Mary Lake

Where Mary Lake is

Mary Lake lies in the District Municipality of Muskoka, within the Town of Huntsville.

How deep is Mary Lake?

Mary Lake reaches a maximum recorded depth of 58.8 meters, determined from 20,380 soundings taken during Ontario’s 2003 survey.

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.

Mary Lake Aerial Photography

Ontario photographs the province from the air on a rolling schedule.

The photograph below is from the SCOOP2023 collection, flown in 2023 and 2024.

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.

Mary Lake has 3 Lake Partner monitoring sites. Two have reported phosphorus for the three or more years the province requires before a status is assigned. All are classified oligotrophic.

The longest record is held by site 3032/2 at N of Crown Is., sampled over 24 springs to 2025. Its long-term mean phosphorus concentration is 7.88 micrograms per litre, within the oligotrophic band, which extends to 10. The site’s 161 clarity readings give a median Secchi depth of 3.2 meters.

Water temperature

Ontario’s aquatic resource records class Mary Lake as a cold-water lake, with a mean depth of 25 meters against its maximum of 58.8.

A deep lake separates into layers through the summer. Sunlight and wind warm and mix the upper few meters, and beneath a band of rapid cooling, the thermocline, the water stays cold through the whole season. A mean depth of 25 meters places most of Mary Lake’s volume in that cold layer, which is the condition lake trout and lake whitefish require.

No provincial monitoring station records water temperature on Mary Lake. The province’s longest in-water record is kept on eight research lakes near Dorset, sampled since 1976. A reading is taken at the surface, at one meter, at each meter to seven and then every two meters to the bottom, about every two weeks through the open-water season and about monthly under ice. The record holds 5,240 readings at a depth of one meter.

At one meter the eight lakes track one another closely. On a day when several were sampled, the warmest and the coolest lake differed by about one degree in every month of the year, and a 12 meter deep lake and a 37 meter deep lake read within 0.2 degrees of each other from June to October. The weather of the year, and not the lake, sets most of the width of the band below.

Satellite thermal imagery gives a surface reading on clear days: the Landsat sensors pass over Mary Lake every eight days at a resolution of 100 meters. In the province’s record the surface reading and the one meter reading agree to within half a degree nine times in ten in summer.

The water a few meters down is colder than any figure on this chart, in every month of the year.

How depth changes the temperature

The chart below is the province’s record on Harp Lake, 37 meters deep, drawn as the median temperature at each depth for each month over 49 years. In July the water at one meter is 23 degrees, at five meters 15, at seven meters 9, and from 13 meters to the bottom under 6. By October the warm layer has deepened to seven meters and cooled to 11 degrees. In February the whole column sits between 1 and 4 degrees under the ice, coldest at the top.

A shallower lake in the same record warms sooner. Heney Lake, six meters deep, reads about 1.8 degrees warmer than Harp Lake at one meter from January to May, 0.8 degrees warmer from June to August, and the same or cooler from September.

Fish in Mary Lake

Ontario’s aquatic resource records name 20 fish species in Mary Lake. The province classes the lake as cold water. The record took effect on 23 September 2016, and it compiles what field biologists had reported by then. The province’s other records for the lake name two further species, brook trout and rainbow trout. They appear in the records below.

Twelve of the 22 species in the province’s records

• Lake Trouttrout and whitefish
• Lake Whitefishtrout and whitefish
• Northern Pikepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Largemouth Basssunfish
• Black Crappiesunfish
• Rock Basssunfish
• Pumpkinseedsunfish
• Burbotcod
• Brown Bullheadcatfish

Spawning areas

Ontario maps 15 fish spawning records within the Mary Lake outline, for four species, all of them mapped areas. Every record is recorded as verified and presently suitable, and none is classed as sensitive. Three of the areas are one outline mapped for white sucker and rainbow smelt alike. An outline filed for more than one species is drawn once, striped in each of their colours.

By number of records

By area mapped

• Lake Trout8 areas1.7 ha
• White Sucker3 areas3.5 ha
• Rainbow Smelt3 areas3.5 ha
• Smallmouth Bass1 area0.2 ha

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.

Stocking history

Ontario’s stocking records for Mary Lake run from 1947 to 2012 and hold 78 releases. Lake Trout account for most of the fish: 110,556 released in 41 of the years between 1947 and 2012. The last recorded release was 2,000 lake trout yearlings in 2012.

The province’s current stocking programme, recorded from 2016, lists 6,850 lake trout released into Mary Lake between 2016 and 2025.

Eating the fish

The 2025 Guide to Eating Ontario Fish carries consumption advice for seven species taken from Mary Lake, tested for contaminants at a sampling location in Stephenson Twp., Muskoka D.M. 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

Fisheries Management Zone 15

Mary Lake lies in Fisheries Management Zone 15. Ontario sets its fishing seasons and catch limits by zone. Zone 15 covers central Ontario from Georgian Bay to the Ottawa River, bounded to the north by the Pickerel River, Highway 522 and the northern edge of Algonquin Park. It takes in Parry Sound, Huntsville, Bracebridge, Bancroft, Pembroke and Renfrew, and the whole of Algonquin Provincial Park. The province describes it as a zone of many natural lake trout and brook trout lakes, with a well-developed road network and moderate angling effort.

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. No waterbody exception names Mary Lake.

Mary Lake Ice Fishing

An ice hut on Mary Lake must be removed by March 31. Mary Lake lies in fisheries management zone 15, one of the twenty zones Ontario sets its fishing rules by, and one removal date applies across the whole zone.

An ice hut must also be removed before the ice breaks up, whether or not that date has passed. The deadline clears huts from the ice ahead of the thaw and does not close the fishing season.

Registration is required in zone 15 for an ice hut built of anything other than fabric, and for a fabric tent covering seven square meters or more. Registration is free, and the number is displayed on the outside of the hut.

When the ice comes and goes

No ice record is kept on Mary Lake. The province’s nearest long record covers eight lakes near Dorset, about 25 kilometers away. It holds a date for every winter since 1975, observed by eye and from cameras set on the shore.

On those eight lakes the median date of complete ice cover is December 7 and the median date of complete ice loss is April 20, a median season of 134 days.

The date the ice forms has moved later through the record: a November 26 average across 1971 to 1980, and December 11 across 2011 to 2020. The date it leaves has moved by less than a day each decade over the same period. The two charts below are drawn to one scale of 26 days, so they can be read against each other.

Ice conditions

No public record reports the thickness or the condition of the ice on Mary Lake on any given day. Ontario publishes ice safety guidance for anyone going out on the ice, at ontario.ca.

Getting on the water

Four access marks are mapped on Mary Lake: one boat launch, one shoreline access point, one marina and one beach. One is Ontario’s own record, one is from the Overture Places dataset and two are operator entries, and the list under the map says which is which. Boat launch is the province’s class for a ramp; shoreline access is its class for a place the public can reach the water on foot.

Islands and names

Mary Lake contains 8 islands in the Ontario Hydro Network’s outline, zero of them over 10 hectares and zero under a tenth of a hectare.

Official names

Ontario’s geographic names register lists 7 official names on the water and its islands: 6 islands. A further 1 named point stands on the mainland shore. The register records the origin of one name: Forrest Island, “Named after William Duncan Forrest who purchased the island in 1889.”

Points on the shore 1

• Breezy Point

Communities on the lake 1

• Port Sydney

Nearby lakes

• Lake Vernon11 km away
• Fairy Lake11 km away
• Skeleton Lake16 km away
• Peninsula Lake17 km away

All lakes

Questions about Mary Lake

• How deep is Mary Lake?

  Its maximum recorded depth is 58.8 meters, determined during Ontario’s 2003 survey.

• How big is Mary Lake?

  Its surface covers 10.6 square kilometers and it holds 8 islands. Of its 24 kilometers of shoreline, 21 kilometers is the mainland shore and 3 kilometers is the shore of the islands.

• What fish are in Mary Lake?

  Ontario’s aquatic resource records name twenty species, among them lake trout, walleye, smallmouth bass, largemouth bass, northern pike and lake whitefish. The lake’s spawning, consumption and stocking records also name brook trout and rainbow trout.

• Is the water clear?

  All two monitoring sites with a phosphorus status are classified oligotrophic, the clearest of the province’s three bands.

• Is there a public boat launch?

  Records mark one boat launch, one shoreline access point, one marina and one beach on Mary Lake.