Signs increasingly indicate that something is not quite right for striped bass, one of the most revered fish in the Chesapeake Bay and along the Atlantic Coast. In this in-depth series, we examine how a potent mix of fishing pressure, invasive predators, changing climate and habitat conditions, and disease is impacting the population—and what can be done to help.
Imagine going for a two-mile run in the middle of summer. Add a heat wave. And then imagine someone suddenly removes much of the oxygen from the air around you—especially in the cooler, shady areas.
This is essentially what happens to striped bass and other fish when they encounter low-oxygen dead zones that form in the Chesapeake Bay during the summer months. And it is these conditions that the smaller, younger striped bass who stay in the Bay year-round must endure until the dead zones dissipate with the return of cooler weather.
We have to look at this holistically. Over the long term, reducing pollution, improving habitat, and addressing climate change are all going to be an important part of ensuring striped bass have a future in the Chesapeake Bay.
It is yet another stress for a species already facing immense pressure from fishing, invasive predators, and changing conditions in the tidal Bay rivers where it spawns. For the past five years, annual surveys of young striped bass in the Bay found numbers well below-average. This recruitment failure is especially daunting because the Chesapeake is the spawning ground for upwards of 70 percent of the striped bass population along the entire Atlantic Coast.
The struggles of young striped bass, along with other worrying signs, including a persistently low number of large female striped bass since 2005, prompted tighter fishing regulations coastwide this year. Further actions to protect the fish could be necessary after managers review an updated assessment of the population later this fall.
Yet, while management actions are the most immediate way to help striped bass, addressing chronic problems like the dead zone remains critical to the species’ long-term prospects, says Allison Colden, CBF’s Maryland Executive Director and a fisheries expert.
“We have to look at this holistically,” she says. “Over the long term, reducing pollution, improving habitat, and addressing climate change are all going to be an important part of ensuring striped bass have a future in the Chesapeake Bay.”
Dead Zones in the Chesapeake Bay
Dead zones are areas of water with depleted oxygen levels created by the decomposition of large algal blooms. Excess nitrogen and phosphorus pollution from human activities, which drives the blooms, has increased the number and size of dead zones worldwide. Reducing the dead zone in the Chesapeake Bay is a primary target of restoration efforts.
The good news is that those efforts are making a difference, and over the long term, the Bay’s dead zone has been trending smaller. Nonetheless, low-oxygen areas continue to plague large swaths of water in the Bay’s main channel and its tidal rivers, making it difficult for fish and other animals to survive.
Since June, for example, the volume of water in the Bay considered ‘hypoxic’, the technical term for low-oxygen, was mostly above 1.9 cubic miles, according to modeled data from the Virginia Institute of Marine Science at William & Mary (VIMS). That’s equivalent to roughly 3.2 million Olympic-sized swimming pools, or nearly 12 percent of the Bay’s total volume.
The dead zone’s size fluctuates from year to year based on weather conditions, especially the amount of rainfall throughout the watershed in spring, which washes pollution into rivers and streams. Overall, this year’s dead zone is expected to be only slightly larger than the long-term average, despite stream and river flows that were 23 percent higher than average during the early part of the year, according to the annual forecast released by the Chesapeake Bay Program in June. It indicates that efforts to reduce pollution are helping, but much more is needed as climate change brings wetter, warmer weather.
What Happens to Fish in a Dead Zone?
When fish encounter a dead zone, there are a number of different things that can happen, says Krista Kraskura, a postdoctoral researcher at Towson University who has studied how striped bass respond to low oxygen levels in the water.
Initially, the fish will typically try to avoid the low-oxygen area.
“The first thing is to move to a place where there is oxygen,” Kraskura says. “They will behaviorally try to go to another place and seek refuge.”
If that option isn’t available, however, changes begin to happen within the fish themselves. Fish, like humans, need oxygen for all sorts of basic functions, like finding food and swimming. When there is too little oxygen in the water, the fish’s body has to work harder to perform these tasks and keep everything else regulated.
“If they’re going to keep their metabolic rate at a constant level, their first response will be just like if you go up a mountain—they’re going to breathe harder,” says Jay Nelson, a professor at Towson University who has studied extensively how fish respond physiologically to changes in their environment, including low oxygen levels.
Over time, their cardiovascular systems may begin to make shifts to their heart rate and change how blood is circulated through their gills.
“They make a whole series of physiological adjustments,” Nelson says.
In a series of studies on juvenile striped bass, Nelson and Kraskura found that these physiological changes can impact critical behaviors, such as how effectively a fish can capture prey or avoid a predator itself. The studies also found that some individual fish are able to tolerate low oxygen levels much better than others.
“If I take 20 striped bass that I just caught in the Bay and put them in a tank and lower the oxygen down to five percent, there may be a couple of them that lose equilibrium within 20 minutes,” Nelson says. “But there are going to be other ones that don’t lose equilibrium for six or seven hours. It’s an incredible variation.”
In addition, all of the fish—regardless of their tolerance to begin with—ended up improving their ability to withstand low oxygen levels after repeated exposures, much like a person might improve their fitness by going to the gym.
The mechanisms behind the variation in individual tolerance, and a fish’s ability to improve it, are still uncertain. It’s also unclear what fish give up in order to survive in low-oxygen waters. For example, increasing the production of red blood cells may help fish take advantage of what little oxygen there is, but more of these cells also make blood harder for the heart to pump. Both are areas that need more research, say Kraskura and Nelson.
“It is about tradeoffs, very much so,” says Kraskura. “They have to kind of make the choice—can they move? Can they change their physiology or morphology if they can’t move? How costly metabolically is that, and what are the tradeoffs? Let’s say they gain hypoxia tolerance, can they maintain their immunity? Can they maintain their digestive ability? We haven’t tested all of those straight out.”
How Striped Bass Get Squeezed Between Low Oxygen and Warming Waters
There are also critical tradeoffs between avoiding dead zones and avoiding other stresses in the environment, such as higher temperatures and disease.
“It’s a multi-stressor environment,” says Kraskura. “And one stressor can interact with another stressor in various ways.”
As climate change warms the Bay’s surface waters, and also makes it more susceptible to dead zones, this squeeze will likely worsen.
For example, a study published in 2021 by researchers at the University of Maryland Center for Environmental Science found that striped bass in the Bay end up moving to surface waters in the summer months of June through September, when dead zones are most prevalent. However, these surface waters are warmer than deeper waters—often above 77 degrees Fahrenheit (25 degrees Celsius), the threshold considered to be stressful for striped bass. Moving to the surface, along with changes in their diet and an increased metabolic rate, can also make striped bass more susceptible to fishing.
The result is that the habitat available to striped bass becomes compressed; the fish are squeezed into smaller areas by trying to avoid both the low-oxygen bottom waters and the intolerably hot surface waters. As climate change warms the Bay’s surface waters, and also makes it more susceptible to dead zones, this squeeze will likely worsen. Average annual tidal water temperatures in the Bay are estimated to increase 1.8 degrees Fahrenheit (1 degree Celsius) between 1995 and 2025, and could warm between 3.6 and 10.8 degrees Fahrenheit (2 to 6 degrees Celsius) by the end of the century, according to a report released last year by the Chesapeake Bay Program’s Scientific and Technical Advisory Committee.
In addition, striped bass infected with the prevalent disease mycobacteriosis are likely even more vulnerable to the combination of low oxygen and warmer water temperatures. Mycobacteriosis is a bacterial infection that, among other symptoms, affects the fish’s spleen, an organ involved in regulating blood cells. A study published in 2014 by researchers at VIMS found that while the disease alone did not necessarily reduce the ability of striped bass to withstand low-oxygen waters, the disease in combination with higher temperatures did.
“Our data indicate that striped bass performance under hypoxia is impaired, and that elevated water temperature, hypoxia, and severe mycobacteriosis together reduce aerobic scope more than any of these stressors acting alone,” the authors concluded.
How to Help Striped Bass During the Summer
Because striped bass in the Bay are so vulnerable to these various environmental pressures during summer, one of the most immediate ways people can help the fish is simply to avoid targeting them. Even though many anglers practice catch-and-release when fishing for stripers, many of the fish—an estimated 9 percent—still die due to stress. In the summer, with all the other stressors, this number can be higher.
Giving striped bass a summer break is one way we can help ensure the future of these fish, and also of the fishery.
Some states have responded by prohibiting striped bass fishing during certain times of year. Maryland, for example, closed its season between July 16 and July 31. The season reopened on August 1, but anglers can still check the heat warnings posted by the Maryland Department of Natural Resources to avoid fishing for striped bass on days when temperature stress is particularly acute.
Targeting other species all together can be a better option, says CBF’s Colden. Blue catfish, an invasive species that prey on native species like striped bass, oysters, and blue crabs, are a particularly good choice and can be caught throughout the Bay any time of year, she says. Employing safe fish handling techniques, releasing the fish in the water, and utilizing gear that is less likely to cause injury are also important steps to help more fish survive.
“Sustainable fisheries require long-term forethought and, sometimes, short-term sacrifice,” says Colden. “Giving striped bass a summer break is one way we can help ensure the future of these fish, and also of the fishery.”
