If you were along the shores of Riverside this past weekend, you may have seen something shocking.

Thousands of dead fish — American gizzard shad — have once again washed up onto the shores and around Lakeview Park Marina as part of a natural phenomena that is no cause for concern according to the Essex Region Conservation Authority.

"There's been no reports of spills or contamination, and it is this time of year we run into that," said Tim Byrne, ERCA's director of watershed management services. "It's the exact same phenomena."

According to ERCA, the fish die due to thermal shock. (Submitted by Marguerite Hewer)

Still, the sight was something to see for resident Marguerite Hewer, who was at Lilly Kazzilly's Sunday afternoon for lunch.

"We noticed what initially I thought was new gravel stone," said Hewer. "Then I realized that it was thousands and thousands of fish."

American gizzard shad typically do not swim further north than New York, but in recent years, they have been swimming as far as Canada and quickly dying, according to ERCA.

Thousands of dead fish washed up at the Lakeview Park Marina on Sunday, March 8, 2020. (Submitted by Marguerite Hewer)

The problem is shad are highly prone to thermal shock, which is a sudden change in water temperature that immediately kills schools of fish.

Byrne explained the fish are not native to this region and cannot handle drastic temperature changes.

Marguerite Hewer took photos of the dead fish. At first glance, she believed they were fresh stone. (Submitted by Marguerite Hewer)

The amount of floating fish is also reflective of the species that commonly swim in large schools.

Thousands of shad died along the Belle River back in 2014. At the time, officials said the reasons for the massive death also included sewage that bypassed treatment and record rainfall.

"My older brother was with me we grew up in the Riverside area," said Hewer. "We hadn't seen anything like that before."