A perfect storm in the water
Lake Simcoe was once a a major recreational fishery, with Lake Whitefish (Coregonus clupeaformis) as one of its most prized catches. In the 1960s, the fishery was thriving, but by the early 1970s, catches had plummeted. This collapse was not caused by one factor, but a cascade of interacting environmental stressors that pushed the ecosystem beyond a critical threshold. The lake became a real-world laboratory for understanding how multiple, seemingly separate issues can combine to trigger a sudden and dramatic ecological shift.
The problems began with successful efforts to reduce phosphorus pollution from agriculture and urban runoff. While this improved many aspects of the lake, it led to the next phase of the collapse. In 1995, invasive zebra mussels (Dreissena polymorpha) established themselves, followed by quagga mussels (Dreissena bugensis) in 2004. These mussels are incredibly efficient filter feeders; together, they can filter a volume of water equivalent to the entire lake every five days. This filtering dramatically increased water clarity, allowing sunlight to penetrate deeper. This altered the thermal structure of the lake and fundamentally re-engineered the base of the food web.
Caught in the squeeze
The changes driven by invasive mussels were compounded by other invaders and the effects of a warming climate. The spiny water flea (Bythotrephes longimanus), a predatory zooplankton native to Eurasia, arrived around 1993. Its long, barbed tail spine protects it from being eaten by small fish, allowing it to decimate populations of native zooplankton that young whitefish depend on for food. The round goby (Neogobius melanostomus), another bottom-dwelling invader, competes for food and preys on the eggs of native fish. The collapse was also accelerated by the near-disappearance of the deepwater sculpin (Myoxocephalus thompsonii), an important, energy-rich food source for whitefish and other predators.
Climate change added the final pressure. Warmer surface waters force the cold-loving whitefish into deeper, cooler parts of the lake. Simultaneously, the vast amounts of waste produced by the invasive mussels sink to the bottom and decompose, a process that consumes oxygen. This creates a large, hypoxic (low-oxygen) zone in the deep, cold water the fish need to survive. The whitefish became trapped in a "thermal squeeze"—caught between surface waters that are too warm and deep waters with too little oxygen. This combination of food web collapse, habitat change, and climate stress triggered the population's rapid decline, a situation from which it has not recovered. Today, the whitefish population is sustained largely by annual stocking programs.