By Lisa Ballard, reprinted with permission from Colorado Outdoors.

Last July, Colorado Parks and Wildlife enacted a voluntary fishing ban on a 120-mile stretch of the Colorado River, from Kremmling to Rifle. Voluntary fishing bans, and sometimes mandatory fishing bans, are a tool that aquatic biologists use to save fish when conditions become life-threatening, such as when the water temperature get too warm. What raised eyebrows last summer was how early it happened. Ongoing drought, augmented by record hot weather and runoff from wildfires, left fish on the brink of survival.

When you get hot, you shed clothing and seek shelter in air-conditioned buildings and cars. Fish aren’t so lucky. They bear the discomfort until the situation becomes extreme enough that they die. If you spend time around lakes or rivers in Colorado, you’ve may have witnessed one of these die offs, like two summers ago, when the water temperature in Sloan’s Lake near Denver reached 79 degrees (F), and 800 fish perished. A large, stinky fish kill happened again last summer at Wonderland Lake in North Boulder

Dead fish in Sloan’s Lake and Wonderland Lake grabbed headlines because of their urban proximity, but they were only two examples of the many places around the state that grappled with waterways that were dangerously warm for aquatic wildlife.

Cause of Death. While too much heat stresses fish, they ultimately die due to a lack of oxygen. They suffocate because, as water heats up, its oxygen level decreases. Wildfires exacerbate the situation. After a wildfire, the newly denuded ground is vulnerable to erosion. Sediment washes into the water, causing oxygen levels to plummet even more.

That happened last summer on the Colorado River. The river was at half its typical flow – 1,250 cubic feet per second (cfs) compared to 3,000 cfs – when multiple mudslides from wildfire in Glenwood Canyon dumped sediment into the river. In such conditions, fish couldn’t see to feed, and they became lethargic from higher water temperatures, over 70 degrees (F). Under those conditions, even if an angler caught and released a fish, its odds of survival were low. Already barely hanging on, it wouldn’t have the energy to recover from getting hooked and lifted from the water, even for a short time.
There’s more. Low water levels concentrate pollutants, which also has a negative impact on the health of an aquatic ecosystem, adding to the negative effects of warm water and low flow not only on the trout, bass, bluegills and other fish we like to catch, but also plants, algae, planktons, macro-invertebrates and non-game fish. Rarely is just one of these factors present in isolation, as one usually triggers another.

Warming Water. As water temperature changes, at first 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 organism has a thermal death point, both on the warm end and the cold end of its tolerances. As the temperature of the water approaches those death points, its ability to perform basic metabolic functions diminishes, 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.”

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

What’s more, as water warms up, a fish’s metabolic rate increases, which requires more oxygen. On average, fish respiration rate doubles for every 18-degree (F) rise in temperature. For example, if the water temperature in a lake increases from 35 degrees (F) in early May to 53 degrees (F) in late June, the fish in the lake take in twice the amount of oxygen in late June. Their oxygen needs jump again in July when the lake temperature hits 70 degrees (F). Unfortunately, warmer water contains less oxygen at time when fish need 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. Multiple fish species of conservation value are impacted when water conditions change, such as transitioning to warmer water, in part because other species move in.”

The situation with native cutthroat trout, which rely on cold water, has been well-documented in Colorado’s conservation and angling communities. When downstream portions of cutthroat habitat warm up, predacious smallmouth bass, pike and other warmwater species move in. The trout go higher in the system if they can. Unfortunately, their upstream migration is often blocked, or there’s simply no place higher to go. Instead, they either get eaten or can’t reproduce effectively.
All trout, not just cutthroats, are susceptible to warming water and will eventually disappear from a waterbody, even brown trout, which tolerate slightly warmer temperatures and are guilty of taking over cutthroat habitat. Warmwater bass and pike have a temperature limit, too, albeit higher than trout. If the water continues to warm up, they’ll die, too.
“Warmwater 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 USDA Forest Service. “There’s winners and losers. As water warms up, it has lower levels of dissolved oxygen. Some fish do better, like carp and catfish. Trout and most native species have narrower niches, so they do worse.”

Lower Water Levels. When water levels decrease, an aquatic habitat can become fragmented, sometimes seasonally and sometimes for long periods of drought. Many fish can’t survive even if they find a deep, cool pocket, because they can’t reproduce, or due to predation because they’ve no place to hide.

They also can’t get enough food. The riffles in a river are the equivalent to 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.

“It’s a bottleneck,” explains Gillespie, “Low water flows have always happened, but now it’s happening more frequently.”
Pollution. Lower flow rates concentrate water contaminants. As water toxicity increases, it has a serious physiological impact on piscine because of their increased metabolism and osmoregulatory rate. Put simply, they absorb more unhealthy stuff.

Then there’s nitrogen. Nitrogen occurs naturally. It 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 algal blooms, and it increases the amount of toxic ammonia because of the way it combines chemically with H2O. At colder water temperatures and neutral pH levels, nitrogen combines with water to make ammonium, but as water temperature and pH increase, nitrogen combines with water to make toxic ammonia. For every 18-degree (F) increase in temperature, the ratio of ammonia to ammonium doubles.

There’s more. Just a minor increase in pH level can cause an oligotrophic (oxygen-rich) lake to become eutrophic (oxygen-poor). A eutrophic lake might have plant life in it, and lots of algae, but not many fish.

Turbidity. The amount of suspended solids in a body of water, known as turbidity, also contributes to higher water temperatures. While not necessarily poisonous, these particles absorb heat from the sun, then conduct that heat into the water, accelerating the warming trend. Again, certain fish species tolerate high levels of turbidity and others don’t. It also affects the ability of some aquatic species to find food.

“Trout are visual feeders,” explains Dan Dauwalter, Trout Unlimited’s Fisheries Science Director. “If turbidity increases, it’s harder for trout to find food, whereas catfish use their whiskers [to find food]. The turbidity also shades out the sun, causing reduced vegetation and invertebrate production.”

In other words, the higher the turbidity, the harder it becomes for native fish to see food, and there’s less of it, which is why the huge amounts of sediment and debris that wash into waterways after wildfires concern biologists, especially when water temperatures are high and flow rates are low.

What You Can Do. 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 the that watershed occur, particularly due to warming temperatures, lower flows and increased levels of pollutants, we ultimately suffer, too.

As an angler, the best thing you can do to conserve fish when the water temperature gets too warm is to stop fishing until conditions cool off. But what’s too warm? That depends on the species and the condition of the water in terms of water level, turbidity and purity. For example, rainbow trout and brown trout begin to feel heat stress when the water temperature reaches the high 60s, and they decline quickly above that temperature.

You’ll know things are heating up too much when the low air temperature at night stays hot. When the nights don’t cool off, the water doesn’t either. That’s a clue that aquatic conditions are becoming stressful. Consider a self-impose “no fishing” ban when water gets above that mark. If you give the fish a break, they might feel the heat, but they have a better chance of bouncing back when conditions return to normal.

Lisa Ballard is a regular contributor to Colorado Outdoors, Lisa Ballard is an award-winning freelance writer and photographer. She hangs her waders in Red Lodge, Montana, but has fished throughout the Rockies for most of her adult life.