Nutrients, like phosphorus, feed plants, which are a food source for invertebrates. Plants and invertebrates, in turn, provide food and habitat for fish like yellow perch. From the late 1940s to the 1970s, human activities led to a large amount of nutrients entering the Great Lakes and St. Lawrence River. These included poorly managed sewer systems and the use of detergents containing phosphates.   

Nutrients are important for the river ecosystem. But too many nutrients can lead to negative impacts, including algal blooms. In 1972, the governments of Canada and the United States signed the Great Lakes Water Quality Agreement to help restore and protect water quality. This agreement led to actions to reduce phosphorus pollution.  

Looking at the data for both fish and nutrients over time, yellow perch populations rise with the increase in nutrient levels. Yellow perch population then also decline as nutrient levels decline. This data helps to answer our question about why yellow perch populations appear to peak in the 1970s. The increase in nutrients helped the river support unusually large yellow perch populations. 

Cormorants are another potential reason yellow perch populations declined after the 1970s.   

In the 1950s and 1960s, the number of double-crested cormorants in the Great Lakes and St. Lawrence River plummeted. A major reason for this drop was high levels of toxic chemicals in the bird’s diets, including the pesticide DDT. This pesticide led to thinner eggshells and other health problems. In the 1970s, new regulations helped to reduce the use of chemicals like DDT. Cormorant populations started recovering, and their numbers have increased rapidly.   

Up until the early 2000s, yellow perch were an important food source for cormorants in the Upper St. Lawrence River, making up about 46% of the bird’s diets. But over the 2000s, an increase of round gobies (an invasive species) has led to a shift in cormorant diets. By the late 2000s, cormorant diets were eating about 49% round goby and only 20% yellow perch.  

Fishing can contribute to long-term declines in fish populations if it is not done in a way that is sustainable. And this appears to be the case for yellow perch in some areas of the Upper St. Lawrence River. 

The amount of yellow perch caught by recreational fishers in the Ontario section of Lake St. Francis peaked in 2009, with an estimated 110,000 lbs harvested. After 2009, the quantity of yellow perch caught per hour began to drop.  

When we look at commercial fishing in the same area, we find a similar spike in 2010. In the case of commercial fishing, this spike was due to a change in regulations. In 2010, data available at the time showed that perch populations were recovering. Based on this information, the Ontario government changed its fishing regulations to allow commercial fishers in the Ontario section of Lake St. Francis to harvest three times more yellow perch than before. This increased the total to 18,832 lbs, which is still well below the estimated catch of recreational fishers in the area.  

Differences in fishing regulations could help explain the contrasting population trends in different areas of the river. On the Quebec side of Lake St. Francis, for example, there is no commercial fishery and live bait isn’t allowed. These factors could help make fishing more sustainable.