How Ned Maps grades lakes
Every letter grade on this site comes from one rubric applied to Minnesota DNR lake-survey data. The same rubric runs on the statewide map, the county pages and the best-lakes rankings. No fishing reports, no reviews. If the DNR didn't measure it, it isn't in the grade.
The data
The DNR surveys managed lakes on a rotating schedule using standardized gear: trap nets, gill nets and electrofishing. It publishes the catch rate (catch per unit effort, or CPUE) and the length of every fish measured, by species. Ned Maps uses surveys from the last 20 years. A lake whose newest survey is older than that is still listed from the historical record, but it is not graded.
The main ranking
For each species, every eligible surveyed lake in Minnesota is scored on three components. Each one is log-scaled and percentile-ranked against the whole statewide pool.
- Total abundance, 60%. The survey catch rate in the species' index gear, multiplied by the square root of the lake's acreage. Size counts, because a big lake at a modest catch rate holds more catchable fish than a small lake at a high one, and this ranking is about picking a destination. It counts with diminishing returns rather than in full, because nobody fishes a 100,000-acre lake in a day. Under straight acreage, size alone could carry a lake that was mediocre for the species being ranked.
- Total quality-size fish, 25%. The same total, scaled by the share of measured fish at or above the species' quality length.
- Biggest fish, 15%. The longest fish of that species measured in the survey window, as a statewide percentile.
Hidden gems
The gems list uses a different mix: 40% catch rate (scored against the DNR's own abundance benchmark bands where the species has them), 20% quality-size share, and 40% trophy rate (fish at or above the species' trophy length per 10,000 measured, smoothed toward the statewide rate on thin samples). Using rates instead of totals scores a lake on how it fishes rather than on how big it is, and only shallow, littoral-heavy lakes qualify.
The letter curve
The 0–100 score is a weighted average of percentiles, so most lakes land near the middle. An exam-style scale (A at 90, F below 60) would put nearly every lake below passing, so the cutoffs are spaced against the real score distribution instead. Across roughly 22,000 lake-species pairs the median lands at C+, about 6% grade F, and about one pair in 200 earns an A+.
| Grade | Score |
|---|---|
| A+ | 96–100 |
| A | 88–95 |
| A− | 78–87 |
| B+ | 66–77 |
| B | 55–65 |
| B− | 48–54 |
| C+ | 42–47 |
| C | 33–41 |
| C− | 25–32 |
| D | 15–24 |
| F | 0–14 |
Same lake, same grade, anywhere
Scoring is always statewide. Filtering to a county or a radius slices the ranked list rather than re-scoring it, so a lake carries the same grade wherever you look it up from.
What a grade is not
- Not a promise. Surveys are samples taken years apart, so a grade describes long-run odds, not any single day.
- Not the same as a lake page's fishery labels, like "Elite fishery". Those use the DNR's absolute benchmark bands, meaning whether the catch rate is high on its own terms. The letter grade is relative to every other lake in the state. Both are shown because they answer different questions.
- Never based on fishing reports, user submissions or anything else people typed in.
Questions about the rubric: david@nedmaps.com.