This decrease has led to reduced numbers of krill, the main source of food for some penguins, as well as for many of the fish species that penguins feed on. But it is not just climate change and melting ice that is putting penguins at risk. Other human activities are having serious impacts on penguin habitats and their food supplies. These include over fishing, guano mining, and rampant coastal development. In addition, oil pollution from drilling and shipping industries has impacted penguin populations throughout the southern hemisphere. Canaries were used as sentinels in coal mines because they were more sensitive to toxic gases than humans. Their deaths warned miners to get out before it was too late. Similarly, penguin populations respond rapidly to changes in the marine environment, and serve as a warning that we are doing a poor job of managing our oceans, which could lead to potentially catastrophic consequences.