Mercury In Fish

Mercury in Fish

& Fish Consumption

Guidelines for the

Saskatchewan, Souris,

Assiniboine, Red and

Winnipeg River Systems

Water Quality Series Number 4

Mercury in Fish:

Problems and Solutions

Monitoring Of Mercury

Although Manitoba has some of the cleanest waters

in the world, mercury contamination of fish does

occur.

The Manitoba Department of Environment has

monitored mercury levels in fish from some major

water systems since 1978. Monitoring has focused

on the major waterways in southern Manitoba.

Monitoring sites are located in or near industrial

areas, urban centers, agricultural zones and

reservoirs.

What Is Mercury?

Mercury is a naturally occurring metallic element

that is generally found in low concentrations in the

air, water and soil.

Where Does Mercury Come

From?

Mercury often occurs in association with other

mineral deposits such as lead, silver and copper.

Mercury is released into the air and water from

these sources under natural conditions, but

considerable amounts may also be added to the

environment due to industrial activities.

Former industrial uses that released mercury to the

environment included the production of chlorine and

caustic soda. Mercury was also used as an

ingredient in slimicides for the pulp and paper

industry and for the control of fungi on seed grain.

Current uses of mercury include mercury based

fungicides for golf courses, dental amalgams, paint

and pharmaceutical industries, and the

manufacturing of thermometers, mirrors, batteries

and electrical switches.

Scientific evidence also indicates that long-term

flooding following construction of reservoirs

accelerates the release of naturally occurring

mercury into the water.

The prime source of elevated mercury levels in

Manitoba waters is from natural sources.

Why Is Mercury A Problem

In Fish?

Mercury is present in the environment in several

forms. Over many years, mercury from atmospheric

deposition or from the watershed accumulates in

lake and river sediments. Micro-organisms coming

into contact with inorganic mercury can convert it to

an organic or methyl-mercury form.

Methyl-mercury can be easily absorbed by fish

either directly through their gills or indirectly from

organisms they consume in their diet. Likewise,

people can ingest methyl-mercury from the fish they

eat.

Because both fish and people eliminate methylmercury

slowly, consuming food with high mercury

levels will gradually increase the accumulation of

mercury in the body.

The highest mercury levels will be found in fish that

have been exposed to mercury for extended periods

of time or in fish that consume large amounts of

mercury contaminated food.

Because of the food they eat, predatory fish such as

walleye and northern pike, contain higher

concentrations of mercury than bottom feeding fish

such as whitefish or common suckers.

Consuming fish with high mercury levels could lead

to health concerns such as mercury poisoning.

Symptoms of mercury poisoning include reduced

co-ordination, inability to feel objects properly,

numbness of lips and mouth, tunnel vision or night

blindness.

What Is A Safe Limit?

The acceptable limit of mercury in fish for

unrestricted consumption in Canada is 0.5 micrograms

of mercury in each gram of fish flesh (mg/g).

Manitoba guidelines follow federal recommendations

and are consistent with other provincial

jurisdictions. These are based upon recommendations

from the World Health Organization which

were derived from medical observations of mercury

poisoning in Japan and Iraq.

Mercury In Fish

Consumption Guidelines

The guidelines in this pamphlet can assist anglers

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