Slim

Temperature and dissolved oxygen measurements were collected from the deepest basin in Slim Lake on August 6th, 2025. This was done to evaluate the quantity and quality of cold, oxygenated water (i.e., oxythermal habitat) available to Cisco, also known as Tullibee, the most sensitive coldwater species present in this lake. 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. Based on data collected from the deepest basin (maximum sampled depth was 49.2 feet), the water temperature decreased to 21.2 degrees C (70.2 degrees F) at 15.0 feet and the dissolved oxygen concentration decreased to 3.0 mg/L at 34.8 feet at the time of sampling. This layer in the water column was 19.8 feet, indicating oxythermal habitat for Cisco was present. Further, the temperature at which dissolved oxygen decreased to 3.0 mg/L (i.e., TDO3, a single variable to characterize oxythermal habitat) was 8.6 degrees C (47.5 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 the survey. Data collected by the Minnesota Department of Natural Resources during the period of maximum oxythermal stress (July 26th to Aug 24th) in 2016, 2023, 2024 also indicated suitable oxythermal habitat in all years surveyed, with the least favorable conditions recorded in 2016 when the suitable oxythermal habitat layer was 11.2 feet thick. When adequate oxythermal habitat is not available, Cisco are squeezed into unfavorable, warmer water conditions and experience higher levels of stress.

Slim is a 312-acre lake located 8 miles northwest of Ely in the Boundary Waters Canoe Area Wilderness. Its clear water fills a simple, two-mile long basin encased by high bedrock ridges carpeted in dense pine. Aquatic vegetation is rare and most of the lake bottom drops off rapidly from the shoreline. The U.S. Forest Service maintains four campsites. Access is by a 102 rod portage from the Slim Lake Entry Point parking lot. A 47 rod portage exists from Rice Lake to the west. The 2023 survey consisted of nine gill net sets. Cisco and walleye dominated the catch, followed by white sucker and smallmouth bass. Low numbers of yellow perch and a green sunfish were also sampled. Northern pike were conspicuous in their absence. Walleye abundance was low at 1.2/ gill net, but the fish present were large. Walleye lengths ranged from 21 to nearly 28 inches with an average of 24 inches. Ages ranged from 5 to 23 years with no predominant year classes. Walleye growth was fast compared to other lakes in the area. Walleye fry are stocked annually. Smallmouth bass abundance, at 0.9/gill net, was the highest ever sampled from Slim. The smallmouth were mostly small, but a few decent fish were present. Lengths ranged from 6 to 16.5 inches with an average of 9.5 inches. Gill nets often underrepresent smallmouth, but this catch would suggest an increasing trend in their abundance. Cisco numbers (1.7/net) were above average compared to previous surveys. Cisco lengths ranged from 7 to 14 inches with a mean of 10.5 inches. No ciscoes were present in predator stomachs, but their presence is likely a factor in the growth and size of the walleyes in Slim.

Slim Lake is located approximately 9 miles NW of Ely in the BWCAW. This scenic lake has high pine covered ledgerock hills surrounding it's shoreline. There is a 140 rod portage from a parking lot located off the North Arm Road for public access. There are 3 designated USFS campsites located around the lake for overnight use. Slim Lake is a popular destination for campers and day trippers alike. Slim lake is deep and has very clear and soft (unmineralized) water. Lake bottom substrates along the shoreline of Slim Lake are mostly rock and boulder, with little aquatic vegetation. The outlet stream has a waterfall that prevents upstream fish migration. Results from the 2016 survey of Slim Lake indicate a fish community with low relative abundances and low species diversity. Cisco was the only species sampled in above average numbers. Low numbers of walleye, northern pike, smallmouth bass and white sucker were also sampled. There have been numerous previous surveys completed on Slim Lake with the most recent in 2006. Cisco numbers (5.2/gill net) were above the historical average on Slim Lake. The average cisco sampled was almost 9 inches, slightly more than the historic average length. The largest cisco sampled in 2016 was 12.5 inches. Walleye numbers (1.5/gill net) were below the historic average of 6.4/gillnet for Slim Lake and below average compared to other similar lakes in the area. The 9 walleye sampled in 2016 were large and old with ages ranging from 10 to 18 years old. Walleye fry are currently stocked during even-numbered years in Slim Lake. In 2016, three of the nine walleye sampled came from non-stocked years. The largest walleye sampled in 2016 was 25 inches long.

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

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

Slim has a maximum depth of 49 feet, per Minnesota DNR data.

The most recent Minnesota DNR survey data for the main sport fish in Slim is from 2023.

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