Hundreds of dead fish wash up along shores of Lake Ontario

Photo and videos of dead fish on the shores of Lake Ontario have been making the rounds on social media. (John Musselman)

Videos and images of scores of dead fish near the shores of Lake Ontario have been making the rounds on social media and alarming some residents but experts say that the “die off” is a natural phenomenon which happens periodically in the Greater Toronto Area.

The images circulating on social media have shown dozens of dead fish near the shoreline at both Marie Curtis Park and Humber Bay Park.

The Ministry of Natural Resources (MNR) is aware of this situation. All the dead fish have been identified to be Alewife, a member of the herring family which are found in large numbers in Lake Ontario, a spokesperson from the ministry told CTV News.

These fish spend most of the year in deep, cold water. However, they move to shallow water to spawn during the months of May and June.

“It is stressful for the fish, because a lot of fish are spawning in the spring,” Janet Koprivnikar, professor in the Department of Chemistry and Biology at the Toronto Metropolitan University. told CTV News on Tuesday.

Koprivnikar suggests that this “die-off” is likely due to a combination of a number of factors.

“They’re not unusual, and they often occur when you’ve had a particularly cold winter. And I know we’re not used to cold winters anymore,” she says. “But this past winter actually was colder than the ones we’ve been having for a number of years.They’re spawning or (dealing with) rapid temperature changes, they might have been dealing with low oxygen this past winter. It’s just all too much, and that’s probably what’s going on here.”

The ministry also noted weakened conditions after a long, cold winter, stress related to spawning and rapid temperature changes as the reasons for these die-offs.

A statement from Rick Portiss, senior manager of Aquatic Monitoring and Management at Toronto and Region Conservation Authority. confirmed that the dead fish are Alewife.

“They are highly sensitive to rapid temperature changes,” Portiss says. “When strong winds or storms cause cold-water upwellings, the sudden temperature shift can disorient or even kill the fish.”

Donald Jackson, professor in the Department of Ecology and Evolutionary Biology at University of Toronto, also told CTV News that the overall effect of temperature changes may have resulted in higher mortality than is typical for species.

However, this phenomenon is more likely to be attributed to the dynamics of this species Jackson says.

“Depending on how healthy they are going into winter (i.e. how much fat reserve they have), they may be more vulnerable,” he says.

“There is less food available to this species and others that inhabit the offshore areas of the Great Lakes due to the impacts of zebra and quagga mussels that are very effective at extracting most of the nutrients and food production (algae and zooplankton) within the Great Lakes.”

Are there any health concerns associated with it?

While the dead fish might be an unseemly sight for some residents, Jackson says that there are no major health concerns as a result of the die offs.

The gulls will feed heavily on the dead fish, along with other scavenger birds and wildlife which will effectively wipe out many of the carcasses.

The rest of them will break down and decompose quickly due to the size of the fish, he says.

Although, oxygen can become more limited in the water when the water temperature rises during late summer, leading to an increase in algae and submerged plants that consume oxygen at night, Jackson explains.

“This can contribute to issues in some wetlands or protected areas along the shoreline,” he says. “In these protected areas of the shoreline during late summer there are sometimes events referred to as ‘harmful algal blooms’ that are due to cyanobacteria that can cause health issues for pets and humans.”

Meanwhile, no health concerns are associated with the dead fish, although it may produce a noticeable odour, Portiss states, which can be uncomfortable for anyone residing near the shores where the fish are.

“The dead fish are typically removed quickly as birds and other wildlife forage on them, which is nature’s way of returning nutrients to the ecosystem,” he says.

However, Koprivnikar warns against handling or coming in direct contact with any of these dead fish.

She also warns against letting your pets sniff around or come in direct contact with them, since there could be secondary bacteria growing on it which can pose health risks.

“Just be assured that this is a natural phenomenon that does happen occasionally and 99 per cent of the time, it is not anything to be concerned about,” Koprivnikar says.

“Messy, unpleasant, yes, but not usually anything to be concerned about.”