Health Canada has sent out a warning to consumers to limit their intake of certain types of fish because of high mercury levels in them.

The department had previously warned consumers last year about eating shark, swordfish and fresh and frozen tuna. A news release from the department says it just wants to reiterate that advice.

The advisory says Canadians:

• should limit consumption of fresh and frozen fish to once a week
• pregnant women, women of child-bearing age and young children should eat the fish no more than once a month

Mercury is a toxin that can attack the nervous system. It accumulates in the body and can affect fetal development, cause blindness and other birth defects.

Mercury in lakes, streams and oceans is transformed into a more toxic form: methyl mercury. It bonds with proteins in fish tissue and accumulates. Predatory fish species have higher levels than non- predatory fish.

The fish have been exempted from testing because they are listed as "gourmet." An investigative report last year by an Ottawa newspaper found excessive levels of mercury in the fish.

Health Canada says not enough is eaten to constitute regular checks. However, it does caution consumers about eating too much of the gourmet fish.