Grindstone Lake is a coldwater, two-story lake in east central Minnesota. Assigned to lake class 23, Grindstone Lake is currently managed for Lake, Brown, and Rainbow Trout. The lake has a mean depth of 60 feet and a maximum depth of 153 feet. During the spring of 2022 a special assessment was conducted on Grindstone Lake by MN-DNR Fisheries staff in order to examine stomach contents of Rainbow Smelt. Stomach content analysis provides important insight into fish feeding patterns and quantitative assessment of food habits is an important aspect of fisheries management. In an attempt to sample smelt effectively, fisheries staff used spring trap nets during the spawning run to collect smelt data. Due to time constraints and limited staff, we missed the spawning run and fished ten nets for only one day in May and captured 24 smelt. All smelt captured were dissected and stomach contents examined with a dissecting scope. Eight smelt stomachs had food materials present while the remaining 16 smelt stomachs were empty. Most food materials were partially digested which made forage identification difficult. Food materials found included unidentified fish and zooplankton remains. Forage that were positively identified were Rainbow Smelt, Yellow Perch, and Mysis Relicta. Length distribution of smelt ranged from 4.4 to 5.6 inches with an average length of 5.2 inches. CPUE for smelt was 2.4 per net lift. Future surveys should be conducted in conjunction with the spawning run. Previous surveys implemented during the spring spawning run have yielded good numbers of smelt.
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Management Grindstone Lake, Pine County, is one of few coldwater lakes in the lower half of the state of Minnesota. The Minnesota Department of Natural Resources (MDNR), Section of Fisheries has directed its attention toward trout management and research on Grindstone Lake for the past 50 years. Trout stocking was to make optimum use of the large amount of cold-oxygenated water furnished by this lake (Groebner 1969). The stocking of troutprovided angling opportunities for those who did not want to travel to the northeastern part of the state where trout lakes are more common. The original fish population of Grindstone Lake was composed of warmwater species. From 1908 to 1945, various trout, salmon, and coolwater fish species were stocked into Grindstone Lake. Between 1938 and 1946, some lake investigations and short creel surveys were conducted. Trout stocking was started on a regular basis in 1949, the same year the first full lake survey was recorded. Brown trout were stocked from 1950 to 1959, discontinued until 1975, then stocked annually to today. Emphasis on stocking focussed on rainbow and lake trout from 1955 to 1975. Rainbow trout have been stocked annually to present day since 1955. Lake trout stocking took place from 1955 through 1959, a gap from 1960 through 1965, then followed by annual stocking until 1977 except for two gap years (1968 and 1972). One additional stocking occurred in 1986. The MDNR fisheries lake management plan dated 8/12/96 was revised to include lake trout stocking of yearling fish on an annual basis. In both 1996 and 1997, lake trout were stocked. The MDNR introduced kokanee salmon to the lake during the 1960's in hopes of establishing a spawning population. This short lived fish (4 years) showed promise but average lengths of the adult fish were only from 10 to 11 inches. The program was discontinued due to poor spawning success and the low rate of return to the angler. Splake stocking was also tried in the 1960's and early 1970's but with poor rates of survival andreturn to the angler. In 1965, special regulations were enacted on Grindstone Lake. These regulations allowed fisheries managers to manage Grindstone as an experimental trout lake (lake and stream trout). The regulations were: 1) a continuous open angling season for all species.2) the use of three hooks on one line.3) a minimum size limit of 30 inches for lake sturgeon.4) spearing was prohibited. These special regulations were to increase angling opportunity, increase the harvest of stocked trout species, allow fishing for smelt since there was no open season in inland waters, and to ban spearing as to protect trout from being speared. These regulations had an end date of March 1, 1994. In 1993, the MDNR had a public information meeting to discuss if the special regulations were still necessary on Grindstone Lake. It was concluded that the regulations return to the statewide norm effective after March 1, 1994 for the following reasons: 1) Easier enforcement. Seasons will be the same as statewide seasons for warmwater species and trout species in inland lakes. 2) MDNR will be able to stock the trout 3-4 weeks before their season opener giving trout a chance to disperse, acclimate, and begin feeding on natural food before anglers can start fishing for them. This should make them less vulnerable to the angler. 3) Fishing pressure and harvest of the stocked stream trout is well established already. We do not need the extended seasons to promote further harvest as was the goal in 1965.4) There is a winter stream trout season now. In 1965 there was no winter stream trout season. The current regulations and seasons for Grindstone Lake are the same as for warmwater species and trout species in inland lakes. Dark house spearing is now permitted. At present, MDNR management plans for Grindstone Lake still call for 5,000 rainbow trout (yearlings), 2,500 brown trout (yearlings), and 5,020 lake trout (yearlings) to be stocked annually in late April. Status of the Fishery Grindstone Lake is ecologically classified as a trout lake and managed as a lake and stream trout lake (lake class 23). Fish populations consist of a mixture of coldwater and warmwater species. This is typical for a lake of this type, commonly called a \"two-story\" lake. The uppermost region of the lake (termed the epilimnion) usually holds warmwater species and the region below this (metalimnion) usually contains more of the coldwater species. The lowermost region of the Grindstone Lake, the hypolimnion (zone below metalimnion), usually has sufficient dissolved oxygen to about 110 feet during the warmest months of the summer. However, these depth zones can be utilized by any fish species anytime. Generally speaking, fish species prefer atemperature range suitable to them that contains an available forage base. According to recent DNR test netting (1987, 1992, 1997) coldwater fish species present in Grindstone Lake were tullibee (cisco), rainbow trout, brown trout, lake trout, and rainbow smelt. Warmwater fish species present were northern pike, brassy minnow, common shiner, spottail shiner, bluntnose minnow, fathead minnow, creek chub, white sucker, golden redhorse, shorthead redhorse, greater redhorse, black bullhead, yellow bullhead, brown bullhead, rock bass, pumpkinseed, bluegill, largemouth bass, smallmouth bass, black crappie, Iowa darter, johnny darter, and yellow perch. The tullibee (cisco) population has been low in recent years. Competition with a larger population of rainbow smelt seemed to have kept tullibee numbers in check since the mid 1970's. They were not sampled in the 1997 survey. The rainbow trout population has been maintained only by stocking, as they are not native to this area nor do they successfully reproduce here. The 1997 survey sampled their abundance higher than in past surveys, however, the catch was solely yearling fish from the 1997 stocking. Following annual stocking each spring, a large portion of the stocked fish are caught by anglers. Research has found that rainbow trout are more easily caught than brown trout. This is one reason why the proposed season change for trout would give rainbows a few weeks to disperse throughout the lake. Other factors known to reduce the stocked rainbow trout are migration out of the lake, predation by larger species (e.g., northern pike), and natural mortality. Recent MDNR studies have found that some fish leave the lake via the outlet of the North Fork of the Grindstone River. Downstream temperatures of this river reach lethal levels (above 80?F) for trout to survive. Northern pike stomach samples have documented rainbow trout as one of the species in their diet. Generally, rainbow trout in Grindstone Lake rarely survive long enough to reach lengths greater than 15 inches. To date, strain changes have not shown improvement for increasing the size of rainbow trout in this lake. Brown trout were sampled at normal abundance relative to previous survey catches, although, their lengths ranged from 10.7 to 19.9 inches. Ages one through five were recorded in the sample. The brown trout population has the same factors influencing their numbers as the rainbow trout, but with migration not as prevalent. Brown trout have typically been harvested in great numbers by anglers before reaching 15 inches, but this fish has and can grow to more than 10 pounds. There have been more reports of larger brown trout vs. rainbow trout through the years. Growth rates are on the faster side of normal with ample forage of rainbow smelt and yellow perch. In 1961, a 16.5 pound brown trout was caught and was the state record until the mid 1980's. Lake trout were sampled in somewhat high abundance but of those observed, 87 percent were one year olds from the spring stocking (7.5-8.0 inch fish). Four larger lakers measuring 27.0 to 32.5 inches were sampled. They ranged in age from 9-11 years old. Future stockings and surveys will better determine if the lake trout population can maintain adequate levels. Although contrary to past MDNR survey results, aged lake trout from recent survey data have found some lake trout in Grindstone Lake to be from non-stocking years. This shows a small amount of successful natural reproduction but probably not enough to sustain a viable population acceptable to anglers. One must recognize the limiting factors that play upon a fragile lake trout population: angler harvest, competition with other species, predation by other species, and natural mortality. Rainbow smelt were first found in test netting in 1963. Their method of introduction into Grindstone Lake was not by the MDNR. One belief was the disposal of smelt entrails from a Lake Superior fishing/netting trip. The other would be the direct stocking of smelt. At any rate, they have reached a well-established population and act as a good forage base for the trout. Angling for the smelt through the ice became popular in the 1970's. The 1997 catch sampled smelt at abundance levels typical of past surveys. They ranged from 6.4 to 9.3 inches, while averaging 7.4 inches. Northern pike have always maintained population levels normal for this lake type. This survey sampled pike from 18.1 to 38.2 inches. Their average weight was a hefty 4.4 pounds that places them heavier than the average for lakes of this type. Removal of this species was once an issue for many years. In order for the trout to flourish, removing a primary predator such as northern pike was thought to have positive results. Therefore, removal efforts were tried but met with poor success because spawning movement in and out of the lake was too wide spread. Barrier maintenance became difficult to control and alternative spawning habitat preferences within the lake were nearly impossible to monitor. Program time and costs far outweighed the removal results, so the project was discontinued. As before, northern pike provide a good opportunity to catch trophy fish. Growth rates are fast and the fish are large for their age. Smallmouth bass were present in fair abundance during the 1997 survey. They ranged between 3.7 and 15.0 inches, while the average length was 7.9 inches. Angler reports and some MDNR visual observations have noted smallmouth bass to attain sizes more than five pounds in Grindstone Lake. Normally, a low priority species sought after in this lake, they can provide excitement and pleasure to most anglers who find one at the end of their line. All other warmwater species in Grindstone Lake have fairly low populations. The lake lacks the suitable shallow water habitat necessary for producing and holding large amounts of these species. Aquatic vegetation is one of the primary habitats found in variably low abundance. Black crappie were apparently found in higher numbers during the 1950's and sizes were considerably large (2 pounds). The present crappie population was below average but that has been the case for 40 years. One angler reported during the survey that he can still go out and catch 16 inch crappie, although admits to just catching two or three per trip. Prepared by: Jack Lauer, Area Fisheries SpecialistREFERENCESGroebner, J. 1969. Progress Report. Grindstone Lake. Minnesota Department of Natural Resources, St. Paul, Minnesota.
","surveyType":"Standard Survey","surveyDate":"1997-08-04","headerInfo":{"wb_alt_name":"","survey_type":"Re-Survey","wb_id":"58012300","id_date":"1997-08-04","survey_id":"114979","wb_type":"Lake","county":"Pine","wb_name":"Grindstone"}},{"surveySubType":"Targeted Survey","lengths":{"NOP":{"maximum_length":32,"fishCount":[[11,1],[20,2],[21,1],[23,1],[26,1],[32,1]],"minimum_length":11},"LAT":{"fishCount":[[15,1],[19,2],[20,2],[22,1],[23,2],[24,2],[27,1],[32,1]],"maximum_length":32,"minimum_length":15},"BNT":{"maximum_length":15,"fishCount":[[13,1],[15,2]],"minimum_length":13},"RBS":{"minimum_length":3,"fishCount":[[10,6],[3,3],[4,32],[5,695],[6,25],[7,25],[8,17],[9,3]],"maximum_length":10},"WTS":{"maximum_length":11,"fishCount":[[11,1]],"minimum_length":11},"YEP":{"minimum_length":4,"maximum_length":4,"fishCount":[[4,1]]}},"fishCatchSummaries":[{"quartileWeight":"N/A","species":"YEP","totalWeight":20,"totalCatch":1,"gear":"fine mesh forage gill net","averageWeight":"0.04","gearCount":16,"CPUE":"0.06","quartileCount":"N/A"},{"species":"WTS","quartileWeight":"N/A","totalWeight":305,"gear":"Special gill nets","totalCatch":1,"averageWeight":"0.67","CPUE":"0.17","gearCount":6,"quartileCount":"N/A"},{"species":"BNT","quartileWeight":"N/A","totalWeight":1711,"totalCatch":3,"gear":"Special gill nets","averageWeight":"1.26","CPUE":"0.50","gearCount":6,"quartileCount":"N/A"},{"quartileCount":"N/A","CPUE":"0.33","gearCount":6,"averageWeight":"2.58","totalCatch":2,"gear":"Special gill nets","totalWeight":2340,"species":"LAT","quartileWeight":"N/A"},{"totalWeight":38,"species":"RBS","quartileWeight":"N/A","gearCount":6,"CPUE":"0.50","averageWeight":"0.03","gear":"Special gill nets","totalCatch":3,"quartileCount":"N/A"},{"totalWeight":22148,"species":"LAT","quartileWeight":"N/A","quartileCount":"N/A","CPUE":"0.62","gearCount":16,"averageWeight":"4.88","totalCatch":10,"gear":"fine mesh forage gill net"},{"quartileCount":"N/A","averageWeight":"2.91","CPUE":"1.17","gearCount":6,"totalCatch":7,"gear":"Special gill nets","species":"NOP","quartileWeight":"N/A","totalWeight":9225},{"gear":"fine mesh forage gill net","totalCatch":804,"averageWeight":"0.01","CPUE":"50.25","gearCount":16,"quartileCount":"N/A","species":"RBS","quartileWeight":"N/A","totalWeight":4628}],"surveyID":"19842303132078000","narrative":"Grindstone Lake is a coldwater, two-story lake near Sandstone, Minnesota. The lake is one of the few coldwater lakes in the lower half of the state. Grindstone is a 526-acre lake managed for Rainbow, Brown, and Lake Trout. The stream trout species are currently maintained through annual yearling trout stocking, and Lake Trout stocked every-other-year. Grindstone Lake provides a unique fishing opportunity for anglers to catch many different species of warm and coldwater fish throughout the year.
\n
\nDuring the summer of 2022 a special assessment was conducted on Grindstone Lake by MN-DNR Fisheries staff in order to monitor the Rainbow Smelt population. Rainbow Smelt are an important prey fish for Lake Trout and other gamefish species in Grindstone Lake. Rainbow Smelt also provide ice fishing opportunity for anglers. Anglers are also allowed to use seines and dip nets to harvest smelt in the spring.
\n
\nDuring the survey, fisheries staff deployed 4 small-mesh gill nets on Grindstone Lake for a 4-day period in August. A total of 806 smelt were captured during the survey. Length distribution of smelt ranged from 3.7 to 10.1 inches with an average length of 5.5 inches. Catch rates for smelt was 55.0 per net lift compared to 66 per net in 2021. Other game fish caught during the survey were Lake Trout, Brown Trout, Johnny Darter, Rainbow Trout, Smallmouth Bass, and Yellow Perch.
\n
\nGrindstone Lake anglers should expect moderate ice fishing pressure for Rainbow Smelt late in the winter season. Grindstone Lake should provide anglers with harvestable sized Smelt from 5 to 8 inches.
\n
\n
Figures are for all 27 gillnet sets. Depths not separated. See individual depth figures. All trout, smelt, herring, and smallmouth bass figures are compared against state medians. All other species against the local median and respective. gear. The brown, rainbow, and lake trout had slightly low population figures as did the smallmouth bass. The smelt dropped from 1.64 in 1976 to 0.48 in 1980. The herring dropped from 0.96 in 1976 to 0.30 in 1980. The northern pike was low at 2.22 per set for total sets but was high at 4.89 in the upper gillnet sets. The northern redhorse, white sucker, and yellow perch ranged from slightly low to very low respectively. The black bullhead and bluegill sunfish had low figures while the pumpkinseed and black crappie were very low. The rock bass was high at 8.33 per set. The walleye is known to be present but none were taken in 1976 or 1980.
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nets","CPUE":"0.33","gearCount":6,"averageWeight":"1.80","quartileCount":"0.2-0.7","totalWeight":3.6,"quartileWeight":"1.0-2.8","species":"WTS"},{"totalWeight":9.4,"species":"BLC","quartileWeight":"0.3-0.5","totalCatch":32,"gear":"Standard trap nets","gearCount":6,"CPUE":"5.33","averageWeight":"0.29","quartileCount":"0.5-2.2"}],"narrative":"In assessing the catch of Grindstone Lake gillnet sets, ranging from 10 to 120 feet deep, it is necessary to recognize that different species inhabit different depths. Obtaining data (fish/set) that is comparable to standard medians requires eliminating net sets that are not in a particular species normal range. The following table (table1) specifies the preferred depths and number of sets used to calculate comparative data for each species.\nTable 1. Specie preferred depths and number of gillnet sets.\nSpecies, Depth Selectivity for Relative Abundance(feet), Number of Sets\nCisco, 40 to 120, 18\nRainbow Trout, 10 to 70, 18\nBrown Trout, 10 to 70, 18\nLake Trout, 40 to 120, 18\nRainbow Smelt, 40 to 120, 18\nNorthern Pike, 10 to 30, 9\nSmallmouth Bass, 10 to 30, 9\nYellow Perch, 10 to 70, 18\nAll other species, 10 to 30, 9\n When compared to respective state or local medians the fish population is as follows: The brown trout (0.72/set) shows a slight increase from the 1980 survey (0.44/set) and is just above the state median (0.66/set). The majority of the brown trout population consists of small fish (median length group 11.5-11.9 inches) that represent recent stocking efforts. A small number of large individuals exist from past stocking. Brown trout are presently stocked on an annual basis (5,000 yearlings). Of the trout species present in Grindstone Lake, brown trout currently provide the greatest angling success (1985 creel census and recent verbal reports). Lake trout (0.17/set) are down considerably from the 1980 survey (0.83/ set) and are well below the state median (1.00/set). Past lake trout stocking has resulted in poor survival and growth, stocking was discontinued from 1978 until 1986, when stocking was again tried with yearling fish. Two of the 3 lake trout taken in the current survey are probably from the 1986 stocking with the 3rd being a relic from past stocking. Although unlikely, the possibility does exist that these fish are the result of natural reproduction. Rainbow trout were not taken in any test net gear during the current survey. Yearling rainbows, basically Madison strain until the changeover to Kamloop in 1987, have been stocked at an annual rate of over 5,000 fish for the past 25 years. Many of these trout are harvested immediately after stocking each year. Some have been confirmed to migrate from the lake down the North Fork of Grindstone River. Only occasionally is an adult rainbow trout taken by angling. The northern pike (3.89/set) has decreased somewhat from 1980 (4.89/set) but maintains a level very similar to the local median (3.88/set). Northern pike exhibit good year class representation with a median length group of 20.1-20.9 inches and a mean weight of 3.3 lbs. Smallmouth bass (0.33/ set) are down from the 1980 net catch (0.56/set) but remain far above the local median (0.06/set). Nine YOY (young of the year) smallmouth were recorded in shoreline seining. Angling for smallmouth bass is reported to be excellent. The yellow perch (3.06/set) shows a real increase from 1980 (0.17/set) yet remains well below the local median (24.27/set). Yellow perch have historically been caught in very low numbers. This survey indicates the highest population since 1949. Shoreline seining recorded 213 YOY perch. The rainbow smelt net catch (2.78/set) increased from 1980 (0.56/set) and is well above the local median (1.64/set). Exactly what role smelt play in relation to other species in Grindstone is unknown; they have, however, become quite popular with anglers by providing an excellent opportunity for late winter angling. Natural reproduction seems readily available in some of the inlets. Only 1 cisco (0.05/set) was caught in the gillnets. This is well below the local median (0.96/set) and reflects a continuing decline observed in previous surveys. The bluegill test net catch (14.00/set) has increased from 1980 (9.83/set) but remains below the local median (25.67/set). They tend to be small (median length group 5.0-5.4 inches) as they have been in previous surveys. Shoreline seining efforts produced 290 YOY bluegills. The black crappie (5.33/set) remains far below the local median (18.45/set) but the size is generally good (median length group 8.0-8.4 inches). Shoreline seining recorded 113 YOY crappies. Other species taken in the test nets (white sucker, golden redhorse, black bullhead, yellow bullhead, brown bullhead, rock bass, and pumpkinseed) do not appear to be remarkably above or below local medians nor have they undergone any noteworthy changes from the previous survey.\n
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Lake, with a maximum depth of 150 feet, is unique among lakes in east central Minnesota. The long, narrow 528 acre lake has enough oxygen in its deep cold waters to support coldwater species such as trout as well as cool and warmwater species such as bass, panfish, and northern pike. Brown trout and rainbow trout are stocked annually in the spring, and additional rainbow trout are stocked in the fall. Lake trout are stocked every other year to provide one of two southern most lake trout fisheries. Rainbow smelt, which were not introduced by the DNR, provide an additional angling opportunity in winter and spring. The public access is located on an inlet on the north side of the lake. During periods of low water, boat launching can be difficult for medium to large boats.
\n
\nAlthough 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, rainbow, brown, and lake trout may be difficult to locate. Winter angling is very popular and can be very productive. Open water anglers have the most success right after the May trout season opener before the surface water has warmed up and trout can still inhabit the shallower water.
\n
\nTest netting in 2012 showed lower abundance of all three trout species than the 2007 netting. All brown trout netted were from the May yearling stocking, and the one rainbow trout appeared to be from the 2011 stocking. Lake trout mean size improved from the previous survey, with an average of 15.4 inches and trout up to 24 inches seen in nets. Stocked lake trout are marked by clipping one of the lower fins; only two unclipped lake trout were seen in this survey, indicating little natural reproduction.
\n
\nNorthern pike have consistently maintained normal population levels with above average size in Grindstone Lake. Northerns netted in 2012 had an average weight of 4.4 pounds and ranged from 18.5 to 32.6 inches. In recent years anglers have caught or speared northern pike over 40 inches.
\n
\nSmallmouth bass up to 17.6 inches were captured by electrofishing, and large numbers of smaller smallmouth bass indicate good natural reproduction. Largemouth bass were sampled for the first time in 2012; the six largemouth sampled ranged from 8.5 to 20.6 inches in length. Both bass species will likely increase in abundance if the trend towards warmer summer surface water temperatures continues.
\n
\nBlack crappie remained above normal levels in 2012 while bluegill abundance was typical for the lake type. Most crappies exceeded 8 inches and a larger portion of the bluegill catch exceeded 7 inches than in the previous assessment.
Grindstone Lake, with a maximum depth of 150 feet, is unique among lakes in east-central Minnesota. The long, narrow 528 acre lake in lake class 23 has adequate oxygen and cool enough water to support trout as well as warmer water species such as bass, panfish, and Northern Pike. Brown Trout and Rainbow Trout are stocked annually in the spring, and additional Rainbow Trout are 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. The public access is located on an 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 out in the fall of 2016.
\n
\nAlthough 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 opener in May, before the surface water has warmed up and Trout still inhabit the shallower water.
\n
\nThe Lake Trout catch in 2020 (1.22 per net) was typical for the lake class but relatively low compared to recent Grindstone surveys. Average length of Lake Trout sampled was 12.5 inches and 2 year old fish made up three fourths of the catch. However, Lake Trout up to 24 inches were present. Rainbow Trout were not sampled and only one Brown Trout was captured.
\n
Grindstone Lake is a coldwater, two-story lake near Sandstone, Minnesota. The lake is one of the few coldwater lakes in the lower half of the state. Grindstone is a 526-acre lake managed for Rainbow, Brown, and Lake Trout. The stream trout species are currently maintained through annual yearling trout stocking, and Lake Trout stocked every-other-year. Grindstone Lake provides a unique fishing opportunity for anglers to catch many different species of warm and coldwater fish throughout the year.
\n
\nDuring the summer of 2024 a special assessment was conducted by MN-DNR Fisheries staff to monitor the Rainbow Smelt population. Rainbow Smelt are an important prey fish for Lake Trout and other gamefish species in the lake. Rainbow Smelt also provide ice fishing opportunity for anglers. Anglers are also allowed to use seines and dip nets to harvest smelt in the spring.
\n
\nDuring the survey, fisheries staff deployed 5 small mesh gill nets on Grindstone Lake for a 3 day period in early August. A total of 441 smelt were captured during the survey. Length distribution of smelt ranged from 3.2 to 8.0 inches with an average length of 6.2 inches. The catch rate for smelt in 2024 was 31.5 per net compared to 55.0 per net in 2022.
\n
\nThere were 12 Lake Trout Captured during the survey ranging in length from 12.7 to 33.5 inches. Average length of Lake Trout was 24.4 inches. All the Lake Trout captured were clipped demonstrating little or no natural reproduction occurs. Other game fish caught during the survey were Black Crappie, Common Shiner, Brown Trout, Largemouth bass, Smallmouth Bass, Spottail Shiner, Northen Pike, and Yellow Perch.
\n
\nGrindstone Lake anglers should expect moderate ice fishing pressure for Rainbow Smelt late in the winter season. Grindstone Lake should provide anglers with harvestable sized Smelt from 5 to 8 inches.
\n
Grindstone is a 526 acre lake managed for rainbow, brown, and lake trout and assigned to lake class 23. The lake is extremely deep with adequate oxygen levels to 110 feet. Although Grindstone is managed for trout, it is not a designated trout lake. Therefore, anglers only need a trout stamp if they plan on possessing trout. The three trout species are currently maintained through annual yearling stockings. The lake was netted in August 2002 to assess lake management goals and current stocking protocols. Twenty seven gillnet sets were made in three depth stratums: 10-30', 40-70', and 80-120'. Gillnet catch rates reported in this summary are relative to species temperature tolerances. Hence, catch rates for warmwater species are calculated from the 9 shallow sets while the coldwater species are from the 18 sets deeper than 30 feet. Lake trout abundance, based on the gillnet catch rate of 18.7/net, was high compared to the 1997 rate of 4.2 and to other lakes in class 23. Lake trout were small overall averaging 11.6 inches and 3/4 of a pound but fish over 30 inches were present. Annual stocking started in 1996. The relatively young population, dominated by the 1998 and 2000 year classes, contributed to the small overall size structure. Lake trout were the most abundant species in the winter creel of 2002. However, Over two thirds of the catch was released due to size. The rainbow trout catch rate, 0.7 per gillnet, was lower than the 2.1 reported in 1997. Sampled rainbows ranged from 9.7 to 13.5 inches and averaged half a pound. All rainbows were three years or less in age, the result of recent yearling stockings. Shore angling for rainbows is popular early in the season until the warming of surface water pushes fish off shore into deeper water. Although brown trout were stocked in May of 2002 and anglers reported catching them early in the season, none were sampled during the netting.Rainbow smelt were first sampled in the lake in 1963. How smelt were introduced into the lake is unknown but the population has become well established. Although they provide a forage base for northern pike and larger trout there may be some adverse effects. The tullibee (cisco) population has declined since the establishment of smelt which compete for the same forage. Tullibee have not been sampled in Grindstone Lake since the 1992 assessment. It is also believed that smelt compete with young trout for forage. A total of 86 smelt were caught in 2002, more than in any previous sampling. Netted smelt ranged from 2.8 to 10.4 inches.Northern pike have always maintained normal population levels compared to other lakes in the class. Historically northerns have been larger than normal. The current netting was no exception with a catch rate of 4.6 northerns per net and an average weight of 4.6 pounds. Sampled northerns ranged from 18.1 to 33.4 inches. Northerns have provided a trophy fishery with several fish over 40 inches caught in recent years.Yellow perch abundance has historically been low. The 2002 catch was near the 75 percentile for the lake class after an abscence from the sampling in 1997. Over 90% of the sample fell between 6 and 7 inches, a size unacceptable to most anglers. Based on the night electrofishing catches, smallmouth bass abundance is fairly low. In 2002 only 13 bass were sampled for a catch rate of 8.8 per hour. Lengths ranged from 8 to 15.4 inches with means of 12.6 inches and 1.1 pounds. The 1997 catch was higher but bass were noticably smaller. Fishing pressure directed at smallmouth is believed to be light.Black crappie were sampled above normal levels for the first time. Size was below normal for the lake class with a 7 inch mean. Bluegill were also small, but with a normal abundance. None of the sampled bluegill reached 7 inches in length.Shoreline seining revealed natural reproduction of bluegill, rock bass, smallmouth bass and yellow perch.Others species present in the lake included golden redhorse, pumpkinseed sunfish, rock bass, river redhorse, shorthead redhorse, and white sucker.
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electrofishing","averageWeight":"0.00","CPUE":"0.56","gearCount":1.80111111111111},{"totalWeight":0,"species":"BLC","quartileWeight":"N/A","quartileCount":"N/A","totalCatch":58,"gear":"50-ft beach seine","gearCount":14,"CPUE":"4.14","averageWeight":"0.00"},{"gearCount":14,"CPUE":"1.36","averageWeight":"0.00","totalCatch":19,"gear":"50-ft beach seine","quartileCount":"N/A","totalWeight":0,"quartileWeight":"N/A","species":"WTS"},{"quartileWeight":"N/A","species":"SMB","totalWeight":0,"quartileCount":"N/A","gear":"Backpack electrofishing","totalCatch":16,"averageWeight":"0.00","gearCount":1.80111111111111,"CPUE":"8.88"},{"quartileCount":"N/A","CPUE":"1.86","gearCount":14,"averageWeight":"0.00","totalCatch":26,"gear":"50-ft beach seine","totalWeight":0,"species":"YEP","quartileWeight":"N/A"},{"species":"JND","quartileWeight":"N/A","totalWeight":0,"quartileCount":"N/A","averageWeight":"0.00","CPUE":"2.43","gearCount":14,"gear":"50-ft beach seine","totalCatch":34},{"totalCatch":18,"gear":"Backpack electrofishing","averageWeight":"0.00","CPUE":"9.99","gearCount":1.80111111111111,"quartileCount":"N/A","species":"CSH","quartileWeight":"N/A","totalWeight":0},{"quartileCount":"N/A","averageWeight":"0.00","CPUE":"5.00","gearCount":1.80111111111111,"gear":"Backpack electrofishing","totalCatch":9,"quartileWeight":"N/A","species":"IOD","totalWeight":0}],"lengths":{},"narrative":"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.
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\nThe 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.
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\nThis 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.
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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.
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\nPast 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.
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\nIn 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.
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\nOther 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.
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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.
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\nAlthough 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.
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\nIncluding 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.
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\nA 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.
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\nRainbow 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.
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\nOnly 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.
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\nAn 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.
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\nThe 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.
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\nSmallmouth 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.
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\nNighttime 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.
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\nThe 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.
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\nThe 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.
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\nYellow 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.
\nAdditional 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.
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GRINDSTONE LAKE IS CLASSIFIED AS A TROUT LAKE AND MANAGED AS AN EXPERIMENTAL TROUT (LAKE AND STREAM) LAKE WITH SPECIAL REGULATIONS. MOST RECENT STOCKING PLANS INVOLVE ANNUAL STOCKING OF 5000 RBT AND 5000 BNT (YEARLINGS). IN AUGUST 1992, A FISH POPULATION ASSESSMENT WAS CONDUCTED ON GRINDSTONE LAKE. TO DETERMINE THE STATUS OF THE FISH POPULATION 27 GILLNETS AND 6 TRAPNETS WERE SET AND 3 SHORELINE STATIONS WERE SEINED. THE GILLNETS WERE SET IN THREE DIFFERENT DEPTH ZONES (9 IN EACH); 10'-30', 40'-70, AND 80'-120'. THIS WAS DONE TO ENABLE US TO SAMPLE DIFFERENT SPECIES WHICH INHABIT DIFFERENT DEPTHS. THE FOLLOWING TABLE SPECIFIES THE SPECIES PREFERRED DEPTHS AND THE NUMBER OF GILLNETS USED FOR COMPARISONS. BASED ON RESULTS FROM THESE EFFORTS, THE STATUS OF THE FISH POPULATION COMPARED TO PREVIOUS SURVEYS AND TO LAKES OF THE SAME CLASSIFICATION ARE DESCRIBED. THE TLC CATCH (0.11/SET; 18 SETS) HAS REMAINED AT RECENT LOW LEVELS. A DROP IN NET CATCHES SINCE 1976 HAS CONTRASTED TO A RISE IN THE RBS NET CATCH. BOTH SPECIES PLAY AN IMPORTANT ROLE AS FORAGE FOR THE TROUT AND NOP IN GRINDSTONE LAKE. THE RBT CATCH (1.06/SET' 18 SETS) HAS SHOWN A REAL INCREASE FROM THE PREVIOUS SURVEY IN 1987 WHERE NO RBT WERE CAPTURED. IT IS STILL UNCLEAR TO WHY NO RBT WERE SAMPLED IN 1987. IT IS KNOWN THAT MANY OF THESE TROUT ARE HARVESTED IMMEDIATELY AFTER STOCKING AND THAT SOME HAVE BEEN CONFIRMED TO MIGRATE FROM THE LAKE DOWN THE NORTH FORK OF GRINDSTONE RIVER. IN THE YEARS PRECEDING 1987, MADISON STRAIN RBT WERE MAINLY STOCKED. SINCE THEN, KAMLOOP AND ARLEE STRAINS HAVE BEEN STOCKED. THE STOCKING RATE OF APPROXIMATELY 5000 YEARLING RBT ANNUALLY HAS REMAINED THE SAME SINCE 1978. THE 19 RBT CAPTURED IN 1992 WERE ALL TAKEN IN THE 10'-30' GILLNET SETS. THEY RANGED FROM 10.5 TO 14.9 INCHES. AGE DETERMINATION FOUND TWO AGE CLASSES PRESENT, AGE 1 (10 FISH) AND AGE 2 (9 FISH). AVERAGE LENGTH AT CAPTURE FOR THE AGE 1 FISH (11.94 INCHES) SHOWED A 31% LENGTH INCREASE FROM THE TIME OF RELEASE (9.14 INCHES), WHICH WAS 60 DAYS PRIOR TO THE SURVEY. WE CONTACTED THE HATCHERY IN LANESBORO, MN, WHERE THESE AGE 1 CAME FROM, AND ASKED THEM TO CALCULATE WHAT THE MAXIMUM LENGTH INCREASE WOULD BE FOR THESE SAME FISH IF THEY WERE HELD IN THE HATCHERY FOR THE SAME 60 DAY PERIOD. THEY CALCULATED A MAXIMUM 0.18 INCH/DAY INCREASE WHICH WOULD PUT THE FISH AT 10.24 INCHES. THIS SHOWS THAT THE RBT IN GRINDSTONE LAKE WOULD BE OVER 1.5 INCHES LONGER THAN THE HATCHERY REARED TROUT DURING THE SAME TIME PERIOD OF 60 DAYS. THE DIFFERENCE IN LENGTHS IS MOST LIKELY DUE TO WARMER WATER TEMPERATURES IN GRINDSTONE LAKE. AVERAGE BACK-CALCULATED LENGTHS OF THE AGE 2 FISH SHOWED A 72% LENGTH INCREASE FROM LENGTH AT AGE 1 (6.49 INCHES) TO LENGTH AT AGE 2 (11.17 INCHES). ALSO, THESE AGE 2 FISH HAD A 17% LENGTH INCREASE FROM LENGTH AT AGE 2 (11.17 INCHES) TO TIME OF CAPTURE (13.08 INCHES). PREVIOUS SURVEYS HAVE NOT RECORDED A RBT OVER 14 INCHES IN GRINDSTONE LAKE. THE BNT CATCH (1.00/SET; 18 SETS) WAS HIGHER THAN THE 1987 CATCH (0.72/SET; 18 SETS). SINCE THERE IS NO STANDARD LAKE CLASS MEDIAN CATCH FOR BNT, A NONPARAMETRIC COMPARISON OF THE 1987 AND 1992 CATCH WAS EVALUATED. A RANK SUM TEST (ALPHA LEVEL OF 0.20) WAS USED WHICH COMPARED THE BNT CATCH IN EACH OF THE 18 GILLNET SETS FROM 1987 TO 1992 (SCHUPP 1992). THE TEST RESULTS FOUND THAT THE TWO-TAILED P-VALUE FOR NORMAL APPROXIMATION WAS 0.10 WHICH IMPLIES THAT THE BNT CATCH SHOWED A STATISTICALLY SIGNIFICANT CHANGE (INCREASE) FROM 1987 TO 1992. THE 19 BNT CAPTURED IN ALL GILLNETS RANGED FROM 10.5 TO 21.9 INCHES. AGE DETERMINATION FOUND AGES 1-6 PRESENT. THE SAME GROWTH TREND WAS IDENTIFIED FOR BNT AS MENTIONED EARLIER ABOUT THE RBT. THE AGE 1 BNT (8.84 INCHES) RELEASED IN GRINDSTONE LAKE ON APRIL 20, 1992 AVERAGED 11.15 INCHES AT TIME OF CAPTURE IN AUGUST 1992. THIS WAS A 26% LENGTH INCREASE. THE 5 OTHER AGE CLASSES AVERAGED A 14% LENGTH INCREASE FROM LAST ANNULUS FORMATION TO TIME OF CAPTURE IN AUGUST. AN INTERESTING ASPECT TO BNT LOCATION IN GRINDSTONE LAKE WAS ADDRESSED ON PAGE 10 IN THE SUPPLEMENTAL FISH SAMPLING SECTION OF THIS SURVEY. FUTURE POPULATION ASSESSMENTS MAY NEED TO USE THIS TYPE OF SAMPLING. THE LAT CATCH (0.28/SET; 18 SETS) WAS HIGHER THAN THE 1987 CATCH (0.17/SET; 18 SETS). THIS WAS A SIGNIFICANT INCREASE AS DETERMINED BY THE RANK SUM COMPARISON TEST OF 18 SETS FROM 1987 TO 1992, (P-VALUE 0.10 WAS Grindstone is a 526 acre lake managed for rainbow, brown, and lake trout and assigned to lake class 23. The lake is extremely deep with adequate oxygen levels to 110 feet. Although Grindstone is managed for trout, it is not a designated trout lake. Therefore, anglers only need a trout stamp if they plan on possessing trout. The two stream trout species (brown and rainbow) are currently maintained through annual yearling stockings. Two thousand five hundred brown trout are stocked each spring. Five thousand rainbow trout are stocked each spring with up to 3,000 more added in the fall when available. Yearling lake trout are now stocked biennially. The lake was netted in August 2007 to assess lake management goals and current stocking protocols. Twenty four gillnet sets were made in three depth stratums: 10-30', 40-70', and 80-120'. Gillnet catch rates reported in this summary are relative to species temperature tolerances. Hence, catch rates for warm-water species are calculated from the 9 shallow sets while the coldwater species are from the 15 sets deeper than 30 feet. Grindstone Lake, with a maximum depth of 150 feet, is unique among lakes in east-central Minnesota. The long, narrow 528 acre lake in lake class 23 has enough oxygen in its deep, cold waters to support coldwater species such as trout as well as cool and warmwater species such as bass, panfish, and Northern Pike. Brown Trout and Rainbow Trout are stocked annually in the spring, and additional Rainbow Trout are 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. The public access is located on an 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 out in the fall of 2016.
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\nLake trout abundance, based on the gillnet catch rate of 4.2 was well above the lake class norm, down from the 2002 rate and identical to the 1997 catch. Size structure has climbed some with a mode and mean between 13 and 14 inches and a 1.2 pound average. Measured lake trout ranged from 7.6 to 30.0 inches in length. Lake trout relative weights, a comparison of fish plumpness to a species-specific standard, were in excess of 100 for all length groups. This indicates that the forage base is likely being under utilized. Yearling lake trout were stocked annually from 1996 through 2005 when biennial stocking began.
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\nThe rainbow trout catch rate, 0.6 per gillnet, was similar in 2002. Sampled rainbows ranged from 12.1 to 21.4 inches and averaged 1.1 pounds. Nine of the ten rainbows sampled were from recent yearling stockings. The one larger fish was the result of the stocking of 77 surplus hatchery brood fish in 2006. Shore angling for rainbows is popular early in the spring until the warming of surface water pushes fish off shore into deeper water.
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\nAlthough yearlings were again stocked in May of 2007, the 6 sampled brown trout were from the 130 surplus brood fish stocked in the fall of 2006. These fish averaged 19.5 inches with a mean weight of 3.1 pounds.
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\nRainbow smelt were first sampled in the lake in 1963. How smelt were introduced into the lake is unknown but the population has become well established. Although they provide a forage base for northern pike and larger trout there may be some adverse effects. The tullibee (cisco) population has declined since the establishment of smelt, which compete for the same forage. Tullibee have not been sampled in Grindstone Lake since the 1992 assessment. It is also believed that smelt compete with young trout for forage. A total of 102 smelt were caught in 2007, more than in any previous sampling. Netted smelt ranged from 3.8 to 9.7 inches.
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\nNorthern pike have always maintained normal population levels compared to other lakes in the class. Historically, northerns have been larger than normal. The current netting was no exception with a catch rate of 3.9 northerns per net and an average weight of 3.4 pounds. Northerns ranged from 11.1 to 33.0 inches and have provided a trophy fishery with several fish over 40 inches caught or speared in recent years. Pike weights were very close to the standard until they reached 600 mm (24 inches). At this point weights began to exceed the standard, i.e. relative weight values began to exceed 100. Pike mouth gape (roughly 60 mm for a 600mm northern) allows predation on forage up to 80mm in depth. Rainbow trout depths don't begin to exceed 80mm until they reach 12 inches in length.
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\nYellow perch abundance has historically been low. The 2007 catch was near the 50th percentile for the lake class, down slightly since 2002. Perch were between 6 and 7 inches, a size unacceptable to most anglers.
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\nBlack crappie were again sampled above normal levels. Size has improved some with and 64% of the catch exceeding 8 inches. Bluegill abundance was below the lake class norm. Size structure was not great with relatively few fish over 7 inches available.
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\nOthers species present in the lake included greater redhorse, pumpkinseed sunfish, rock bass, smallmouth bass, yellow bullhead, and white sucker.
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\nAlthough 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 May trout season opener, before the surface water has warmed up and trout still inhabit the shallower water.
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\nLake Trout catch (1.16 per net) was the lowest it has been since the 1996 survey but is still typical for the lake class. Average length increased from the previous survey in 2012 from 15.4 to 20.7 inches. This change in average length is likely due in part to a lack of numerous smaller fish that were present in previous surveys. The smallest Lake Trout sampled in this survey was 13.7 inches. Several larger individuals were present with the largest caught exceeding 32 inches.
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\nThe Brown Trout gill net catch was the highest it has ever been. The majority of Brown Trout were between 12 and 16 inches but several larger individuals were sampled, with the largest nearly 20 inches. Only 4 Rainbow Trout were sampled in gill nets. Based on their size, these fish were likely from the most recent spring stocking. It is rare to see larger individuals of either stream trout species in the lake. It is unknown whether this is due to heavy exploitation, escapement, or natural mortality. Both species were also sampled during spring night time electrofishing efforts.
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\nNorthern Pike catch at 2.4 per net was slightly lower than the historical rate but still typical for lakes similar to Grindstone. Mean weight was higher than that of other lakes in the class at 2.9 compared to 2.2 pounds. Northerns averaged 23 inches with the largest sampled at 32 inches. They have good growth and are exceptionally heavy for their length. The potential exists to catch trophy pike in this lake.
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\nThe Black Crappie catch was similar to previous surveys at 2.9 per trap net. This is high for the lake class but not for typical panfish lakes. Nevertheless, the potential for quality crappie fishing exists. Black Crappie averaged 7.8 inches and forty-two percent of crappies sampled exceeded 9 inches with the largest being 12.4 inches long. Bluegill catch was significantly higher than the historical average at 17.6 fish per trap net, and size was typical for similar lakes with a mean length of 6 inches. The largest Bluegill sampled was 7.9 inches.
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\nNighttime electrofishing sampled 11 Largemouth and 67 Smallmouth bass. Largemouth were first sampled in the previous survey and they seem to be establishing themselves. The largest sampled were around 15 inches, but two significantly larger fish were sampled in a separate spring seining assessment. One of those fish was exceptionally large at over 20 inches long. While the Smallmouth catch was the highest it has been since electrofishing began on this lake, fish size was small with an average length of 5.5 inches. One 18.5 inch Smallmouth was sampled in a gill net and anglers have reported catching them over 20 inches. It is likely that Grindstone Lake contains a quality bass fishery and the lack of large fish in our electrofishing sampling may be due to the steep shorelines of the lake making them less vulnerable to our equipment.
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