When it comes to important but unheralded Chesapeake Bay species, it may be time to praise the polychaete.
The bottom-dwelling worms turn out to be the most important food for a wide range of important fish predators, and their abundance seems to be holding steady in recent decades.
On the other hand, there may be reason for concern about lowly mysids, small shrimplike crustaceans that are a tasty treat for many Bay fish but have been in decline for years.
One thing is certain: There is cause to worry about the shoreline hardening around the Bay and its tidal tributaries. Despite efforts to promote “living shorelines,” the spread of bulkheads and riprap is taking a bite out of critical habitats for polychaetes, mysids and many other forage species that constitute the lower levels of the Chesapeake’s food web.
Those are some of the findings from a recent Forage Status and Trends Report from the state-federal Bay Program. It’s the first attempt to analyze if the Chesapeake is producing enough food for striped bass, weakfish, summer flounder, Atlantic croaker and other predators.
“We still can’t answer that question directly,” acknowledged Bruce Vogt of the National Oceanic and Atmospheric Administration Chesapeake Bay Office, who chairs the workgroup that produced the report. “What we say is that we have a better handle on it.”
The reason for the murkiness is that the Bay’s food web, which fuels more than 500 million pounds of seafood harvests a year, is incredibly complex.
The prey near the bottom of the web includes such things as bay anchovy, polychaetes, mysids, amphipods, sand shrimp, macoma clams and other species that most people would never recognize and have likely never heard of. Yet they are critical for the Bay’s productivity and constitute the bulk of the diet of its predatory fish.
As far back as 1993, a Bay Program report recognized the role that zooplankton, silversides, hogchoker and a host of other “ecologically valuable species” play in the Chesapeake ecosystem.
“No matter what is done to control nutrients and improve habitat conditions,” it warned, “there must be an adequate base of zooplankton, forage fish and [benthic animals] to support healthy and productive populations of recreational and commercial finfish.”
But it wasn’t until the 2014 Chesapeake Bay Watershed Agreement that forage species received a goal of their own. The agreement recognized the importance of protecting “habitats and ecological relationships” to sustain fisheries and called for an assessment of the Bay’s forage supply.
Studies cited in the new report suggest that the overall abundance of forage over the last few decades has been relatively constant, but what makes up that food base changes over time.
It found that the total amount of invertebrate biomass — things like clams, insects and worms, which make up the vast majority of what is consumed — appears stable in recent decades, though there were changes within the group.
Polychaetes, which studies suggest are the most widely consumed forage, have remained steady. Meanwhile mysids, which are also very important in predator diets when available, have declined. The abundance of small forage fish, bay anchovy and menhaden has generally been low for the last two decades.
The report suggests that predators appear to adjust their diet depending on what’s abundant and available.
The reasons for some of the increases and declines are unclear, but in some cases the changes seem to be tied to specific weather patterns that can persist for decades. Water quality and a changing climate play a role, too.
“I don’t think there’s any red flag right now to say that, aside from maybe mysids, that we have a real problem when it comes to forage availability,” Vogt said. “But we can’t say more directly that there’s plenty of forage out there for the predators that we are trying to manage. We can’t make that leap yet.”
That’s a daunting challenge. Simply knowing the abundance of forage doesn’t necessarily mean it’s adequate. It also must be available at the right place and right time for predators. Different types of forage may have different nutritional values.
“There are a lot of variables here,” said Mary Fabrizio, a fisheries scientist with the Virginia Institute of Marine Science who worked on the report. “A number of factors all have to line up.”
The report suggests that more work is needed to determine if the health of predatory fish is changing over time, something that could provide a signal about whether changes in diet could affect their overall condition.
“Body condition would be something that’s fairly easy to track,” Fabrizio said. “You could look at that and any changes through time.”
Still, that wouldn’t explain why the abundance of certain types of forage are increasing or decreasing. And it wouldn’t show whether the timing and location of forage production shifts as climate changes — which could have major impacts.
“We maybe don’t spend the time that we need to learn about all these components because they are complex,” Fabrizio said. “When you start talking about long-term populations like this, and responding to environmental change, that’s hard.”
The report highlights that reducing nutrient pollution alone — the focus of much of the Bay restoration effort — is not enough to ensure that adequate habitats exist for forage species, citing studies showing the negative impact of shoreline hardening.
Many of the most sensitive forage species are gone when 10% of an area’s shoreline is hardened, and even many of the more-tolerant species disappear when that amount reaches 30%.
“Overall, hardened shorelines negatively affect key forage species at both Baywide and tributary scales,” the report states.
The lower James River, much of the tidal Potomac River, the lower Patuxent River, and most of the upper Bay from the Choptank River to the Gunpowder River are areas where shoreline hardening puts forage abundance at risk, according to the report.
That echoes the finding of a report from the Bay scientific community last year that suggested putting more focus on improving shallow water habitats to increase direct benefits to aquatic life.
“That’s one of the major findings that we’re trying to communicate,” Vogt said. “This is another indicator that hardened shorelines are not great for the ecosystem.”