The Great Lakes population of double-crested cormorants was devastated during the 1960s, primarily by the effects of chemical contamination from heavy metals such as mercury, organochloride pesticides like DDT, and other chlorinated compounds like PCB’s and dioxins. Cormorants are fish-eating birds at the top of the food chain and can live up to 20 years old. These two factors caused adults to accumulate large amounts of pesticides and other toxins from their prey. The chemicals did not kill the birds outright, but caused reproductive failure or extreme deformity in young that hatched. Cormorants were listed on the Wisconsin list of threatened and endangered species and then also listed for federal protection in 1972.
Today, the Great Lakes population of double-crested cormorants is at historic highs. The abundance of this species in the Great Lakes region has become a concern due to the destruction of vegetation at some nesting sites, and perceived competition with humans for fish resources. Biologists do not expect the population to grow continuously. Instead, the number of birds should decline eventually, and then stabilize due to limiting factors of available nesting habitat.