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B

Boy Lake

Cass County, MinnesotaMesotrophic

On the LakeGrade scale Boy Lake grades a B, with clarity at 8.9 ft and 21 µg/L of phosphorus placing it above the Minnesota median. With clarity and phosphorus scored, the sub-grades cluster within a letter of each other, pointing to stable conditions rather than one specific stressor.

The lake sits in the mesotrophic zone — the band where most well-managed Minnesota lakes fall. The lake bottoms out at 45 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. With 3,308 acres of surface and 21.6 miles of shoreline, Boy Lake is a large water by Minnesota standards — recreational pressure is spread across a long shoreline. Within the 132 graded lakes of Cass County, Boy Lake sits at rank 114, near the bottom of the county list.

Boy Lake is on the Minnesota infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. Muskie are on the species list at Boy Lake, among the lake's 20 documented fish. Public access is available — the lake is on the Minnesota PCA public-access list. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.

Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-07. Grade methodology: LakeGrade methodology.

Reviewed by LakeQuality Editorial Team · Updated

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Swimming Safety

Good for swimming, clear water with low algae levels

Water Quality Grade: B, Good

Moderate clarity, visible to about 8.9 ft. Phosphorus level: 21 µg/L. Trophic State Index: 47.

MetricValueGrade
Water Clarity (Secchi Depth)8.9 ftC
Phosphorus21 µg/LB
Chlorophyll-a (Algae)No data
Trophic State Index (TSI)47Mesotrophic

Moderate nutrients, good water quality

Lake Details

CharacteristicValue
Maximum Depth45 ft
Surface Area3.3K acres
Shoreline Length21.6 mi
Littoral Zone61%
Public AccessYes

Fish Species

black bullhead,black crappie,bluegill,bowfin (dogfish),brown bullhead,burbot,greater redhorse,hybrid sunfish,largemouth bass,muskellunge,northern pike,pumpkinseed,rock bass,shorthead redhorse,smallmouth bass,tullibee (cisco),walleye,white sucker,yellow bullhead,yellow perch

Click a species to see all Minnesota and Wisconsin lakes where it is found.

→ Best fishing times for Boy Lake (14-day solunar calendar)

→ Is it safe to eat fish from Boy Lake? (mercury & PFAS guide)

Boy Lake fishing regulations (limits, seasons, special rules)

Invasive & Introduced Species

Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.

zebra mussel

Water Quality Trend: Improving

Based on 6 years of monitoring data (2020-2025).

MetricTrendChange/YearYears
Water Clarity Improving+0.124 m/yr6
Phosphorus Improving-1.7 µg/L/yr5
See year-by-year chart →

Location

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County Ranking

Ranked #114 of 132 lakes in Cass County

Cleaner Lakes Within 30 Miles

Boy Lake holds Grade B. 5 nearby lakes hold higher grades.

See full comparison →

Nearby Lakes in Cass County

DNR Fisheries Survey Summary

11 surveys on file from MN DNR Fisheries. Most recent: 2025-07-28 (Standard Survey).

Top Species by Catch Rate

SpeciesAvg CPUEAvg Weight
Yellow Perch7.530.16 lb
Bluegill5.580.31 lb
Northern Pike5.381.34 lb
Black Bullhead4.780.75 lb
Pumpkinseed3.780.2 lb
Walleye3.432.6 lb

CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.

Length Distributions

Number of fish caught at each inch class in the most recent survey that recorded lengths. Red dashed line marks an approximate trophy threshold for that species.

Yellow Perch

61 fish · 59 in · 2025-07-28
2814056789

Bluegill

447 fish · 19 in · 2025-07-28
117590trophy 10123456789

Northern Pike

209 fish · 1030 in · 2025-07-28
482401012141618202224262830

Black Bullhead

2 fish · 1112 in · 2021-08-02
101112

From the 2025-07-28 survey

Boy Lake is a 3,466-acre lake with 2,007 littoral acres, 26 miles of shoreline and a maximum depth of 45 feet located north of Longville, MN. A DNR owned public access is located on the southwest shore. The DNR has classified Minnesota's lakes into 43 different classes based on physical, chemical and other…

Source: MN DNR LakeFinder Fisheries Lake Survey

DNR Reports & Resources

Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Boy Lake. 3 reports on file.

EPA Impairment Status

Boy Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).

Causes of impairment

Mercury

Source: EPA ATTAINS assessment unit MN11-0143-00 · Official waterbody report

Data Sources

Water quality data from the EPA Water Quality Portal

Impairment status from EPA ATTAINS 303(d) database

Grading methodology based on Metropolitan Council standards

Lake details from Minnesota DNR LakeFinder

Most recent sample: 2025-10-07

Monitoring stations: 4