Credit Valley Conservation (CVC) has received a number of enquiries about a recent fish die-off in Port Credit and along the Mississauga shoreline. The emails and phone calls have described large numbers of dead or dying silvery fish, in the 15 centimetre (6”) range, along the shoreline and rivermouth areas.

These fish are alewife and are not native to Lake Ontario. Every year in the late spring, they move into warm rivermouths and shoreline areas to reproduce. Changes in wind direction can result in cold water from the lake moving inshore and quickly increasing or decreasing water temperatures. Unusually warm spring weather can also create large thermal differences between off and nearshore waters. Because the urge to reproduce is so strong, fish are unwilling to leave these warm areas and may get caught in areas of rapid temperature change. Alewife have not evolved to adapt to these rapid temperature changes and many may die as a result.*

This year’s fish kill may be more obvious than recent years due to the number of alewife in LakeOntario. A 2014 report published by the Ontario Ministry of Natural Resources estimates that there are 681 million yearling-and-older alewife in Lake Ontario, which is 2.5 times higher than those observed in 2012 and 3.7 times higher than the previous ten-year average.*

For more information, contact Jon Clayton, Aquatic Biologist for CVC at 905-670-1615 ext. 502 or [email protected].


Conservation Authorities are a provincial/municipal partnership. CVC was established by an Act of the Province of Ontario in 1954 with a mandate to protect all natural resources, other than minerals, in the area drained by the CreditRiver. We have been working for 60 years with our partner municipalities and stakeholders to protect and enhance the natural environment of the CreditRiver watershed for present and future generations. CVC is a member of Conservation Ontario.

*Ontario Ministry of Natural Resources. 2014. LakeOntario Fish Communities and Fisheries: 2013 Annual Report of the LakeOntario Management Unit. Ontario Ministry of Natural Resources, Picton, Ontario, Canada.


Media Contact
Baljit Sharma
Marketing & Communications Specialist
Credit Valley Conservation
905-670-1615 ext. 224
[email protected] 

Information Contact:
Jon Clayton
Aquatic Biologist
CreditValley Conservation
905-670-1615 ext. 502
[email protected]

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