The catch-and-release restriction on the Red Deer River effective Tuesday is to remain for the rest of this season and possibly into the next, said Dave Ealey of Alberta Environment.
"It's a proactive effort on our part to enable us to do a better job to see if there was any impact from the oil spill," he said.
On June 7, a pipeline owned by Plains Midstream Canada leaked up to 475,000 litres of oil into the river near Sundre. The leak fouled shorelines and closed a popular downstream recreational lake to fishing and swimming for weeks.
Plains Midstream says water quality in the river has returned to near normal and the impact on wildlife has been minimal. Gleniffer Lake reopened the July long weekend.
But fishing guides and landowners along the river fear the spill may have killed off the insects fish feed on and reduced the number of young fish that survived.
Ealey said scientists want to determine if there are any changes to the age distribution of fish populations.
"If we had an effect, it's potentially on the younger fish," he said. "We don't want to not be aware of any change in age structure."
Fish samples will also be assessed for contaminants and there will be long-term monitoring, said the department.
Cleaning up damage to habitat is the responsibility of the company, said Ealey.
In its most recent update, Plains Midstream said the affected area is returning to its original condition and oil contamination has been removed from the site.
"Remediation of the site is complete," it said on Friday.
Company vice-president Stephen Bart has said compensation will be available to landowners.
A class-action lawsuit has been filed over the spill.
The catch-and-release restriction applies to all sections of the Red Deer River upstream of the Dickson Dam, as well as its tributaries.
The Red Deer is a popular river for both Albertan and foreign anglers, who come to its waters for important sport fish such as mountain whitefish and brown trout.