Nutrients, such as phosphorus, feed plants and invertebrates, which, in turn, provide food and habitat for yellow perch. From the late 1940s to the 1970s, the use of phosphate-based detergents, poorly managed domestic sewer systems, among other factors, led to a dramatic increase in nutrient levels in the Great Lakes and St. Lawrence River. While nutrients are important for the river ecosystem, too many nutrients can lead to negative impacts, including algal blooms. In order to restore and protect water quality, the governments of Canada and the United States signed the Great Lakes Water Quality Agreement in 1972, which led to actions to reduce phosphorus pollution. When looking at the historical data, we see that yellow perch populations rise along with the increase in nutrient levels, and then decline as nutrient levels decline. This suggests that the increase in nutrient levels in mid-20^th century helped the river to support unusually large yellow perch populations.

Prior to the 1970s, double-crested cormorant populations in the Great Lakes and St. Lawrence River had declined significantly, in part due to the impacts of pesticides such as DDT. In the 1970s new pesticide regulations allowed cormorant populations to begin to recover, and they have now reached unprecedented numbers. Up until the early 2000s, yellow perch were an important food source for cormorants in the Upper St. Lawrence River, making up roughly 46% of their diet. Over the course of the 2000s, however, a steady increase of round gobies (an invasive species) has led to a shift in cormorant diets. By the late 2000s, cormorant diets were made up of 49% round goby and only 20% yellow perch. So, what we learn from this data is that the growing number of cormorants in the region may be putting pressures on yellow perch (contributing to declines in yellow perch populations). However, cormorants are also starting to rely more on round gobies, and this alternative food source may lessen their impact on yellow perch populations.

If it is not sustainably managed, commercial and recreational fishing can contribute to long-term declines in fish populations. This appears to be the case for yellow perch in some areas of the Upper St. Lawrence River. The quantity of yellow perch caught by recreational fishers in the Ontario portion of Lake St. Francis peaked in 2009, when over 110,000 lbs of yellow perch were harvested. After 2009 the quantity of yellow perch caught per hour began to drop. When we look at commercial fishing, we find a similar spike in 2010. In 2010, government regulations changed to allow three times more perch to be harvested by commercial fishers in the Ontario side of Lake St. Francis, raising the total to 18,832 lbs. The patterns documented by the monitoring and harvest data suggest that the combined pressure of recreational and commercial fishing activities on the yellow perch population in that area were not sustainable given the current conditions.