Cormorants on Mississippi Lake:

Understanding Their Impact and What Can Be Done

by Lise Boulay, Environment Chair

Cormorants have been a natural part of Ontario's wildlife for many years. Recently with a reduction in pesticide use, their numbers have been growing quickly on our lakes and islands. 

Presence on Mississippi Lake: When large numbers of birds gather and nest in one concentrated location—such as the 100+ cormorants observed this year on Loon Island in Mississippi Lake—their presence can have noticeable effects on the island, surrounding water and other wildlife.  In previous years, we have seen cormorants inhabiting other islands and this year we've noticed a large number of cormorants swimming and diving throughout the lake.

It is important to clarify that cormorants themselves are not poisonous or "toxic." The concern is primarily the enormous quantity of droppings produced when a large colony occupies a relatively small area. The sheer volume of their accumulated waste, called guano, can transform ecosystems as it kills the trees and vegetation. 

The Impact of Cormorant droppings: Cormorant guano, which is 80% uric acidic,  also contains high concentrations of nutrients, particularly nitrogen and phosphorus. When large quantities accumulate on an island and enter the surrounding water, these nutrients can contribute to changes in water quality and encourage the growth of algae and aquatic vegetation.

A large colony can also dramatically alter the island itself. Nesting, trampling and the accumulation of guano can kill vegetation beneath nesting areas. Over time, this can leave areas of bare soil, contribute to erosion and change habitat used by other species.

There is also a potential human-health concern associated with accumulated bird droppings. Bird feces can contain bacteria, parasites and other microorganisms capable of causing illness. People should avoid handling the guano and should use appropriate precautions when working around heavily contaminated areas.

Predators:  Adult cormorants have very few natural predators due to their size and aquatic defences, but their eggs and chicks are heavily targeted.  Main predators are: bald eagles and hawks; gulls, crows and ravens (which steal eggs and small chicks); raccoons, foxes and coyotes (which raid ground or low-tree nests). As for the cormorants, they consume large quantities of small fish, including perch.

Lifespan: Cormorants average about 11 to 15 years in the wild.

Can Cormorants Be Moved From Loon Island?

Local and Individual Actions:  Under provincial wildlife rules, local municipalities or private property owners can take legal steps to control nuisance animals, which can include hiring designated control officers or removing roosting trees when birds cause localized environmental damage. 

Techniques used in wildlife management can include controlled noise, distress calls, visual deterrents and other forms of harassment. The objective is to make the location unattractive enough that birds choose another site.

Timing is extremely important: Clearing out old nests or trimming tall nesting trees during the off season before birds return will help. Deterrence is generally much more effective when undertaken in the spring, before a colony becomes firmly established or before nesting begins. Once birds have established nests and young, simply disturbing the colony may not solve the problem and can create additional wildlife-management concerns.

An important consideration for Mississippi Lake: Moving birds away from one island does not likely mean eliminating the problem from our lake. If suitable habitat exists elsewhere, the birds simply establish their colony on another island.

Jurisdiction: Unlike certain other migratory birds covered under federal framework, double-crested cormorants in Ontario lack specific federal protection under the Canada Migratory Birds Convention Act. That places their management and regulation under the authority of the Ontario Ministry of Natural Resources (MNR).

Hunting Season: The province has established an annual provincial hunting season running from September 15 to December 31 to manage local populations and protect habitats from guano damage. Hunters are permitted to harvest up to 15 cormorants per day, provided they have a valid Outdoors Card and small game license. Any carcasses must be retrieved and either buried or taken to an authorized disposal site.

A Responsible Approach for Mississippi Lake

The Mississippi Lakes Association is monitoring Loon Island by taking regular water samples and with the help of the community doing a regular visit to the island. We count the number of cormorants, their nesting activity, the changes in the vegetation, when the birds migrate south, and of course for next year, when we first see them on Loon island or any other island on the lake.  It is only by monitoring and tracking will we better understand these birds. 

Next Steps: If the colony continues to grow or evidence demonstrates significant ecological impacts, there will be pressure to discuss available management options—including non-lethal deterrence and, where legally authorized and scientifically justified, more intensive population-control measures.

The objective: To protect the health and ecological balance of Mississippi Lake while respecting Ontario and Canadian wildlife laws.

Cormorants are not "toxic birds," however a large, concentrated colony can produce substantial amounts of nutrient-rich guano and can significantly alter the environment in which it establishes itself. Understanding the problem, documenting the impacts and exploring legally available management options will allow our community to make informed decisions about their future on Loon Island and Mississippi Lake.

Please reach out to us if you would like to join the cormorant tracking team. Residents who observe significant changes associated with the cormorant colony are encouraged to document what they see and report concerns to us at environment@mlakes.org