A 10-year study of Chesapeake Bay fishes by researchers at the Virginia Institute of Marine Science provides the first quantitative evidence on a bay-wide scale that low-oxygen “dead zones” are impacting the distribution and abundance of “demersal” fishes — those that live and feed near the Bay bottom.
The affected species — which include Atlantic croaker, white perch, spot, striped bass, and summer flounder — are a key part of the Chesapeake Bay ecosystem and support important commercial and recreational fisheries.
The study, published in a recent issue of Marine Ecology Progress Series, was authored by Andre Buchheister, a Ph.D. student in William & Mary’s School of Marine Science at VIMS, along with VIMS colleagues Chris Bonzek, Jim Gartland, and Dr. Rob Latour.
All four authors are involved in VIMS’ Chesapeake Bay Multi-Species Monitoring and Assessment Program (ChesMMAP), an ongoing effort to track and understand interactions between and among fishes and other marine life within the Bay ecosystem.
Buchheister says “This is the first study to document that chronically low levels of dissolved oxygen in Chesapeake Bay can reduce the number and catch rates of demersal fish species on a large scale.” He notes that other studies have looked at the effects of low oxygen on fishes within the water column and on demersal fishes within individual Bay tributaries.
Low-oxygen conditions — what scientists call “hypoxia” — form when excessive loads of nitrogen from fertilizers, sewage, and other sources feed algal blooms in coastal waters. When these algae die and sink, they provide a rich food source for bacteria, which in the act of decomposition take up dissolved oxygen from nearby waters.
In Chesapeake Bay, low-oxygen conditions are most pronounced in mid-summer, and in the deep waters of the Bay’s middle reaches. “This appears to displace fish biomass toward the northern and southern edges of the bay’s mainstem channel,” says Buchheister.
“The drastic decline we saw in species richness, species diversity, and catch rate under low-oxygen conditions is consistent with work from other systems,” he adds. “It suggests that demersal fishes begin to avoid an area when levels of dissolved oxygen drop below about 4 milligrams per liter, as they start to suffer physiological stress.”
The fishes’ response at this value is interesting, says Buchheister, “because it occurs at levels greater than the 2 milligrams per liter that scientists formally use to define hypoxia.” Normal coastal waters contain from 7-8 milligrams of oxygen per liter.
Previous research suggests that oxygen-poor waters can stress fish directly, through increased respiration and elevated metabolism, and also by affecting their prey.
“Low levels of dissolved oxygen stress or kill the bottom-dwelling invertebrates that demersal fishes rely on for food,” says Buchheister. “Prolonged exposure of these invertebrates to hypoxic conditions in the mid-Bay represents a substantial reduction in the habitat available for foraging by demersal fishes baywide, and could reduce the quality of foraging habitat even after bottom waters become re-oxygenated.”
The authors caution, however, that the limits on fish abundance and distribution brought on by low-oxygen conditions are to some degree balanced by the positive effects that nutrients have on production of mid-water and surface-dwelling fishes elsewhere in the Bay. The nutrient-rich waters that encourage dead-zone formation also fuel algal growth, thus turbocharging the base of a food web that ultimately supports fish and other predators. Read more….