Could Our Waters Be Causing Fish Kill?

Fish need conditions to be just right to be healthy. If their aquatic environments change too drastically, large numbers of them can die unexpectedly, a phenomenon known as a fish kill. Recently, we’ve seen several of these events occur, even in usually healthy waters like those in Yellowstone National Park.

Occasionally, accidents or sudden changes cause these die-offs. If they don’t happen persistently, populations can usually recover. However, scientists worry that they may become a more regular occurrence, which could wreak havoc on our planet and our way of life.

Temperature Changes

Sudden or extreme changes in temperature can harm fish populations, as can gradually changing temperatures. Sometimes, human activity directly causes temperature changes. Power plants, for instance, often take water from rivers and lakes to use for cooling and then discharge that water back into its source. If that water isn’t cooled again to the temperature of the lake or river, it will change the body of water’s temperature and harm its aquatic life.

This may also be occurring gradually across many of the world’s waters due to climate change. This temperature change is leading to and worsening many of the other problems that hurt fish populations. For instance, it speeds up their metabolism and increases the amount of oxygen they need.

Oxygen Levels

When water temperatures rise, fish need more oxygen to survive. Rising temperatures may also be making oxygen in our waters scarcer. According to one study, every degree that the temperature lowers means that oxygen levels dip by two percent.

Large algae blooms can throw off a body of waters’ oxygen levels. During the day, algae and other plants produce oxygen. If these algae suddenly die off, oxygen levels may become too low. This can quickly kill fish.

Typically, during the summer, lakes stratify. This means that the bottom section of water has little oxygen. This normally occurs gradually and doesn’t cause a problem. However, heavy storms and other disruptions can cause this deoxygenated water to mix in with the rest of the lake or river, depriving fish of oxygen.


Climate change is also lowering the pH of the ocean’s water, which harms fish. The ocean absorbs around 30 percent of the atmosphere’s carbon dioxide. When CO2 levels in the ocean go up, its makeup changes.

These changes can make it difficult for fish to survive. The resulting drop in blood pH changes many of their body’s functions and means they need to exert more energy to normalize their system. It also hampers some fishes’ ability to detect predators and navigate the water. This could throw off the food chain and ecosystem these fish live in. It also damages coral, an important habitat for a huge portion of the ocean’s life.


As we release emissions and into the atmosphere, dump waste and use more chemicals, our oceans are becoming dirtier. Agriculture runoff from pesticides and fertilizers can find its way into waters, changing its pH and introducing toxic chemicals.

Floating trash presents a hazard to aquatic life and can even block sunlight from reaching into the water if it gathers in one place. Occasionally, a larger spill event, such an oil spill, occurs, which can desolate entire ecosystems for long periods.

When left to their own devices, oceans, lakes, rivers and streams typically provide the perfect environment for fish to live in. As humans exert more and more influence over the planet, though, those conditions are changing. Unless we turn things around, it will likely continue to become more difficult for the world’s fish populations to survive.

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Emily is a freelance conservation journalist who feels passionately about protecting endangered species and preserving the wilderness landscapes.

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