MONTREAL—Scientists have recorded an “unprecedented” spike in the fish deformities in the wake of the deadly 2013 train derailment and oil spill in Lac-Mégantic, Que., according to a provincial government report.

The report into the effects of the disaster on the 185-km-long Chaudière River, which begins in Lac Mégantic, found that in some parts of the river as many as 47 per cent of the fish they collected had an external deformation.

The rate of deformations greatly surpassed that recorded in a similar fish population study in 1994. The study also found a “marked drop” in the river’s fish biomass, or total weight.

“There is no hypothesis other than the oil spill of July 6, 2013 that can explain these results,” says the report, which got little attention when it was released last November. It was brought to wider attention Wednesday when resurrected by Montreal’s Le Devoir newspaper.

The derailment and ensuing explosion, in which 47 people were killed, decimated the picturesque small town in eastern Quebec and turned its downtown strip and waterfront into an oil-soaked wasteland.

The 72-car train was carrying nearly 8-million litres of highly combustible crude oil that was bound for a refinery in New Brunswick. An engine fire that occurred when the train was left unattended on the main tracks about 11 km from Lac-Mégantic resulted in the air brakes failing and the unattended train hurtling into town. It derailed near a popular bar, the site where most of the dead were found.

About 100,000 litres of crude oil is estimated to have washed into the Chaudière River and settled as contaminated sediment on the riverbed. The expert committee’s report said there are some encouraging signs that the worst contamination is limited to the first 10 km of the river, whereas traces were found some 80 km away in testing conducted right after the incident.

But a whole ecosystem has been affected. The insects, worms and other organisms that live on the sediment and upon which fish feed were affected by the oil spill but are showing signs of recovery after testing conducted in 2014.

Crude oil coming to rest on the riverbed can prevent fish from accessing food and can result in the death of fish eggs or embryos. The population drop could also be attributable to other factors such as more active predators or lower reproduction rates, the report noted.

But the contaminated sediment is the most likely explanation for the alarmingly high rate of external deformities recorded among the sample of 900 fish collected for study. The most common problems were lesions and infection-induced breakdown of the fins, which can occur when a fish comes into direct contact with the sediment, leaving it vulnerable to bacteria, fungus and parasites that eat away at the tissue.

The widely held standard is that if more than five per cent of fish in the sample show signs of external deformities, the habitat is considered to be contaminated by toxic substances.

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Perhaps as a result, fish populations are estimated to be 66 per cent smaller and the biomass — the total weight of the fish stock — is down 48 per cent.

“The weak biomass observed in 2014 is difficult to attribute to anything other than the oil spill,” the report concluded.

Scientists have now set their sights on a longer-term monitoring plan and a fish-population survey they hope to carry out in 2016. One of the things they will be looking for are skeletal malformations — a widely recognized consequence of exposure to petroleum hydrocarbons.

Their interest in this stems from a laboratory study in which the eggs of two types of fish — the fathead minnow and the brown trout — were exposed to contaminated sediment from the oil spill.

The exposure had no effect on mortality rates or the time it took for the eggs to hatch. But the eggs of the brown trout that were exposed to the most contaminated sediment showed a higher rate of scoliosis, an abnormal lateral curvature of the spinal column.

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