Researchers peg Mobjack Bay osprey woes on food shortage

On busy city bridges and along shorelines throughout the Chesapeake Bay region, osprey appear to be fish-feasting, breeding and thriving all summer. But researchers that track dense concentrations of the iconic raptors in Virginia’s Mobjack Bay say the birds’ nests in that location are beginning to tell a different story.

In “the good days” a few years ago, each of the 83 active osprey nests monitored in Mobjack Bay might have had one hatched egg by late spring, said Michael Academia, a researcher with the Center for Conservation Biology at the College of William & Mary in Virginia.

But recently, the nests have produced a total of just 10–15 hatchlings per year. And, during a check this June, there appeared to be only three young ospreys in all of the nests combined.

“Something’s not adding up,” he said.

In a paper published in the Frontiers of Marine Science in April, Academia contends that the cause of these dips in nest numbers is a shortage of food — namely, Atlantic menhaden. The center has been tracking the health of local osprey populations since the 1970s and sees “an inextricable link” between the birds and the nutrient-rich fish that travel in schools near the water’s surface in the ocean and estuaries.

A study in the mid-1980s first identified that menhaden often make up nearly 75% of an osprey’s diet. A 2009 study showed that is still the case the closer the ospreys are to the mouth of the Chesapeake, while upper Bay birds tend to have a more varied diet.

Mobjack Bay is located directly off the lower Chesapeake, near the area where Chesapeake menhaden harvesting is most active.

Besides feeding predators like osprey, menhaden are the focus of one of the largest commercial fisheries on the Atlantic Coast. The health of that fishery is measured and managed as a coastwide population, making it difficult to determine how local or Chesapeake populations of the fish may be faring.

The Bay is also home to the world’s largest breeding population of ospreys. Many of them migrate into the area in early March to breed before heading south in mid-August for the winter.

For Academia, who grew up as the son of a commercial fisherman in Hawaii, ospreys “are the proverbial canary in the coal mine” for menhaden abundance.

“We were taught before technology to look at birds to locate fish,” he said. “If the birds are doing OK, then there’s a lot of fish.”

Because he lacked abundance data for menhaden in Mobjack Bay, Academia and Bryan Watts, director of the center, conducted an experiment in 2021 and published the results this year.

They divided breeding pairs of ospreys into two groups, supplementing the diets of one group with additional menhaden to see whether there was a relationship between more food and more hatchlings surviving. A nest was considered a success if it had one survivor (female osprey typically lay three eggs in late spring).

Among the nests that received food supplementation, 81% succeeded, compared with 33% in the control group. The supplemented nests had an average productivity rate of 1.13 young per active nest, close to the 1.15 rate that’s needed to offset mortality in the osprey population and make it self-sustaining.

The control group’s fertility rate of .47 young per active nest was lower than the fertility rate for ospreys in the 1960s, when the widespread use of the pesticide DDT pushed populations of ospreys, eagles and other birds to the brink. A 1972 ban on DDT helped Chesapeake ospreys recover from an estimated low of 1,450 breeding pairs. The breeding pairs recovered to about 3,500 by the mid-1990s. Watts has estimated that as many as 12,000 pairs today consider the Chesapeake region home.

The Atlantic States Marine Fisheries Commission is responsible for managing the Atlantic menhaden population along the East Coast, assessing populations and setting catch limits. In 2022, the governing

body found harvests to be abundant and approved a 20% increase of the previous catch quota. A longstanding cap on harvests in the Chesapeake remained unchanged, at 51,000 metric tons, but conservationists and sport anglers still worry about the impact of large-scale menhaden fishing near the mouth of the Bay on local populations.

Academia’s paper argues that the health of osprey should be considered an ecological reference point for menhaden and that the harvest quotas should account for the impact on osprey fertility rates. The paper figures into a lawsuit filed by a recreational fishing group challenging Virginia’s management of menhaden.

In a case filed May 10 in Richmond Circuit Court, the Southern Maryland Recreational Fishing Organization contends that the Virginia Marine Resources Commission, in approving an increased catch for the Omega Protein fishing fleet, failed its legal obligation to protect the menhaden population from overfishing and to take into consideration the subsequent impacts on species that depend on them for food, including Atlantic striped bass and osprey.

“We just want some responsible regulation here,” said David Reed, an attorney with the Chesapeake Legal Alliance, which filed the suit on behalf of the fishing group. “You’ve got to put some aside for the ecosystem.”

A VMRC spokesperson declined to comment on the litigation, as did Ben Landry, vice president for Omega Protein’s fishing fleet. But Landry did note that scientific assessments of the coastwide stock of Atlantic menhaden found that they are not overfished. He dismissed as “unfounded” contentions by anglers and some scientists that there is a localized depletion in menhaden in the Bay because of Omega’s large-scale harvesting there. 

Timothy B. Wheeler contributed reporting to this article.