Suspected avian botulism outbreak in Christchurch
Toll reaches over 4300
Over the past month there has been a dramatic decrease in Christchurch’s birdlife. The Avon/Heathcote Estuary and Bromley area has suffered a real loss of more than 10 per cent of the area’s bird population; a devastating blow to one of the country’s top ten sites for wetland birds. Over 4300 dead birds have been collected in the area; this number surpasses the death toll of the Rena oil spill last year.
White-faced heron (9.61% of population killed)
Immediately following February’s earthquake large volumes of untreated wastewater were discharged into the lower Avon and Heathcote Rivers as well as the Avon-Heathcote Estuary.
A bird survey taken last year, a week after the February quake showed surprisingly that there were approximately 5000 more birds on the Estuary than the previous year (before the quakes), and initial fears that there would be nothing left on the Estuary due to liquefaction were quelled. The initial influx in bird numbers directly after the quake could have been due to birds flocking to the area as ‘quake refugees’.
You only have to look at the estuary and the effects of the quakes are immediately evident - the estuary is now a physically different place. The northern part of the estuary (Avon River mouth) has subsided by 0.2 - 0.5 metres from its previous level and the Southern part (Heathcote River mouth) has risen by 0.3 – 0.5 metres. According to a NIWA report there is an average elevation rise of 0.14 metres which is enough elevation to keep a million cubic metres (400 Olympic-sized swimming pools) of water from flowing in and out of the estuary, that’s one-sixth of its original flow.
I remember being struck by the amount of liquefaction on the estuary after the February earthquake. Some parts of the estuary resembled the surface of the moon, with large crater like liquefaction mounds covering the mudflats for kilometres! NIWA estimate that up to 40 per cent of the estuary was covered by liquefaction mounds after February! Liquefaction has the potential to smother estuarine biota and I’ve noticed a dramatic reduction in epifauna especially mudsnails and mudcrabs. Crab burrows once covered much of the Avon/Heathcote estuary and there has been a visible reduction of these burrows due to liquefaction cover at my bird photography sites. Mud crabs are an important food source for birds like the kingfishers and herons and there has been a noticeable reduction in kingfishers and herons at these once popular feeding spots.
A NZ Kingfisher with a mudcrab, epifauna such as mudcrabs have lost habitat due to liquefaction
On the 9th of February an article in The Press expressed its concern with the finding of 600-800 dead birds around the Christchurch estuary, in particular Bromley oxidation ponds and the eastern wetlands area. According to council ornithologist Andrew Crossland the deaths are more to do with the fluctuating water levels and the changes in water quality due to the earthquakes. Over the weekend another article reported that the death toll now is above 4300 birds! The fatalities are thought to be a result of a suspected avian botulism outbreak due to an increase in sewage levels. The Council is working with the Department of Conservation, vets at Massey University, Ministry of Agriculture and Fish & Game to determine the cause of deaths.
The worst affected species are the paradise shelducks, which lost more than 85 per cent of their population (1415 birds), the mallard/grey ducks (49 per cent or 385 birds), and the grey teal (13 per cent or 495 birds), NZ scaup (8.54 per cent), NZ shoveler (3.5 per cent). These staggering numbers mean that there will be a substantial decrease in numbers around the estuary and Bromley, as well as at source areas and migration/dispersal destinations. Duck species are not the only ones that have suffered losses - white-faced herons, royal spoonbills, black-backed gulls and the endangered black-billed gulls, just to name a few, although these species' populations should recover in one to three years. In yesterday’s Press there was a photo of a royal spoonbill being rescued by a DOC officer, suspected of having avian botulism. Providing mildly affected birds with fresh water, shade and protection from predators may help them recover from the intoxication.
Male paradise shelduck (85.34% of population killed)
Mallard (49.18% of population killed)
Grey teal (13.33% of population killed)
NZ Scaup (8.54% of population killed)
Avian botulism is a paralytic disease caused by the ingestion of a toxin produced by the bacteria, Clostridium botulinum. The bacteria require warm temperatures, a protein source and an anaerobic environment to produce the toxin, and the increased sewage levels due to the earthquakes may have been responsible for aiding in this intoxication. Birds can either ingest the toxin directly or feed on invertebrates such as maggots containing the toxin. Invertebrates are not affected by the toxin but store it up in their bodies. The spread of avian botulism happens when maggots feed on animal carcasses and ingest the toxin, birds that consume toxin-laden maggots can develop botulism after eating as few as 3 or 4 maggots, therefore the swift removal of dead birds will help minimise the spread of the disease.
NZ shoveler (3.50% of population killed)
Further facts about avian botulism :-
- A paralysing, often fatal disease that is caused by a botulism bacterium.
- Birds suffering from the toxin will have sluggish movements, and will struggle to use their wings, raise their heads or feed as it affects the nervous system.
- Avian botulism cannot be transferred to humans.
The good news is that the numbers of dead birds being found is slowly decreasing. Regardless of the cause of mass deaths let’s hope that this decline in deaths will continue and that Christchurch will still remain one of the top ten national sites for wetland birds!
South island pied oystercatcher (0.09% of population killed)