Aquatic forage species—the small fish, worms and other out-of-sight species that feed all the others—are the lifeblood of the Chesapeake Bay. So, to better assess the health of nation's largest estuary, scientists are getting better at measuring the pulse of these smaller species and explaining why they matter.
The call for increased study of forage species was made in the 2014 Chesapeake Bay Agreement. The Bay's buffet of these species was historically much larger and more diverse, and scientists are trying to understand how resilient this foundation of the food chain might be.
One of the biggest takeaways from their research is to think of forage species broadly and not just as small fish eaten by larger fish. Scientists understand that the food consumed by larger predators in the Bay is a group that goes well beyond small fish to include benthic invertebrates, worms, bivalves and crustaceans.
Each of these species helps transfer energy, starting with the smallest plankton and detritus, up the food chain to larger and larger consumers. In the Bay, forage abundance indicates the health of shallow-water areas, which are the engine that keeps the rest of the ecosystem running.
"We've developed a lot of exciting science in a short time," said Bruce Vogt, ecosystem science manager for the National Oceanic and Atmospheric Administration's Chesapeake Bay Office. "What we're talking about now is tying that to questions [fisheries] managers have."
The work is steered by the Chesapeake Bay Multispecies Monitoring and Assessment Program. Their research, which started in 2002, not only measures the abundance of key species but also identifies the contents of their stomachs. Predators whose stomach contents have been detailed include striped bass, summer flounder, Atlantic croaker and white perch. That dissection provides helpful annual data about food the fish are eating.
Researchers were surprised to learn that Bay anchovies appeared to be a more important forage species than Atlantic menhaden. The anchovies, which measure 2–3 inches in length and live up to three years, are the most abundant fish in the Bay.
But anchovies haven't been as closely monitored as menhaden populations, which are regularly measured and managed as a commercial fishery. That's why we often hear more about them, Vogt said. Along with these species, juvenile fish, before they eventually get larger, also provide a rich source of food to larger predators. These include juvenile spot, weakfish, hake and river herring such as American shad.
Chris Moore, senior regional ecosystem scientist with the Chesapeake Bay Foundation, points out that even prime commercial species, such as blue crabs, can become forage, especially during their juvenile stages in Bay marshes. When pollution or habitat loss harms those species, he said, they impact everything up the food chain, too.
While the abundance of juveniles in many species waxes and wanes throughout the year, there are a few lesser-known foodstuffs that provide regular forage to predators in the Bay. Athough they may not travel in large schools, ready to be gobbled en masse by striped bass and others, they are still an important part of local diets.
Among those creatures are bristle worms, or polychaetes, a class of benthic organisms that can be as small as a few millimeters and are named for the hair-like bristles running down their sides. Mysids, another tiny benthic organism, are also a significant part of larger predators' diets, studies have shown. While not technically shrimp, these mysids are commonly called opossum shrimp because they look like their fellow crustaceans and carry their young in brood pouches like marsupials.
Given the variety in size and abundance of species that fill the bellies of Bay predators, researchers have broadened their work to understand the factors that might be impacting all of them. That has led scientists to focus on changes in overall forage abundance. Doing so also allows them to zoom out from the seasonal fluctuations of one species to look for trends impacting the whole.
This work also points to a shift in the scientific process: away from focusing on improving outcomes for one species at a time and toward a broader, ecosystem-based approach. Such an approach considers links between organisms—whether certain fish or humans are consuming more of a species than they used to, for example—and can more quickly identify trends that impact land and fisheries management decisions.
Joseph Gordon, project director of Conserving Marine Life at the Pew Charitable Trusts, said the relationship of Bay forage species to so many others—from ospreys in the summer to humpback whales off the coast—means it often makes sense to study them as a group.
"Although there are cycles for every species of growth and decline, there are also systemic problems [impacting] a healthy forage base," Gordon said. "It's increasingly important to maintain abundance and respond quickly to declines to maintain a strong foundation" for the entire system.
So far, scientists have identified a handful of factors that influence forage populations in general in the Chesapeake Bay. One study, for example, found that forage species are more abundant when the water warms more gradually from spring into summer.
"There's a relationship between the rate at which the water gets warm and the standing amount of forage during the summer," said Ryan Woodland, associate professor at the Chesapeake Biological Laboratory, part of the University of Maryland Center for Environmental Science. "[Summer] is a really productive time in the Chesapeake Bay, with lots of fish growing and adults feeding."
Scientists also found that the rate at which the waters warm has accelerated over the last half-century. Compared with temperature data that stretches back to the 1950s, Bay waters are reaching their summer warming point about two weeks sooner than they used to.
Woodland said that these shifts could affect the timing of natural processes for both forage species and their predators. For example, if larval fish appear when the water reaches a certain temperature but the blooming of their food—phytoplankton—is based on the availability of light, then the earlier arrival of warm temperatures could result in larval fish appearing weeks before their food supply.
"What's really important from a climate change perspective is if the timing of those events [stops] overlapping. Then you can really have a mismatch when they're starting to feed," Woodland said.
The team also looked at a growing body of research on living shorelines. In the process, they identified a threshold at which the amount of hardened shoreline strongly correlates with a reduction in forage species. Generally, once 10–30% of a shoreline is hardened with concrete or stone "rip-rap," there is a related decrease in forage populations in nearby waters. For Bay anchovies, the threshold appears to be at the lowest end of that range.
"Knowing that threshold … can inform restoration and conservation priorities in the Bay," said Donna Marie Bilkovic, assistant director of the Center for Coastal Resources Management at Virginia Institute of Marine Science. Both Maryland and Virginia require landowners to implement living shoreline practices where they are suitable.
Bilkovic said that when a shoreline is hardened enough to impact forage species, the data show a decline in other resources, too—from benthic invertebrates and fish to marsh birds. Replacing marshy habitats with concrete ones, overall, "diminishes the production capacity of these shallow-water areas of the Bay," said scientist Ed Houde, professor emeritus at the Chesapeake Biological Laboratory.
Other recent research has focused on understanding exactly what conditions are needed for important forage species to thrive. To that end, a research team from VIMS set out to quantify and map suitable habitats for four common species of forage fish: Bay anchovy, juvenile spot, juvenile spotted hake and juvenile weakfish.
Using data from VIMS trawl fishery surveys between 2000 and 2016, the team created computer models to simulate environmental conditions at the sampling sites. Water depth, temperature, salinity, dissolved oxygen and current speeds were analyzed to see if they appear to have an outsized impact on the small species.
"It's kind of like going to see a physician. They don't just look at your heart rate. They look at your blood pressure, weight, height—they take a lot of indicators into account to say, 'OK, you're in good health,'" said Mary Fabrizio, chair of the department of fisheries science at VIMS, who worked on the study.
Overall, the factors that make a suitable habitat varied by species, but some common denominators became apparent. The effort helped to identify shoreline and tributary habitats that consistently provide good conditions for forage. These would be candidates for protection or restoration, Fabrizio said.
In the cases of Bay anchovy in winter and juvenile spot in summer, the study found that good habitat may depend largely on size—a minimum area required to produce a desired abundance of the fish. This sort of data form a baseline for helping fisheries managers evaluate conditions in a given year and understand what's impacting predator populations.
Mandy Bromilow, a fishery science expert at NOAA's Chesapeake Bay Office, is also looking for changes in the total amount of forage by reviewing long-term monitoring surveys.
The next steps for the forage research community will be communicating the latest science to decision-makers. Then, the effort will pivot to figuring out, more specifically, what some of the Bay's most ecologically and economically important fish species are eating.
"It's very likely that, if one key forage [species] isn't available, they are going to switch to another," Bromilow said. "The question is, at what point is [that] not enough to sustain them?"
Whitney Pipkin is a Bay Journal staff writer based in Virginia. You can reach her at email@example.com. This article was first published May 9, 2022, on BayJournal.com and was distributed by the Bay Journal News Service.