of Hinckley and is within the Kettle River watershed. It has a maximum depth of 153 feet, has an average depth of 60 feet, and is 14% littoral (depths less than 15 feet, 71 acres). The lake is well oxygenated from the surface to the bottom, with cold waters that support coldwater species such as trout as well as cool and warmwater species such as bass, panfish, and pike. The main inlet is adjacent to the boat landing in the northwest corner, while the main outlet in the southwest corner is the start of the North Fork of the Grindstone River. A five foot dam is located at the outlet. Eight other small inlets are present, most of which only flow after precipitation events. No other outlets are present. The lake is moderately developed with homes on the south, west, and northern shores, while Osprey Wild Environmental Learning Center maintains most of the eastern shore in a natural condition. Water clarity is good, commonly extending to depths of 9-12 feet. An aquatic vegetation survey in 2018 found a diverse, high quality plant community with vegetation growing to 8 feet. Vegetation was typically limited by substrate type, such as cobble and rubble at deeper depths. Sand, gravel, cobble, and rubble are common throughout most of the lake which provides good spawning substrates for Bluegill and crappie. There is one public accesses on Grindstone Lake in the northwest corner that has a gravel parking lot with ten double spaces and a cement ramp. This access is shallow, which limits the size of boat one can launch. Past standard lake surveys on Grindstone Lake have occurred in early-August. Warm summer water temperatures typically result in larger Bluegills and crappies suspending in open water, which decreases their sampling efficiency. During the last two standard surveys it was noted that Bluegill catch rates had increased, and a few anglers had inquired about the status of the Bluegill population. No prior Bluegill or crappie spawning surveys have occurred on Grindstone Lake. In 2025, a survey targeting spawning Bluegill was conducted using 14 trap nets set over 3 days to evaluate the current Bluegill population (Figure 1). Sampling occurred June 16 - 18, 2025 when water temperatures were 69-70 °F degrees and warming. A total of 1,112 Bluegill were sampled for a catch rate of 37.1/net. Bluegill lengths ranged from 2.2 to 9.1 inches with an average length of 6.0 inches. Only 3% of Bluegill caught in 2025 exceeded 8 inches. Overall, Grindstone appears to support a high-density Bluegill population with below average size structure. Other panfish species sampled in 2025 included Black Crappie and pumpkinseed. A total of 211 Black Crappie were caught for a catch rate of 7.0/net. Lengths ranged from 3.5 to 15.0 inches with an average length of 7.9 inches. Only 9% percent of Black Crappie caught in 2025 exceeded 10 inches; however, five individuals over 13 inches were sampled. In 2025, a total of 30 pumpkinseeds were caught for a catch rate of 1.0/net. Lengths ranged from 3.5 to 7.7 inches with an average length of 6.1 inches.

Grindstone Lake, with a maximum depth of 153 feet, is unique among lakes in east-central Minnesota. The long, narrow 528 acre lake has adequate oxygen and cool water to support trout as well as warmer water species such as bass, panfish, and pike. Water clarity is excellent with visibility down to 13 feet. Rainbow Trout and Brown Trout are stocked annually in the spring, and Rainbow Trout are also stocked in the fall. Lake Trout are stocked every other year (in odd-numbered years) to provide one of the two southern most Lake Trout fisheries in the state. Rainbow Smelt, which were not introduced by the DNR, provide an additional angling opportunity in winter and spring. A gravel public access with room for 7 trucks and trailers, is located at the inlet on the north side of the lake. During periods of low water, boat launching can be difficult for large boats although the access was dredged in the fall of 2016. The lake is heavily developed with 90 homes, a Bible Camp, and an Environmental Learning Center. There is a large section of land owned by Osprey Wilds Learning Center on the East side of the lake with a very natural shoreline and limited development. Although Grindstone Lake is managed for trout, it is not a designated trout Lake, so rules which apply to designated lakes are not in effect on Grindstone. However, a trout stamp is required to keep and transport any Rainbow, Brown, or Lake Trout. Due to Grindstone Lake's depth and steep contours, the Rainbow, Brown, and Lake Trout may be difficult to locate. Winter angling is very popular and can be quite productive. Open water anglers have the most success right after the season opens in May, before the surface water has warmed up and Trout still occur in shallow water. Including the initial survey in 1938, twenty-six fish surveys have been completed to monitor changes in species composition, number, and length distribution. More recent surveys have occurred in 2002, 2007, 2012, 2016, 2020, and 2025. Survey frequency is approximately every 4 years. Gamefish species that are best suited for the lake include Lake Trout, Rainbow Trout, Brown Trout, Northern Pike, Rainbow Smelt, Black Crappie, and Smallmouth Bass. No special regulations are present. A total of twelve Lake Trout were caught in the 2025 survey for a catch rate of 0.4 fish/net, which compares to the average of 1.1 fish/net. Lake Trout ranged from 7.8 to 34.3 inches in length and averaged 18.1 inches, which compares to the 12.5 inch average in 2020. The 2025 catch rate was the lowest recorded in the last seven surveys but two fish over 30 inches were sampled. Estimated ages from otoliths indicated 6 year classes, which included age-1, age-2, age-3, age-5, age-7, and age-8. The population has a low density of moderate sized fish, with a few memorable fish present. Anglers can expect to catch low numbers of Lake Trout in the 8 to 22-inch range, with a chance at a Lake Trout over 30 inches. Rainbow Trout gillnet catches were up in 2025 (0.2 fish/net) when compared with the 2020 (0 fish/net) catch. The 2025 catch was below the average of 0.6 fish/net. Rainbow Trout are not sampled well by either gill nets or trap nets in Grindstone, making it hard to assess the population. Rainbow Trout are managed as a put-grow-and-take fishery, so good angler reports means the goal is being achieved. A total of six Rainbow Trout were sampled in 2025, with fish ranging from 11.8 to 21.3 inches and a mean length of 14.8. Anglers can expect to find moderate to high numbers of 10 to 12 inch fish during the spring and early summer or during the trout ice fishing season after the lake has been freshly stocked. Anglers also have the opportunity for a trophy sized Rainbow that have survived a few years in the lake. Only two Brown Trout were sampled in 2025. The fish measured 12 inches and 15.7 inches. One Brown Trout was caught in 2020. The catch rate of 0.1 fish/net is below the average of 0.3 fish/net. Brown Trout are also managed mostly as a put-grow-and-take fishery and stocked at a rate of 2,500 annually each spring. Anglers can expect to catch low numbers of moderate size Brown Trout. The lake produced a trophy size Brown Trout, in the mid-1900's. The fish was 31 inches and 16 pounds, which was the previous state record. An above average number of Northern Pike were sampled during the 2025 survey. Pike catch rates were 1.6/net in 2025, which compares to the lake average of 0.8/net. Pike catch rates in 2025 were above the third quartile for the lake (1.5/net). The pike sampled ranged from 16.9 to 33.7 inches and had a 22.2 inch average. The average length of pike sampled in 2020 was 23.7 inches. Despite low numbers, large pike over 28 inches have been sampled in all surveys. Anglers and dark house spearers report catching trophy size pike. During the 2025 targeted survey for Bluegill, a 40-inch pike was sampled. Estimated ages from cleithra indicated five year classes from age-2 through age-6 were sampled in 2025. Pike sampled in 2025 had PSD and PSD-P values of 54 and 12, which were slightly below the average of 70 and 17. Pike populations with normal length distributions have PSD values between 30 and 60, indicating this population has an average population of fish over 21 inches. The Rainbow Smelt gill net catch rate remained the same in 2025 as the 2020 catch rate of 1.7/net. The average is 1.5/net. Rainbow Smelt ranged in size from 3.5 to 8.4 inches and averaged 6.1 inches. The last few years of hydroacoustic surveys indicated the Smelt numbers have somewhat declined since the 2021 survey. Anglers can still expect to have angling success catching 6 to 8 inch Smelt. Rainbow Smelt are a major part of the prey base for predatory fish in Grindstone Lake, and they also draw attention from many anglers during the winter and spring months. Smallmouth have been present in Grindstone Lake for many years, but nighttime electrofishing for them had not occurred until May of 1997. In 2025, Smallmouth Bass were sampled at a rate of 41.5/hour, which compares to the average of 34.6/hour. The 2016 survey was the most recent electrofishing survey and had the highest catch rate for Smallmouth Bass of 55.9/hour. Smallmouth ranged from 4.5 to 17.5 inches and averaged 8.2 inches. This compares to the average length of 5.5 inches in 2016. Anglers have reported trophy Smallmouth Bass in Grindstone. Nighttime electrofishing for Largemouth Bass was initiated in May of 1997 and conducted in five of the last seven surveys. In 2025, Largemouth Bass were sampled at a rate of 26.8/hour, compared to the average of 8.1/hour. Largemouth Bass have been sampled in Grindstone Lake for at least 60 years, but at very low densities. In the last 20 years, Largemouth Bass catch rates have increased in each survey; 2002 (0.0/hour), 2012 (4.8/hour), 2016 (9.2/hour), and 2025 (26.8/hour). In 2025, Largemouth ranged from 3.2 to 18.9 inches. Their average length was 8.8 in 2025 compared to 14.7 inches in 2016. Anglers can expect to catch a moderate number of bass in the 5 to 10 inch range, and a chance at a fish over 15 inches. Anglers have reported catching large Largemouth Bass. The Black Crappie gillnet and trap net catches were higher and lower respectively when compared with 2016 catches. The gill net catch was 1.1/net in 2025, which compares to the average of 0.3/net. The trap net catch was 1.1/net, which compares to the average of 3.6/net. Crappie ranged in length from 4.5 to 11.9 inches and had an average length of 8.5 inches for both gears. Crappie over ten inches have been sampled in all of the past seven surveys, while crappie over 12 inches have been sampled in two of the last seven surveys. Crappie exceeding 15-inches have been reported by anglers. Multiple Crappie over 13 inches were sampled in the 2025 Targeted survey for Bluegill. This population has an above average proportion of fish over eight and ten inches. The 2025 Bluegill trap net catch rate of 11.8/net compares to the average of 8.5/net. Bluegill ranged from 3.2 to 8.6 inches with a 5.4 inch average. Bluegills over 8 inches were sampled in only two of the last seven surveys. This population has an average proportion of fish over 6 inches, but a below average proportion of fish over 8 inches. Yellow Perch catch rates have remained consistently low throughout all 26 surveys. Gill net catch rates have only exceeded 5/net three times (1962, 1963, and 1964). Perch gill net catch rates were 0.0/net in 2025, which compares with the average of 1.5/net. No Perch were sampled in 2025 in the gill nets or the trap nets. Perch numbers are too low to offer angler opportunities. Additional gamefish species sampled include Black Bullhead, Hybrid Sunfish, Pumpkinseed, Golden Redhorse, Greater Redhorse, Rock Bass, Silver Redhorse, White Sucker, and Yellow Bullhead. Additional non-game species sampled include Bluntnose Minnow, Black Nose Shiner, Brook Stickleback, Central Mudminnow, Common Shiner, Creek Chub, Fathead Minnow, Golden Shiner, Iowa Darter, Johnny Darter, and Spottail Shiner.

Based on Minnesota DNR survey data, anglers target Smallmouth Bass, Largemouth Bass, Lake Trout, Northern Pike, and Black Crappie in Grindstone. See the full species list above for catch rates, size structure, and quality scores.

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

Grindstone has a maximum depth of 153 feet and a mean depth of 60 feet, per Minnesota DNR data.

The most recent Minnesota DNR survey data for the main sport fish in Grindstone is from 2025.

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