As Ontarians flock to the Great Lakes for a little summer fishing, a new report is warning some categories of fish are unfit for consumption due to high levels of toxic chemicals found in their tissue.

"Fish are still great for our diets, but you need to watch what you eat," said Mike Layton of Environmental Defence, an author of the report.

"For the most part, eating a few small fish every month isn't a big deal but you should be cautious with larger fish that have had more time to accumulate higher concentrations of dangerous chemicals."

The report, entitled "Up to the Gills: 2009 Update on Pollution in Great Lakes Fish," brings together data from fish advisories published by the Ontario environment ministry between 2005 and 2009.

In many Great Lakes areas, high levels of mercury, PCBs, pesticides, dioxins and furanes were found in medium-sized and large fish (55 to 75 centimetres long) such as salmon, trout and carp.

Lake Huron catches showed some improvement but even there, cautions remained for larger fish.

Lake Ontario was less lucky, with 40 per cent of advisories saying eating certain fish in any quantity is unsafe. Many cautions targeted small and medium-sized fish, considered a disturbing trend.

"In the last two years, however, advisories for smaller fish in Lake Ontario levelled off," he said.

The "Up to the Gills" report can be seen at Environmental Defence's website.