Mercury in Fish
& Fish Consumption
Guidelines for the
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
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
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
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
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
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
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
The guidelines in this pamphlet can assist anglers in
determining what species and size of fish are safe
How To Use The
n For the listed water body, find the monitoring
site nearest to your fishing location.
n Check the guide to determine which fish
species have consumption restrictions.
n Measure the fish from the tip...
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