The shores of Lake Ontario have recently become the final resting place for thousands of small, silver fish, alarming residents and sparking concerns over the health of the great waters’ ecosystem online.
The images and videos circulating on social media show hundreds of the fish, known as alewife, dead along the shorelines of Humber Bay Park West, Marie Curtis Park, and parts of Scarborough’s Midland and Kennedy waterfront areas.
“Can anyone explain why there are so many dead fish along the coast of Lake Ontario? Walk my dog here quite a bit and have never seen this,” one Reddit user said.
“I was down by Humber and noticed the same,” another user added.
“I saw a bunch on the Centre Island pier last weekend. I assumed they were just dropped by birds,” another user said.
Despite the fearful sightings, the deaths are being attributed to natural seasonal changes and pose no risk to the wellbeing of the lake or its inhabitants, the Toronto and Region Conservation Authority (TRCA) said in a news release earlier this month.
“Mass die-offs have happened many times, including in Lake Ontario. These events can be upsetting to see (and smell), but they are a normal occurrence in the lake’s ecosystem.”
The tiny, silvery fish – native to the Atlantic Ocean and found in great numbers in the depths of Lake Ontario – tend to migrate from deeper, colder waters to spawn in shallower, warmer areas near the shore.
“Die offs” happen when the alewives, related to herring and similar to smelt, become vulnerable to cold-water upwellings and sudden temperature changes in the water that may shock or kill them. Additionally, prolonged and colder than usual winters can weaken their immune systems, making them more susceptible to disease during the spawning period.
“This does happen every few years, often after a particularly cold winter, and this winter was colder than we’ve had in a while and you get ice build-up, and when you don’t get a lot of water underneath that ice the oxygen depletes over time, right? And that’s stressful for fish and alewife,” Janet Koprivnikar, professor in the department of Chemistry and Biology at Toronto Metropolitan University, told Now Toronto on Tuesday.
While oxygen levels and varying environmental conditions are impacting the spawning behaviours of the fish, the sporadic event may also be due to the species being non-native to Great Lake waters.
“They’re not from here. They’re actually a marine fish so they can live in lakes, but it’s probably a little bit more stressful for them than other fish that are endemic to the Great Lakes. So, this may be why we’re seeing die-offs of alewife in particular,” Koprivnikar added.
Alewives were introduced to the Great Lakes through the Welland Canal in the late 1800s and have become a significant part of the lake’s ecosystem, according to the TRCA. Since their introduction, they’ve experienced many mass die-offs, including one along Lake Ontario’s shores in 2019, according to media reports.
The Ministry of Natural Resources is also aware of the increasing number of fish washing up ashore, citing weakened conditions after a long or cold winter, stress related to spawning, and rapid temperature changes as explanations for the large number of fish ending up onshore.
“In May and June, Alewife move to shallow water to spawn. Spawning takes a lot of energy and can be stressful. Large spring-time mortality events are common with this species in the Great Lakes,” the Ministry of Natural Resources told Now Toronto in a statement on Tuesday.
Meanwhile, the Ministry of Natural Resources and TRCA are expressing that this occurrence, while unpleasant, is part of a natural cycle that will help recycle nutrients back into the environment. Koprivnikar adds that the public shouldn’t be concerned unless there are certain issues like mass species die-offs, summertime mortalities, hemorrhaging, and other signs of illness in fish, including bleeding gills.
“Should the public be concerned? Probably not, right. It’s something that could easily happen again, and it’s just a fairly natural phenomenon.”
