If you were to look below the warming waters of the Chesapeake Bay with a microscope this month, you would enter a bizarre realm exploding with life: a world of glass-like, single-celled algae floating past grains of sediment and detritus; fantastic, translucent creatures sporting feathery antennae and whirling legs; and whip-tailed, iridescent-eyed fish larvae on the hunt.
This is the renowned Bay nursery in action. Every spring, big striped bass and many other ocean-going fish make an epic journey back to the Bay’s shallow waters to spawn, drawn by this rich concentration of life where salt and freshwater mix. Yet while the Bay may be an ideal place to grow up, the nursery can still be perilous. And with concern about striped bass reaching levels not seen since the population collapse of the 1980s, this critical part of the fish’s life cycle is getting renewed focus.
On May 1, regulations took effect across the Atlantic Coast that are meant to reduce fishing pressure and protect large female fish. They are the latest in a string of efforts to rebuild the population following the declaration in 2019 that striped bass are overfished and subsequent warning signs since—including a spike in coastwide recreational fishing pressure in 2022 and several years of well-below average numbers of young striped bass in the Bay.
While reducing the catch and letting more fish live is the most immediate concern, researchers and fishery managers are also investigating other factors that could be at play—including invasive predators, disease, the effects of climate change, and conditions in the nursery areas where young striped bass hatch and live out their first years of life.
“We have stress on both ends of the life cycle now,” says Allison Colden, CBF’s Maryland Executive Director and a fisheries expert. “It’s not just overfishing of large striped bass, it’s also this stickier wicket of things that are affecting spawning and survival in their early life history, things like climate and environmental conditions.”
Giving Spawners a Spring Break
The importance of the Bay as a nursery for striped bass is hard to overstate. Upwards of 70 percent of the Atlantic Coast’s striped bass begin life here.
Last year, annual surveys in Maryland and Virginia that track young striped bass in the Bay found recruitment failure of “young of year” fish—those spawned in the spring that are in their first year of life. It was the fifth consecutive year showing poor reproduction in Maryland waters. The last truly good spawn was in 2015, nearly a decade ago.
While the success of striped bass reproduction is notoriously boom-and-bust due to variable environmental conditions, a sustained downturn is a warning sign for managers—especially when combined with all of the other pressures facing the population.
There is still not a clear, single reason for the recent lows in juvenile fish. Rather, there are likely a number of contributing factors acting simultaneously.
One is the number of mature, female striped bass capable of spawning each year, which since 2005 has remained below the target levels that fishery managers consider necessary for sustaining the population. The larger and older these fish grow, they can produce disproportionately more eggs than their smaller counterparts, according to a recent study by researchers at the Maryland Department of Natural Resources. That can mean a difference of millions of eggs between a small and large fish, underscoring the importance of regulations that protect the biggest, most productive spawners, especially in the spring.
Freshwater Havens
Once the eggs are released, conditions in the water become paramount to their survival.
“The most vulnerable part of this fish’s life is when they spawn,” says Dave Secor, a fisheries scientist at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory who has studied striped bass for decades. “The egg and larval mortality is really high.”
To spawn, striped bass push their way into the freshwater areas of the Bay’s tidal rivers. When everything goes right, these areas provide ideal temperatures for eggs to hatch, plentiful food for the larvae to begin life, and the steep change in salinity between fresh and brackish water keeps them from being swept into the Bay.
But everything rarely goes right. Sudden storms, strong winds, drops or spikes in temperature or pH—all can have lethal consequences for eggs and larval fish. As a result, the success of striped bass spawning is highly variable from year to year. A changing climate and changes to the landscape surrounding nursery areas—if they lead to changes in the timing or intensity of precipitation and temperature shifts—could make it even more so.
On the other hand, certain conditions make a good spawn more likely. Higher rainfall in early spring, before striped bass arrive, can help expand the freshwater nursery areas and buffer them from these shocks, according to a paper Secor and his colleagues authored in 2017.
“Some years are going to be great, but in most years it’s not going to happen,” says Secor.
That’s why it’s so important to alleviate pressures like fishing, to give fish many chances to reproduce.
“If you allow striped bass many times at bat, unless the environment changes radically where we don’t have the right conditions anymore, it just takes one good year for the population to keep grinding along and growing,” he says.
Winter Sets Stage for Spring Survival
Another factor that appears important is the temperature in winter and early spring. Tiny crustaceans called copepods that hatch during this time frame form an important food source for striped bass larvae in later spring.
A study published in 2020 found that winters can affect both the size and timing of the peak spring blooms of copepods. Cooler, wetter winters support a large peak of copepods later in the season, creating a glut of food at the time striped bass larvae begin feeding, says Nicole Millette, Assistant Professor at the Virginia Institute of Marine Science and the study’s lead author.
The colder temperatures essentially slow down the copepods’ development, creating a massive buildup that all mature at the same time when temperatures warm in late spring.
“You have copepods that hatched over an 80-day period that could all be ready to reach similar later stages of development within a 30-day period,” says Millette, “If you’re a fish, all of a sudden, that’s a lot of food available to you.”
Warmer, drier winters, however, can make the peak smaller and more diffuse over time. That, again, raises concerns about the effects of climate change.
“There are of course other factors that influence how many [striped bass] larvae are surviving from year to year, but the study suggests cold winters can help increase the likelihood that there is going to be good recruitment. It starts things off on the right foot,” says Millette. “If you have warm winters, it sets things off on the wrong foot.”
Upwards of 70 percent of the Atlantic Coast's striped bass begin life here [in the Chesapeake Bay].
Fish Food
Young striped bass will stay within the Bay for the first few years of their life. But as the larvae grow into juvenile fish over their first summer and fall, they gradually shift from the freshwater spawning areas to the saltier downstream reaches of the tidal rivers. As they transition through these habitats, their diets also change significantly.
That was the finding of a recent study by researchers at the Smithsonian Environmental Research Center. Using a method called metabarcoding that looks for distinct DNA to identify species, they were able to pinpoint an incredibly diverse array of food items collected from the stomachs of young striped bass living in a variety of river habitats throughout the Bay.
The findings confirmed some expected patterns seen in previous studies from single river systems like the Potomac, says Matthew Ogburn, one of the study’s co-authors.
“They eat a lot of crustaceans, worms, and clams and then transition to more fish as they get larger,” he says.
They also uncovered some surprises—such as larval horseshoe crabs and barnacles, which in their adult stages are too hard for striped bass to eat—and found that insects played a surprisingly large role in the diets of very young fish, especially in freshwater areas.
“A lot of these insects are larvae in the water, but then live on land once they grow up,” says Katrina Lohan, the study’s lead author. “So what they are eating has this land-water connection.”
The Importance of Habitat
The connection between habitat and diet was also strong in other areas. For example, the diets of fish from the Rhode River on Maryland’s western shore included grass shrimp, reflecting the river’s marshy edges and sediment bottom. Similar-aged fish from Harris Creek, the site of a major oyster restoration project on the Eastern Shore, ate reef inhabitants like blennies, gobies, and mud crabs, says Ogburn.
“We can see that in their diets,” he says. “The conservation of natural shorelines, oyster reef restoration in the Choptank River, it’s interesting to look at how those things are influencing the type of prey available.”
In addition to immediate efforts to protect the population by reducing fishing pressure, focusing restoration efforts in areas that feed into striped bass spawning habitat and nursery areas could be one avenue to help recover the species over the medium-term, says CBF’s Colden. It could also help alleviate the pressure from large-scale challenges that reach beyond the watershed, like climate change.
“That’s where it’s important to mitigate the other stressors,” she says. “Making sure striped bass have appropriate forage, making the low-oxygen dead zone as small as possible—we need to do what we can locally to mitigate the impacts.”
Check out other articles from the Spring 2024 edition of Save the Bay magazine.
