Researchers from the University of Guelph’s Department of Pathobiology (Ontario Veterinary College), determined that the cause of the bird deaths was the result of Type E botulism. Botulism is a paralytic condition brought on by the consumption of a naturally occurring toxin produced by the bacterium Clostridium botulinum. Two types are recognized to affect wildlife in the Great Lakes region: Type C and Type E.

 

Type E botulism is connected with the consumption of fish. It occurs mainly in gulls, loons and to a lesser extent, mergansers, grebes, swans and shorebirds.

 

The botulism poison works its way up the food chain. The botulism poison works its way up the food chain. The bacterium C. botulinum Type E, is found in bottom mud, in aquatic invertebrates, and in the gut of fish taken from Lakes Michigan and Huron. The bacterium is picked up by feeding invertebrates. Fish ingest the bacterium, either directly from bottom mud, or from eating invertebrates arising from it. The carcasses of dead fish provide a perfect medium for the growth of the bacterium which produces the toxin. Birds and mammals become poisoned by feeding from the toxic dead fish.

 

Animals affected by botulism exhibit a loss of strength due to muscular paralysis. The degree of paralysis is related to the amount of toxin consumed, and time of exposure. Death is attributed to paralysis of respiratory muscles, or in the case of birds on the water, to drowning from not being able to hold their heads above water.

 

Estimates by local authorities put the numbers of dead birds over one thousand, although that number might be conservative. Most were found on the beaches of Pinery Provincial Park. Park naturalist Terry Crabe has estimated that park staff picked up about 600 bird carcasses. In addition, Alfred Rider, a field naturalist with Lambton Wildlife Incorporated, estimated another 150 in the Kettle Point area. Still more were noted in the area north of the Pinery, up to as far north as Bayfield.

 

While a total number of birds affected by the outbreak has not been determined, at least 700 loons died. About 20,000 loons migrate through this area each year.

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The curious aspect of this outbreak was that there was no coinciding fish die-off, as well as the fact that loons, mergansers and grebes would not be expected to consume dead fish. Researchers speculate in these instances that, with a botulism mortality of loons and other waterbirds in late fall in the absence of a coincidental fish die-off, it is likely the result of the overturn of the lake in the fall, with the suspended invertebrates being ingested by the birds.

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Botulism occurrence may be related to hot, dry summers, and low lake levels which may optimize the conditions for the growth of the botulism bacterium.

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While Type E botulism can pose a serious threat to waterbird populations, it poses little public health concern, since fish-eating birds are not normally eaten by people, and thorough cooking destroys the toxin.

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Fall Overturn

 

In Lake Huron, a fall overturn occurs when the lake begins to cool. It is characterized by the sinking and mixing of cooler, dense water from the surface, displacing warmer and lighter water below.

 

Overturn also causes bottom dwelling invertebrates to move up into the water column where they can be ingested by fish and birds feeding in the area.

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If the botulism bacterium has been picked up by the invertebrates, the toxin can work its way up the food chain.