Thunder Bay District, Ontario

Lac des Mille Lacs

Two hundred and forty-five square kilometers west of Thunder Bay, with 574 islands inside 881 kilometers of shoreline and a deepest charted contour of 27.4 meters from the province's 1974 survey. Cool water, with seven walleye spawning areas recorded.

Surface: 245.2 km²
Shoreline: 881 km517 mainland + 364 islands
Islands: 574
Charted depth: 27.4 m
Depth contours: 949

All figures are derived from Ontario’s survey records.

Where Lac des Mille Lacs is

Lac des Mille Lacs lies in the District of Thunder Bay.

How deep is Lac des Mille Lacs?

The deepest mapped contour on Lac des Mille Lacs is 27.4 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 949 depth contours for Lac des Mille Lacs.

The deepest charted contour on Lac des Mille Lacs, 27.4 meters, from the survey of 1 June 1974. The circle marks the centre of that contour, and the water within it is deeper. Of the survey’s 949 contours, six are its own shoreline contour, one ring around the mainland and one around each island it charted, and are left off, the outline being the Ontario Hydro Network’s; the other 943 depth contours are drawn where the province published them, and along the outline where the survey placed them beyond it. Outline from the Ontario Hydro Network. Each contour is labelled with its depth in meters; zoom in where they run close.

Surveyed Depths

Ontario has surveyed the depths of several thousand of its lakes over more than half a century. Later surveys were conducted by boat with satellite positioning and produced hundreds of thousands of soundings per lake. Earlier surveys were charted by running transects across the water and drawing depth contours by hand.

Water quality

Ontario’s Lake Partner Program is a volunteer monitoring network. Participants collect a water sample each spring for phosphorus analysis and measure water clarity through the summer with a Secchi disk.

Phosphorus is the nutrient that controls how much algae a lake can grow. A lake with little phosphorus stays clear and supports cold-water fish such as lake trout. As phosphorus rises, algae increase, the water clouds, and oxygen in the deep water falls as the algae decay. The province classifies each monitoring site by its spring phosphorus. Below 10 micrograms per litre is oligotrophic: clear water and low algae. From 10 to 20 is mesotrophic: moderately productive water, with more algae and weed growth than an oligotrophic lake but rarely a bloom. Above 20 is eutrophic: nutrient-rich water, prone to algal blooms and low oxygen at depth.

A Secchi disk is a 20 centimeter white or black-and-white plate lowered on a marked line until it disappears from view. The depth at which it vanishes is the Secchi depth, a direct measure of water clarity. Clearer water gives a deeper reading.

Lac des Mille Lacs has 6 Lake Partner monitoring sites. Four have reported phosphorus for the three or more years the province requires before a status is assigned. One of those sites is classified mesotrophic and three eutrophic.

Lake Partner sites on Lac des Mille Lacs with a quotable phosphorus record
Site	Location	Years	Last sampled	Classification
2391/2	Sawbill bay	3	2024	eutrophic
2391/4	South of Pine Point	3	2024	eutrophic
2391/5	Open Bay	3	2024	eutrophic
2391/6	Honkonan Narrows	3	2024	mesotrophic

The longest record is held by site 2391/2 at Sawbill bay, sampled over 3 springs to 2024. Its long-term mean phosphorus concentration is 23.7 micrograms per litre, within the eutrophic band, which begins at 20. The site’s 3 clarity readings give a median Secchi depth of 0.6 meters.

Median Secchi depth by year at site 2391/2, surface at the top. The thin line through each bar spans the shallowest and deepest reading that year, and the row beneath the years gives the number of readings. Years without readings are blank.

Fish in Lac des Mille Lacs

Ontario’s aquatic resource records name 13 fish species in Lac des Mille Lacs. The province classes the lake as cool water. The record took effect on 8 April 2016, and it compiles what field biologists had reported by then.

Seven of the thirteen species in the province’s records

Lake Whitefishtrout and whitefish
Ciscotrout and whitefish
Northern Pikepike
Walleyeperch
Yellow Perchperch
Smallmouth Basssunfish
Burbotcod

The other six species in the province’s records
Family	Species
Perch	logperch
Sucker	longnose sucker, white sucker
Minnow	mimic shiner, spottail shiner
Trout-perch	trout-perch

Spawning areas

Ontario maps 7 fish spawning records within the Lac des Mille Lacs outline: 7 walleye spawning areas covering 2540.29 hectares. Every record is recorded as verified and presently suitable, and none is classed as sensitive.

By number of records

By area mapped

Walleye7 areas2540.3 ha

Spawning records mapped by the province within the Lac des Mille Lacs outline. A record under 100 square meters is drawn as a dot; a spawning area is drawn as the outline mapped. One colour per species, the same as the bars and the list above.

The federal Fisheries Act prohibits the harmful alteration, disruption or destruction of fish habitat without an authorization, so a dock, dredging, infill or shoreline work at a mapped spawning area is reviewed on that basis.

Eating the fish

The 2025 Guide to Eating Ontario Fish carries consumption advice for eight species taken from Lac des Mille Lacs, tested for contaminants at a sampling location in Thunder Bay Dist. Each figure below is the most meals a month the guide advises for a fish of that length. The advice falls as the fish gets longer, because mercury accumulates over a fish’s life. A zero means the guide advises no meals of that fish at that size.

General population adultsSensitive population children under fifteen, women who are or may become pregnant

Lake Whitefish no contaminant listed
Length, cm	30 to 35	35 to 40	40 to 45	45 to 50	50 to 55
General population	32	32	32	32	32
Sensitive population	32	32	32	32	16

Cisco no contaminant listed
Length, cm	25 to 30	30 to 35	35 to 40
General population	32	32	32
Sensitive population	16	16	16

Walleye Mercury
Length, cm	20 to 25	25 to 30	30 to 35	35 to 40	40 to 45	45 to 50	50 to 55	55 to 60	60 to 65	65 to 70
General population	32	16	12	12	8	8	4	4	4	4
Sensitive population	12	8	4	4	4	0	0	0	0	0

Northern Pike Mercury
Length, cm	35 to 40	40 to 45	45 to 50	50 to 55	55 to 60	60 to 65	65 to 70	70 to 75	over 75
General population	32	16	16	16	12	8	8	4	4
Sensitive population	12	8	8	4	4	4	0	0	0

Smallmouth Bass Mercury
Length, cm	25 to 30	30 to 35	35 to 40	40 to 45
General population	32	16	12	8
Sensitive population	16	8	4	4

Yellow Perch Mercury
Length, cm	15 to 20	20 to 25	25 to 30
General population	32	32	12
Sensitive population	16	12	4

White Sucker Mercury
Length, cm	30 to 35	35 to 40	40 to 45	45 to 50	50 to 55
General population	32	32	16	16	16
Sensitive population	32	16	12	8	4

Longnose Sucker Mercury
Length, cm	30 to 35	35 to 40	40 to 45	45 to 50	50 to 55	55 to 60
General population	32	32	32	16	8	4
Sensitive population	32	16	12	4	4	0

Fisheries Management Zone 6

Lac des Mille Lacs lies in Fisheries Management Zone 6. Ontario sets its fishing seasons and catch limits by zone.

Zone 6 seasons and limits, 2026
Species	Open season	Sport licence	Conservation licence
Atlantic Salmon	Open all year	1	0
Brook Trout	4th Sat. in Apr. – Labour Day	5, not more than 1 greater than 30 cm (11.8 in.)	2, not more than 1 greater than 30 cm (11.8 in.)
Brown Trout	Open all year	5	2
Channel Catfish	Open all year	12	6
Crappie	Open all year	15	10
Lake Trout	Jan. 1 – Sept. 30	2, not more than 1 greater than 56 cm (22 in.) from Sept. 1 – Sept. 30; no size limit rest of year	1, no size limit
Lake Whitefish	Open all year	12	6
Largemouth and Smallmouth Bass or any combination	Open all year	4	2
Muskellunge	3rd Sat. in June – Dec. 15	1, must be greater than 91 cm (36 in.)	0
Northern Pike	Open all year	4, not more than 1 greater than or equal to 70 cm (27.6 in.)	2, not more than 1 greater than or equal to 70 cm (27.6 in.)
Pacific Salmon	Open all year	5	2
Rainbow Trout	Open all year	1	0
Splake	Open all year	5	2
Sunfish	Open all year	50	25
Walleye and Sauger or any combination	Jan. 1 – Apr. 14 & 3rd Sat. in May – Dec. 31	4, not more than 1 greater than 46 cm (18.1 in.)	2, not more than 1 greater than 46 cm (18.1 in.)
Yellow Perch	Open all year	50	25

The figures are the number of fish an angler may catch and keep in a day and have in possession, under a sport fishing licence and under a conservation licence. The rows are the whole zone’s, and they apply to every lake in it. Zone-wide, Smelt may not be used as bait or possessed for use as bait except where exceptions apply.

Exceptions for Lac des Mille Lacs itself, 2026
Species	Open season	Sport licence	Conservation licence
Northern Pike	Open from January 1 to April 14 and second Saturday in May to December 31	Zone-wide	Zone-wide
Walleye	Zone-wide	4; must be greater than 33 cm (13.0 in), not more than 1 greater than 46 cm (18.1 in)	2; must be greater than 33 cm (13.0 in,), not more than 1 greater than 46 cm (18.1 in.)

Where a row reads zone-wide, the zone’s own season or limit above applies.

Islands and names

Lac des Mille Lacs contains 574 islands in the Ontario Hydro Network’s outline, 32 of them over 10 hectares and 196 under a tenth of a hectare.

The ten largest islands, numbered to the table. Areas are measured on the province’s outline.

The ten largest islands
	Island	Hectares
1	unnamed	951.6
2	Bull Island	554.7
3	Owl Island	272.8
4	Fisherman's Island	215.2
5	Rabbit Island	189.3
6	Mountain Islands	127.7
7	unnamed	51.6
8	Case Island	46.2
9	unnamed	39.8
10	unnamed	29.3

Official names

Ontario’s geographic names register lists 28 official names on the water and its islands: 9 islands, twelve bays, three narrows and one point on an island. A further 5 named points stand on the mainland shore.

Bays 12

Baril Bay
Blind Bay
Bolton Bay
Boot Bay
Carlstead Bay
Maze Bay
Open Bay
Portage Bay
Reserve Bay
Rock Bay
Sawmill Bay
Shelter Bay

Harbours, cuts and narrows 3

Birch Narrows
Honkonen Narrows
Owl Narrows

Points on the shore 5

Birch Point
Long Point
Pine Point
Poplar Point
Sand Point

Nearby lakes

Upper Shebandowan Lake27 km away
Marmion Lake72 km away
Dog Lake75 km away
Whitefish Lake81 km away

All lakes

Questions about Lac des Mille Lacs

How deep is Lac des Mille Lacs?
Its deepest charted contour is 27.4 meters, and the water within it is deeper.
How big is Lac des Mille Lacs?
Its surface covers 245.2 square kilometers and it holds 574 islands. Of its 881 kilometers of shoreline, 517 kilometers is the mainland shore and 364 kilometers is the shore of the islands.
What fish are in Lac des Mille Lacs?
Ontario’s aquatic resource records name thirteen species, among them walleye, smallmouth bass, northern pike, lake whitefish and yellow perch.

A print of Lac des Mille Lacs

Lac des Mille Lacs has not yet been drawn as a print. The lakes drawn so far are on the lake maps page, and a print of Lac des Mille Lacs can be requested by email; whether it can be drawn is confirmed before any payment.

Ask about a Lac des Mille Lacs print

Looking at a property on Lac des Mille Lacs?

Depth and water quality are two records out of many. Our Rural & Cottage Buyer’s Report gathers what the public record holds about one specific address: shore road allowance, road access, wells, flood mapping, Crown land, wetlands and more, read and checked by a person before it is sent.

See what is in the report

What these figures do and do not say

Depth: A depth reading is the distance from the water surface on the day of survey to the lake bottom. Lake levels vary through the season and between years. The depth figure above is the deepest contour charted and is a minimum; the depth within it was not recorded. The figures for Lac des Mille Lacs are read from the province’s bathymetry records within the Ontario Hydro Network’s outline of the lake: the deepest figure is the deepest record inside that outline, and the count is every record inside it.
Shoreline and surface: The shoreline figure is the length of the mainland shore and of every island shore added together, measured on the Ontario Hydro Network’s outline of the lake. The surface figure is the area of water inside that outline, less the area of its islands. The provincial outline is mapped to within a few meters of the shore, and a shoreline measured from a coarser map comes out shorter, which is why a figure published elsewhere is often lower.
Survey coverage: Not every survey covered every arm of a lake, and the province’s index does not always record where a survey ended.
Water quality: Each figure belongs to a single monitoring site and not to the lake as a whole. A bay receiving a creek can read differently from open water half a kilometer away. Sites are matched to Lac des Mille Lacs by name in the province’s records.
Nearby lakes: Distances are measured between lake centres.
Islands and names: Islands are counted and measured on the Ontario Hydro Network outline; names are the register’s, placed by its own coordinate. The register names islands the outline draws as part of a larger one, and the outline holds islands the register has not named, so the two counts differ. The outline carries some lakes across marsh, so a gap in the marsh is drawn as land, and every such gap is included in the island count. Where the register has not named one and it lies mostly inside a wetland in the province’s Wetland With Significance dataset, the page names the wetland and the table of largest islands leaves it out. Wetland boundaries are mapped at their own scale and can cover a small island whole.
Depth contours: Each contour is drawn where the survey placed it, at the survey’s own positional accuracy. The survey carries its own shoreline, which differs from the Ontario Hydro Network’s outline, so a contour near the shore can run to or past the outline. Such a stretch is drawn along the outline, as a drawing convention, and is not the survey’s line there. The contour count in the stat band is the province’s, and it includes the survey’s own shoreline contour, one ring around the mainland and one around each charted island, which the map leaves off; the map’s small print gives the split.
Location: Municipal boundaries are read from the province’s records as published and can lag a boundary change.
Fish: Ontario’s fish records are compilations of existing knowledge from field biologists and do not represent exhaustive inventories. A species that is not listed was not ruled out. The species list carries one date for the whole record and none for any species.
Spawning areas: A spawning area is drawn where the province mapped it, at the province’s stated accuracy, and the record says nothing about how many fish use it in a given year. A nest record is one nest seen on one day; a count of nests is not a count of fish, and a count of records is not a comparison between species. Whether a given piece of work needs an authorization is a determination made by Fisheries and Oceans Canada on the work itself.
Eating advice: The guide’s advice is for fish taken from the lake at the tested location and applies to the size classes tested. The province’s guide is the authority and is updated each year.
Fishing rules: Seasons and limits are the zone’s, for the year of the dataset the page was built from, and a licence’s own conditions and any waterbody exception apply over them. The province’s regulations summary is the authority.

Where these figures come from

Depth, shoreline, islands and named places: Ontario Ministry of Natural Resources bathymetry and the Ontario Hydro Network, © King’s Printer for Ontario, Open Government Licence – Ontario.
Phosphorus and water clarity: Ontario Lake Partner Program, Ministry of the Environment, Conservation and Parks, Open Government Licence – Ontario. Extract built 2026-08-09.
Aerial photography: Ontario Imagery Web Map Service, Open Government Licence – Ontario, drawing this frame from Northwestern Ontario Orthophotography Project 2017, Forest Resource Inventory T2 (T2) 2018-2027 and Sentinel 2 2025, satellite.
Official names: Geographic Names Ontario, Ministry of Natural Resources, Open Government Licence – Ontario. Marinas: Overture Maps Foundation places data, CDLA-Permissive 2.0.
Lake outline and municipal boundaries: Ontario Hydro Network and Ontario municipal boundary records, Ministry of Natural Resources, Open Government Licence – Ontario.
Fish species and thermal regime: Ontario Aquatic Resource Area records. Spawning areas: Ontario Fish Activity Area records. Both Ministry of Natural Resources, Open Government Licence – Ontario.
Consumption advice: Guide to Eating Ontario Fish advisory database, 2025 edition, Ministry of the Environment, Conservation and Parks, Open Government Licence – Ontario.
Seasons and limits: Ontario recreational fishing regulations data, Ministry of Natural Resources, Open Government Licence – Ontario.
Fish plates: Duane Raver, U.S. Fish and Wildlife Service; Ellen Edmonson and Hugh Chrisp, New York State Conservation Department; Timothy Knepp, U.S. Fish and Wildlife Service; U.S. Fish and Wildlife Service. All public domain.