A new study of contaminated fish in the Great Lakes suggests the waters are filled with toxic chemicals that threaten human health.

The Environmental Defence report followed trends in Ontario's annual Guide To Eating Sport Fish and found consumption advisories are generally getting worse.

"The toxics can really add up," Aaron Freeman, police director for the non-profit lobby and research group, said in a news release.

"Fish from the supermarket, from the chip stand and from the Great Lakes all contain various concentrations of harmful contaminants, which all together can have serious cumulative effects on human health."

The group analyzed fish advisories published by the Ontario Ministry of the Environment for four species of fish in 13 locations across the Great Lakes. Its report suggests many categories of fish are somewhat or completely unfit for human consumption, and other categories of fish are becoming so.

The major chemical contaminants that lead to consumption advisories include mercury, PCBs, pesticides, dioxins and furans, which have been shown to damage  the nervous, respiratory and immune systems, as well as cause cancer.

The analysis showed that while some areas, such as Lake Erie, have seen reductions in toxic contamination over time, a greater number of areas are becoming more polluted.

Lake Ontario fared the worst, with eight categories of fish that became more contaminated between 2005 and 2007, and only one category that improved.

As a general rule, advisory levels are more severe for larger fish, which are usually older and so have accumulated more toxins. But severe consumption advisories have been issued for even small sizes of fish in Lake Ontario.

The report also said the government only lists which fish are dangerous to eat but doesn't adequately reveal why.

Freeman said the government should provide specific data on actual levels of contaminants in fish, as well as historical levels of contamination.

Fishing is a multibillion-dollar industry, and including fish in meals is a key to health, so more must be done to reduce contamination, the report said.

With files from the Canadian Press