The diet of striped bass in Maryland’s portion of the Chesapeake Bay has shifted significantly in recent years years, resulting in smaller — and possibly less healthy — fish, according to a new study.
After reviewing the stomach contents of more than 1,000 rockfish, scientists concluded that striped bass were finding fewer menhaden to eat than in the past. Instead, they are resorting to alternatives such as grass shrimp and blue crabs. As a result, the rockfish are growing more slowly than in the past.
State officials said the study confirms their concerns about the menhaden stock. “There are no surprises,” Harley Speir, chief of the Maryland Department of Natural Resources Biomonitoring and Analysis Program, said of the report. “I think, clearly, if you look at the data, we want more menhaden.”
The population of menhaden, a small oily fish which is eaten by many predators, has fallen dramatically in recent years as their reproduction has dropped for unknown reasons.
That has led to calls by Maryland, sports fishermen and others for the Atlantic States Marine Fisheries Commission — which manages migratory fish populations — to update its menhaden plan and set commercial harvest limits that would ensure enough fish are left to fuel the food chain. The ASMFC is expected to release a draft of a new management plan by late summer.
According to the report by the University of Maryland Eastern Shore’s Cooperative Fish and Wildlife Research Unit, menhaden have declined — both in number and in weight — in the stomachs of striped bass.
In the 1950s, menhaden and a smaller fish, the bay anchovy, dominated the diet of striped bass, each accounting for about 40 percent of the prey items found in the striped bass’ stomachs, according to the report.
The new study, which examined stomach contents during 1998-99, found that sand shrimp were the most common species inside, accounting for 59 percent of the items found. It was followed by bay anchovy at 28 percent, blue crab at 5 percent and menhaden at 3 percent.
Measured by weight, menhaden — which are larger than most other foods — were still the biggest single nutritional source for striped bass, accounting for 48 percent of the biomass consumed.
Nonetheless, the study showed a dramatic shift away from menhaden to other sources of food. It found menhaden were only a small part of the striped bass diet in the spring and summer, while a 1993 study found menhaden accounted for up to 66 percent of the diet during that same time.
The study also found that 15 percent of the striped bass examined had external lesions — a figure similar to findings by state surveys.
Perhaps more troubling was an analysis of a subset — roughly 300 — of the fish which showed half had internal bacterial infections, although most looked healthy on the outside.
“It is not clear whether the changes in the feeding habits of striped bass have caused individuals to become more susceptible to certain bacteria, but we hypothesize that there is a strong correlation between the two,” the report said.
The infections were by Mycobacteria, some forms of which can cause troublesome infections, known as fish handlers’ disease, in humans with open sores who handle the fish. The infections can be treated with antibiotics.
Steve Jordan, director of the state-federal Cooperative Oxford Lab, which is conducting its own striped bass research, agreed that the number of ailing striped bass could be a sign that the population had outgrown its food supply.
Excessive populations of one species in an ecosystem, he said, ultimately are controlled by starvation or disease, which wipe out weaker individuals. “That’s the way the world has worked since the beginning,” Jordan noted.
He said the state is conducting further studies of the impact of bacterial infections on striped bass. Though Mycobacteria and lesions have been seen in fish for the past several years, Jordan said there has been no evidence of die-offs as a result.
Jim Price, president of the nonprofit Chesapeake Bay Acid Rain Foundation, who helped to collect fish for the study, laid the blame for the poor health of striped bass on the Bay states and the Bay Program. Despite their importance, he said, the Bay Program has never attempted to write a management plan for menhaden or any other forage species.
“There’s no doubt in my mind that when the top predator in the Bay has become so stressed that 15 percent have lesions, and 50 percent have a disease, we’ve made a mistake in the way that we manage the fish,” Price said. “We do not have a management plan for forage fish. Other states do. The Bay is being mismanaged.”
The new Chesapeake 2000 Agreement does acknowledge that Bay fishery plans need to better account for interactions between species, and calls for “multispecies management plans” to be developed for targeted species by 2005.
Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, said the report was “very important” because it documents a food web change many had suspected. “This reinforces the need for the ASMFC to continue on the track of incorporating the role that forage fish play into a new menhaden fishery management plan,” he said.
But, Goldsborough said, the striped bass population had hit a “ceiling” not just because of the lack of menhaden, but also because there is not enough habitat — such as oyster reefs and grass beds — which historically were important for the production of other forage fish.