If you own a pond or spend time on lakes or rivers, you may have witnessed fish kills during the summer. One day, the water looks normal. The next day, dead fish wash up on the shoreline and float on the water.

When our surroundings get hot, we shed layers of clothing and seek shelter in our air-conditioned buildings and cars. Fish aren’t  so lucky. They must bear the discomfort of warming waters unless the situation becomes extreme enough that they die. Fish will die from simply overheating.

Fish are also affected by low water levels and pollution, which are related to each other and to warming water conditions. When flow is reduced, water warms up faster. Lower flow also concentrates pollutants. Pollution, in turn, can change the ecosystem – how the community of underwater plants and creatures interact with each other and their surroundings – in a way that causes the water temperature to rise.

Temperature, water levels, and pollution all have an impact on the health of the aquatic food web – not only the trout, bass, bluegills, and other fish we like to catch but also the plants, algae, planktons, macroinvertebrates, and non-game fish they eat. Rarely does one of these factors appear in isolation. One usually triggers another.

Let’s take a closer look at how warming water temperatures, reduced flow, and increased levels of pollution affect fish.

In Hot Water

Warming water affects native fish two ways. First, their bodily functions become stressed (which is hard enough to handle without the other environmental changes that accompany warming water). Second, fish species that are better adapted to warmer water temperatures intrude and often outcompete native species, eventually taking over a waterway.

Fish are cold-blooded, which means their body temperature closely mimics the temperature of the water around them, so they are extremely sensitive to temperature fluctuations. Warmer water changes their hormone levels, nervous system, digestion, respiration, and osmoregulation (the transfer of water and dissolved substances to the inside of the fish and vice versa). When osmoregulation becomes imbalanced, a fish becomes dehydrated or over-hydrated and its electrolytes get out of whack.

When the water gets too warm, fish stop feeding and reproducing. They’re just trying to get through the moment.Helen Neville, Trout Unlimited senior scientist

Electrolytes are minerals, such as sodium, potassium, and calcium, that can conduct electrical impulses when mixed with water. It’s how cells in a fish “talk” to each other, particularly muscles and nerves. When electrolyte levels become imbalanced, the fish has trouble maintaining the proper level of not only hydration, but also acid levels in the blood and blood pressure, and it loses the ability to repair damaged tissue.

As water temperature changes, certain aquatic organisms flourish while others decline. Each one has a thermal sweet spot, a range in which it lives comfortably, performing all bodily functions at an optimal level. Likewise, every aquatic organism has thermal death points on the warm end and the cold end of its tolerances. As the temperature of the water approaches these death points, the organism’s ability to sustain life diminishes. It gets harder and harder to convert food into energy, maintain healthy cells, and eliminate waste, and it no longer behaves normally.

“When the water gets too warm, fish stop feeding and reproducing,” says Helen Neville, senior scientist at Trout Unlimited. “They’re just trying to get through the moment. Hunting for food, scouting for mates, and building nests takes energy. You can’t do that if you’re on the brink of survival.”

There’s more. As water warms up, a fish’s metabolic rate increases, which means the fish requires more oxygen to function, yet warm water contains less oxygen than cold water. On average, fish respiration rates double for every 10°C (18°F) rise in water temperature. For example, if the water temperature in your local lake increases from 40°F in April to 58°F in late May, the fish in the lake must take in twice the amount of oxygen. Their oxygen needs double again by July, when the lake temperature jumps to mid-70°F. That’s not so unusual for a perch, which is adapted to warm water temperatures, but a brook trout would be unhappy because it lives in colder water. In other words, there’s less oxygen in the water when that brook trout needs it most.

“A fish can only handle oxygen-stress to a certain point,” says Neville. “On the bright side, fish are good at finding sources of refuge, like cool springs and places where groundwater is seeping into a lake or stream. But that only helps in the short term… in part because other species move in.”

The situation with native cutthroat trout, bull trout, and brook trout – all of which rely on cold water – has been well-publicized in the conservation and angling communities. When downstream portions of trout habitat warm up, predacious smallmouth bass, pike, and other warm water species move in. The trout move higher into the stream system, if they can, to avoid the predators. Unfortunately, upstream migration is often blocked by impoundments (such as dams or clogged culverts) or there’s simply no place higher to go, so the trout get eaten and/or they can’t reproduce effectively because they are too hot. Eventually the trout disappear permanently, even brown trout, which tolerate slightly warmer temperatures and are guilty of taking over cutthroat habitat.

Warm water species, including bass and pike, have temperature limits too, albeit higher limits than trout. If the water continues to warm up, they’ll suffer and die too.

“Warm water species have an advantage over cold water fish in warming conditions, but they’re still cold-blooded,” says Nat Gillespie, assistant national fish program leader for the U.S. Forest Service. “There are winners and losers. As water warms up, it has lower levels of dissolved oxygen. Some fish do better, like carp and catfish. Trout and native species have narrower niches, so they do worse.”

While most of us focus on the impacts of warming water on popular gamefish like bass and trout, non-game fish are affected too. All fish move, sometimes many miles, in response to changing water temperatures as well as to find cover and food.

“All fish are doing it,” says Gillespie. “We’ve studied sculpin and white suckers. They move a lot more than we ever thought to fulfill their life cycles. When we fragment their habitat, it reduces [their ability] to deal with change in water conditions. These fish have dealt with these stresses for eons, but their ability to deal with it is lower if they can’t move or if their gene pool is too small… Some non-game fish important to the food web haven’t spawned successfully in a decade due to habitat fragmentation.”

Feeling Low

One of the ways aquatic habitats become fragmented, sometimes seasonally and sometimes for long periods of time, is reduced water levels due to drought and withdrawals for irrigation and municipal needs. Reduced flows have become so common in the southwestern United States and California that they rarely make the news nowadays. It’s a growing problem in the southeast, too.

“Many rivers, like the Colorado River and the Rio Grande, don’t reach the ocean anymore,” says Gillespie. “We have X cubic centimeters of aquatic habitat. If you shrink it, populations of fish shrink. They eat each other and don’t reproduce.”

Low water flows have always happened, but now it’s happening more frequently, which is causing both warm-water and cold-water populations to decline in parts of the country.Nat Gillespie, U.S. Forest Service

Every river has a finite amount of fish habitat in it. When humans withdraw so much water from a river that it doesn’t reach the ocean anymore, the available fish habitat is reduced. There are no more fish in the dry stretch. That aquatic habitat is lost. At first, the fish that remain become concentrated in a smaller area, but eventually those populations shrink, and certain species disappear because they get eaten, and/or the habitat that they are pushed into is not appropriate for spawning.

Trout are the poster-fish for the negative effects of low water levels. According to Neville, trout hole up in pools. If those pools become disconnected over time, sometimes the fish survive and sometimes they don’t. In addition to the water temperature increasing and becoming less oxygenated, trout, which normally tuck under a riverbank to avoid predation from birds and animals, have nowhere to hide because those pools are typically not near the former shoreline.

They also can’t get enough food when they are stuck in these isolated pools. Riffles in a river are the equivalent of a food conveyor belt for fish, which is why they fin just below them. However, riffles are the first to dry up when water levels drop. The situation becomes dire when lots of fish become concentrated in small pools because there are too many mouths to feed. Warm water species fare a little better in low flow conditions, because they can tolerate the temperature better, but they are still affected by the food shortage.

“It’s a bottleneck,” explains Gillespie. “Low water flows have always happened, but now it’s happening more frequently, which is causing both warm-water and cold-water populations to decline in parts of the country.”

Which raises the question: how long can these pools be disconnected from each other before the fish die? There’s no single answer, though Trout Unlimited’s Coho salmon restoration project in California has revealed some answers. “We found the number-one most important determinant of fish survival is number of days disconnected,” says Neville. The impact varies by habitat but can happen in as little as 15 days.

Pollution

While it may seem intuitive that shallow, stagnant water tends to be warmer, you might not realize that it’s usually more polluted, too. Lower flow rates not only concentrate water contaminants, they also increase the toxicity of contaminants such as cyanide, zinc, phenol (used in manufacturing), and xylene (a solvent). The toxicity surges as the concentration of a contaminant and exposure time to it increase. For example, xylene can be cleared from a fish’s body by its liver, if the fish absorbs a small amount over a short time span, but large amounts, especially over a sustained period, cause its central nervous system to malfunction, leading to death.

Temperature can matter, too. As another example, mortality rates due to zinc poisoning are higher at water temperatures above 77°F due to a fish’s increased metabolic and osmoregulatory rates. Put simply, they absorb more of the contaminant.

Then there’s nitrogen. Nitrogen occurs naturally and is critical to all living things, but it can throw an aquatic ecosystem off balance if present in unnaturally high amounts – especially if the water temperature is overly warm. It’s another factor that depletes oxygen levels. It causes algae blooms, and it increases the amount of toxic ammonia because of the way it combines chemically with H₂O. At low water temperatures and neutral pH levels, nitrogen combines with water to make ammonium, a nontoxic salt in urine, but as water temperature and pH increase, nitrogen combines with water to make ammonia. For every 18°F increase in temperature, the ratio of ammonia to ammonium doubles.

Acid Rain

Just when you thought acid rain was a thing of the past, think again. According to a report in the “Washington Post” (August 18, 2018), the Affordable Clean Energy (ACE) Rule weakens mercury and air-quality standards, allowing coal-fired power plants to emit 12 times the amount of carbon dioxide, sulfur dioxide and nitrogen oxides compared to pre-ACE standards. It’s the equivalent of adding 75 million more cars to our roadways. These emissions react with other compounds in the air to form acids, which then fall into waterways when it rains or snows.

The optimal pH level for most fish falls between 6.5 and 9.0. (Pure water has a neutral pH of 7.) Acid rain is deadly for all species of freshwater fish. Acid rain disrupts the electrolyte-water balance in fish, which leads to ruptured blood cells and thicker blood, which in turn strains internal organs past their limits. Fish need their watery world to have a normal pH level for normal blood chemistry and to breathe normally. In addition, the more acidic the water, the more chemicals and heavy metals it typically contains, which leads to higher bioaccumulation rates of these poisons in fish.

As heavy metals accumulate in a fish’s body, it loses its sense of smell and thus its ability to find food. It also depends on scent to find mates and avoid predators. In March 2013, “Scientific American” reported on a Canadian study that found perch and minnows introduced to polluted waters showed a 59% to 75% deterioration in their sense of smell within only 24 hours. The researchers concluded that this kind of pollution could lead to the extinction of many different fish species. The challenge is determining which metals are man-induced pollutants versus naturally occurring in a watershed.

Copper

Copper is intensively used as a pesticide and fungicide, says Nathaniel Scholz, an ecotoxicology program manager at the National Oceanic and Atmospheric Administration's (NOAA) Northwest Fisheries Science Center. The metal is found in industrial discharge and near legacy mining operations. It’s even used in brake pads and boat paint.

In fact, copper use has more than doubled in the United States since the 1990s. It’s one of the metals blamed for the decline of Coho salmon on the West Coast. Researchers at NOAA found that even low concentrations of copper are toxic to the salmon’s ability to smell. This leaves juvenile salmon unable to respond to chemical signals in the water – including smells that indicate a predator is attacking other salmon. When a salmon is attacked by a predator, like a grizzly bear, an osprey, or a heron (in the case of a fingerling), the fish’s torn skin releases a scent that warns other salmon of the impending danger. In 2016, a team of NOAA researchers, including Scholz, found that even small amounts of copper in a waterway inhibited a juvenile salmon’s ability to smell this warning, making it more likely to be next on the menu. Though this multi-year study focused on juveniles for the first time, scientists have known for several decades of copper’s negative effects on the olfactory function of adult salmon. What’s most poignant about the 2016 discovery is that it led to environmental protection legislation in British Columbia, limiting runoff into the Salish Sea, prime salmon habitat. This legislation now prevents thousands of pounds of toxic metals, like copper, from washing into the Salish Sea, and is helping with salmon recovery efforts there.

Pesticides

Pesticides such as atrazine and chlorpyrifos also impair sense of smell and can lure fish into thinking there’s food present when there really isn’t. They can also disrupt a fish’s ability to reproduce.

“Smell is a chemical cue,” says Dan Dauwalter, Trout Unlimited's fisheries science director, and “fish are extremely sensitive. For example, salmon cue in on one drop of native water in 500 gallons, which is how they find their way back to where they came from to spawn. If you add pollutants that change those chemical cues, it inhibits their ability to spawn.”

On the bright side, fish recover within 24 hours when returned to cleaner water. And if they hatch in clean water, they have a keener sense of smell – even if they end up in polluted water later. “Fish are in constant bio-flux as water conditions change,” says Dauwalter.

Changing water conditions affect you, too!

When water stays warmer than historical levels, flow rates dwindle and pollutants enter our waterways, we risk losing important native populations of fish and other aquatic life.

When a body of water doesn’t support native fish, it’s a strong clue that it has degraded. If you live downstream, your quality of life degrades, too. It’s a basic issue of water quality.

If the native fish disappear, it impacts the entire food web: the insects, the birds, and the mammals. People, too. Fish are important to us economically, culturally, recreationally, as a source of food, and we drink the water they fin. Their populations reflect the health of a watershed. If changes to that watershed occur, particularly warming, lower flows and increased levels of pollutants, we too ultimately suffer.