Shellfish Biotoxins
Species and Seasonality Aspects of Marine Biotoxins
Eating contaminated Shellfish can be life threatening:
take a look at the
areas that are closed due to PSP (Red Tide)!
Change to Public Notification of PSP (Red Tide) Closures for Bivalve Shellfish
*September 2006*
In B. C., the Canadian Food Inspection Agency (CFIA) is responsible
for analysing samples of shellfish for paralytic shellfish poisoning
toxin (PSP - saxitoxins) and amnesic shellfish poisoning (ASP
- domoic acid). The action levels for PSP and ASP are 80 micrograms
per 100 g of meat and 20 ppm of domoic acid respectively.
As
the basis of the marine biotoxin monitoring program in the Pacific
Region, large mussels are hung in plastic mesh sacks in shellfish
growing areas (in addition to a lesser number of commercial
samples of all species). Samples are withdrawn on a weekly or
biweekly basis and shipped to the CFIA laboratory for analysis.
Shellfish species differ in their tendencies to accumulate and
eliminate marine biotoxins in their tissues. Mussels tend to
be indiscriminate feeders of phytoplankton and are therefore
much quicker to pick up biotoxins than many other shellfish
species. The levels of biotoxin in mussels are frequently up
to 10 times higher than oysters and manila or littleneck clams
growing in the same area. This allows the CFIA to recommend
harvest restrictions (eg. area closures) to Fisheries and Oceans
Canada while species other than mussels, such as clams, oysters
and scallops, are still safe. Mussels also tend to rid their
tissues of biotoxins faster than other species. Therefore, although
harvest restrictions are usually based on mussel analysis results,
samples of the other species in the area are tested before the
harvest restrictions are lifted.
Some bivalve species cause additional concerns. For example,
butter clams are able to retain high levels of PSP toxin for
over a year. For this reason, Fisheries and Oceans Canada, on
the recommendation of the CFIA, put in place a blanket harvest
ban for this species. If a fishery for butter clams is planned
pre-harvest samples are collected to ensure that the levels
of toxin are acceptable prior to lifting the restriction for
the area in question. Education and awareness with respect to
shellfish biotoxins is very important. There have been incidents
in the past in which individuals have harvested and consumed
butter clams under the harvest ban and been hospitalized as
a result of PSP. Many non-commercial harvesters do not take
the time to check with their local Fisheries and Oceans Canada
office, the Pacific Region Fisheries and Oceans website or the
24 hour information line at 604-666-2828 to insure that the
area and time of year are safe for harvest.
The CFIA is also concerned about scallops and geoduck clams.
These species are generally found sub-tidally, well below the
intertidal zone, and they may accumulate biotoxins at different
rates than intertidal bivalves in adjacent areas. Reasons for
this may be that currents expose them to different amounts of
the toxic phytoplankton during a bloom (can be greater or less)
or they may be exposed to greater numbers of dormant cysts of
the toxic dinoflagelates (e.g. Alexandrium catanella.)
These cysts are a part of the algae's life cycle and they are
found resting on the bottom in sediments. Strong currents or
storms may resuspend them and they may then be ingested by filter
feeding bivalves such as scallops or geoduck clams.
In scallops, biotoxins can be accumulated to high levels and
be retained for a considerable period of time. They may still
be toxic even after oysters and manila clams in the area are
safe. The toxins are particularly concentrated in the mantle,
viscera, roe, etc. where we have seen levels considerably greater
than 1000 micrograms. In most species, the adductor muscle rarely
accumulates levels of toxins that would cause illness. An exception
is the native purple hinge rock scallop that has been found
to have elevated levels of biotoxins in the adductor muscle.
These levels are still much lower than in the other portions
of the animal. With smaller species, such as the pink and spiny
scallops, this does not apply as the whole animal is marketed
and consumed and the removal of the adductor muscle from the
rest of the animal is not usually practised.
In geoduck clams the toxin is almost never detectable in the
siphon and body meat. In this species, the toxin is found in
the viscera which is consumed in some markets. These large subtidal
clams are also prone to picking up toxins in the winter as storms
can stir up the cysts of toxic dinoflagelates.
Additional note:
Crabs and predatory molluscs (eg. moon snails) may feed on
toxic bivalves and thus become toxic themselves even though
they are not filter feeders. In crabs the toxins are found in
the hepatopancreas (part of the viscera, under the carapace)
which the CFIA periodically analyses for biotoxins. Although
the action level may be exceeded at times and harvest restrictions
have been instituted, there have been no illnesses reported
from any species other than bivalves. Likewise, there have been
no recorded instances of illness as a result of ASP from any
species harvested from B. C. waters although the CFIA has found
levels of domoic acid in excess of the action level and closed
areas to harvest as a result.
Conclusions
-
Different bivalve species may accumulate and eliminate
biotoxins at different rates and levels.
-
Toxin levels may be very different in intertidal and adjacent
subtidal bivalves due to differences in their growing environment.
-
Some bivalve species will concentrate toxins in a particular
part of their bodies, making some parts much more toxic
than others.
-
Long term retention and resuspension of cysts may result
in toxic shellfish in B. C. at any time of the year even
though the toxic algae blooms generally occur during the
warmer periods of the year.
-
Some non-bivalve molluscs and crustaceans may accumulate
biotoxins that could result in illness.
|