Mark Ridgeway says the lake trout population in Lake Opeongo is under pressure, but the fishery in the area is still productive. (Steve Fischer/CBC)

The lake trout season is over in Ontario, but that hasn’t stopped a group of researchers in Algonquin Park from heading out to catch some fish.

Every fall, thousands of lake trout come out of the depths to spawn along the rock beds near the shore of Lake Opeongo at night, and every fall, researchers from the Ministry of Natural Resources are there to track them.

Scientists are trying to get a sense of how lake trout are faring now — but the study also provides researchers with valuable information about how the lake trout population in Algonquin Park's biggest lake is changing over time.

The Lake Opeongo research project — known as a "creel survey" after a Scottish word for the wicker basket that anglers used to hold their catch — is one of the longest-running studies of population ecology in the world.

Researchers have found the fish population has stayed relatively stable over the past 73 years, but they have noticed the fish are not growing as big. (Steve Fischer/CBC)

The project started in 1936, when a new system of roads had just opened up Algonquin Park to drivers from the city. Park officials decided to initiate a study because they were concerned about how the sudden influx would affect trout and other species in the popular fishing spot.

Over the past 73 years, researchers have caught, weighed and measured thousands of fish.

Ten years ago, researchers started tagging the fish, which allows them to track individual fish and gather more data for population and survival estimates.

When the team captures a fish, they quickly drop it into a holding tank that is filled with a mild anesthetic. That keeps the fish calm while staff weigh it, measure it, take a DNA sample from its scales, and even implant a chip into it using a large needle.

Mark Ridgeway, a research manager with the Ministry of Natural Resources, said the information is entered into a massive database that lets researchers track changes in individual fish — and changes in the fish population.

”When you put all these individuals together, you can determine the growth rate of the population, and you can determine how it’s changed over the decades,” he said.

Ridgeway said the lake trout population in Lake Opeongo is under pressure, but he noted that the fishery in the area was still productive.

“It’s pretty much at its maximum yield, at least as far as we understand it, but it’s chugging along.”

Longer warmer seasons

Researchers have found is that the population has stayed relatively stable over the past almost 75 years, but they have noticed the fish are not growing as big — perhaps because the larger fish are not being released but are being kept by fisherman.

It's believed there are about 20,000 lake trout right now in Lake Opeongo alone, but Ridgeway noted that there is one potential threat on the horizon for lake trout in Algonquin Park.

”Climate change is affecting all lakes, in and out of Algonquin Park,” Ridgeway said. “In this lake, it’s generating longer warmer seasons.”

Ridgeway said the changes in the lake have been a boon for smallmouth bass, which are adapting well to the changing environment, but he said lake trout haven’t adapted as well.

”It changes their spawning time and the survival of young,” Ridgeway said.