Fish and Fish Habitat

Ontario has an abundance of aquatic resources and some of the largest freshwater fisheries in the world. In addition to the Canadian portion of the Great Lakes, there are approximately 250,000 inland lakes in the province and countless rivers and streams. Although the Ministry of Natural Resources and Forestry (MNRF) manages fisheries in Ontario, it cannot do so alone. The health of fisheries depends on the collaborative and coordinated efforts of governments, organizations, communities, and individuals.

Fisheries support human, economic, and environmental health. Recreational fishing is a relaxing pastime that contributes over $2.2 billion annually to Ontario’s economy. Fish are a valued nutritional resource and are important for lake health and ecosystem functioning. They are also good indicators of environmental change. If the health of a lake deteriorates, fish populations can be harmed.

Fish Requirements

All living organisms require oxygen, including fish. On land, oxygen is
continuously rejuvenated in the atmosphere, but this is often not the case in
aquatic environments. Two main sources of usable oxygen in lakes are the diffusion
of oxygen from the atmosphere and the release of oxygen from aquatic plants.
The amount of dissolved oxygen in the water varies from lake to lake and can
affect the type and amount of fish species in the lake.

Each fish species has an optimal temperature in which it can thrive. Unlike
warm-blooded species, fish cannot regulate their body temperatures. When the
lake temperature is outside the optimal range it can harm the fish (e.g., water
that is too warm can kill trout). Water temperatures also have a direct impact
on oxygen levels. As water warms, it loses its ability to carry oxygen.

            Food. A rich variety of aquatic species is important for healthy fish populations. Each species plays a key role in the food web and the health and survival of all species, including fish, depends on the health and survival of others.

            Habitat. The shallow water near the shore (referred to as the littoral zone) can account for 80-90% of the lake’s total fish production. Vegetation, dead wood, and rocks along the riparian are essential elements of fish habitat. Vegetation provides food, oxygen, and protection for many aquatic species, and logs and rocks create ideal spawning grounds and nursery areas for young fish. Rocks and boulders also help dissipate the force of waves on the shoreland, reducing erosion and siltation. Silt or sand from erosion that is carried to spawning areas by wind or water can decrease or destroy fish production. A build-up of sediment can fill crevices in and around rocks that are essential for the development of numerous fish species’ eggs. Silt can also coat the eggs and suffocate the larvae.


Stressors are factors that upset the balance of aquatic communities, degrading the health of the ecosystem. Three types of stressors that can harm fish are physical, chemical, and biological stress.

Physical Stressors

Shoreland alterations can harm aquatic ecosystems. Clearing weeds, rocks, and wood debris and replacing them with sand, docks, and decks can provide better swimming and docking areas for people, but can have a devastating impact on fish communities. Removing aquatic plants along the shores eliminates feeding areas and hiding places important for fish survival. It can also cause fine soil or organic matter to enter the lake, increasing the water’s turbidity (cloudiness) and nutrient input.

By law, it may be required to obtain a permit for work on private and Crown land shores. Government agencies work with property owners to balance the needs of cottagers while maintaining the health of fisheries. To learn more about work permits and harm reduction, please visit the following websites:

Projects near water, Fisheries and Oceans Canada

When you do (or don’t) need a work permit, Ministry of Natural Resources and Forestry

Chemical Stressors

Human activities such as land development, improperly installed or poorly maintained septic systems, and the use of fertilizers can increase the inflow of nutrients into a nearby lake (a process known as eutrophication). Increased nutrients, such as phosphorous and nitrogen, can reduce the availability of oxygen in a lake and raise water temperatures. Eutrophication is a serious problem because can it can lower water quality, cause harmful algal blooms, and kill fish.

There are a number of easy and cost-effective ways that landowners can help reduce the inflow of nutrients and other pollutants into the lake, including:

  • Reduce the use of fertilizers
  • Use natural pest controls (e.g., coffee grounds, crushed egg shells)
  • Do not cut "lawns" along the shoreland- let areas grow naturally
  • Plant or maintain native grasses, perennials, trees, and shrubs

For more examples, please see the Landowner Guide to Protecting Water Quality in the Kawarthas.

Biological Stressors

            Overfishing. Overfishing is a major source of declining fish populations. A few generations of harvesting the biggest fish can also lead to genetic changes – fish mature earlier and become smaller in size. Sparing larger fish for smaller ones can help replenish the fishery because larger fish can produce more eggs and maintain larger growth rates.

Fishing regulations on the number and size of fish being harvested help prevent the harmful impact of overfishing on the aquatic environment. Catch and release fishing is beneficial for both the angler and the fish, allowing for the continued enjoyment of a much-loved pastime while maintaining a healthy fishery. Handling and releasing fish carefully is important in ensuring their survival, however.

Tips to ensure a successful catch and release include:

  • Keep the fish in the water as much as possible
  • Hold the fish horizontally and support its belly
  • Avoid touching its gills or eyes
  • Use pliers to remove the hook as quickly and safely as possible
  • Gently lower the fish into the water
  • Support it in the water until it can swim away on its own

Click here for more catch and release tips.

            Invasive Species. Invasive species are species that have been introduced to a new habitat where there are no natural predators. Without these natural controls, invasive species populations can grow quickly and are almost impossible to remove. Aquatic invaders are harmful to fish and fish habitats because they can prey on native species, out-compete them for resources, and alter the environment so it is no longer suitable for fish. For example, zebra mussels filter water, removing plankton (an important food source) and increasing water clarity. Clearer water allows more sunlight to enter the lake, forcing light-sensitive fish, such as the walleye, to move to deeper and darker environments. More sunlight can also increase the growth of aquatic vegetation, including invasive plants like the Eurasian watermilfoil.

Ontario's Invading Species Awareness Program provides information on identifying, reporting, and preventing the spread of invasive species.

Invasive species action plans can also be found on the MNRF website, which includes tips for anglers and boaters, such as:

  • Use local bait
  • Never dump bait into the water or on the ice (it is illegal to do so)
  • Avoid driving your boat through aquatic plants
  • Clean your boat and gear

Please visit the links and additional resources for more information on protecting the health of fish and fish habitats.

References and Additional Resources

Barneche, D. R., Robertson, D. R., White, C. R., & Marshall, D. J. (2018). Fish reproductive-energy output increases disproportionately with body size. Science, 360, 642-645. doi: 10.1126/science.aao6868

Federation of Ontario Cottagers' Associations (2009). Take the plunge: A guide to stewardship of Ontario's waters.

Lynch et al. (2016). The social, economic, and environmental importance of inland fish and fisheries. Environmental Reviews, 24, 115-121.

Ontario Ministry of Natural Resources and Forestry (2019). Fisheries in Ontario. Retrieved from

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