July 9, 2013
Fish Species That Thrive In Bay Bottoms Are Being Impacted By Dead Zones
April Flowers for redOrbit.com - Your Universe Online
Scientists at the Virginia Institute of Marine Science (VIMS) have completed a 10-year study, providing the first quantitative evidence on a bay-wide scale that the distribution and abundance of "demersal" fishes -- fish species that live and feed near the Bay bottom -- are being impacted by low-oxygen "dead zones."Species affected by these dead zones, such as the Atlantic croaker, white perch, spot, striped bass and summer flounder, are all key to the Chesapeake Bay ecosystem; they support important commercial and recreational fisheries.
Andre Buchheister, a PhD student in William & Mary's School of Marine Science at VIMS, 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." Prior studies have examined the effects of low oxygen on fishes within the water column and on demersal fishes within individual Bay tributaries, according to Buchheister.
Areas of hypoxia, or low oxygen, form when algal blooms in coastal waters are fed by excessive loads of nitrogen from fertilizers, sewage, and other sources. As the algae die and sink to the bottom, they become a rich food source for bacteria. The bacteria cause the algae to decompose, taking up dissolved oxygen from nearby waters.
The low oxygen conditions are most pronounced in mid-summer, and in the Bay's deep water middle reaches. "This appears to displace fish biomass toward the northern and southern edges of the bay's mainstem channel," said 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 added. "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."
At this value, the fishes' response is interesting, "because it occurs at levels greater than the 2 milligrams per liter that scientists formally use to define hypoxia," said Buchheister. The normal value for coastal waters is between 7 and 8 milligrams of oxygen per liter.
Prior studies have suggested oxygen-poor waters can directly stress fish through increased respiration and elevated metabolism. The loss of oxygen can also affect the fishes' prey.
"Low levels of dissolved oxygen stress or kill the bottom-dwelling invertebrates that demersal fishes rely on for food," said 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 limits that low-oxygen conditions place on fish abundance and distribution are partially balanced by the positive effects that nutrients have on production of mid-water and surface-dwelling fishes elsewhere in the Bay. These nutrient rich regions that encourage dead-zone formation also fuel the growth of algae. This turbo charges the base of a food web that ultimately supports fish and other predators.
ChesMMAP AND VIMS
Between 2002 and 2011, the researchers of ChesMMAP performed 48 sampling trips at 3,640 ChesMMAP stations throughout the mainstem of the Chesapeake Bay. The findings of this current study are based on an exhaustive analysis of late juvenile and adult fishes caught and released in the trawl nets of those trips.
Established in 2002, ChesMMAP is part of the growing international recognition that a single-species approach to fisheries management does not fully account for the complex interactions within marine ecosystems. Dr. Rob Latour, head of the Multispecies Research Group at VIMS, said, "The traditional approach to fisheries management looks at a single species as if it were independent, unaffected by other processes and having no effect on other species. In ChesMMAP and our other multispecies research programs we analyze the interactions between species and their environment, including studies of predator-prey dynamics, seasonal changes in distribution, and water-quality parameters such as temperature, salinity, and DO [dissolved oxygen]."
According to the study, salinity is the most important factor affecting the distribution of Bay fishes, regardless of whether they live near the surface or the bottom of the Bay.
"Salinity was the major environmental gradient structuring community composition, biodiversity, and catch rates in our 10-year dataset," said Buchheister. "The saltier waters of the lower Bay and the fresher waters of the upper Bay generally have a more diverse and dynamic fish fauna than the middle Bay, whose brackish waters place physiological demands on many aquatic organisms."
Latour noted that the value of ChesMMAP extends beyond this current study. "Our work provides a 10-year frame of reference that can be used to evaluate future large-scale changes in the composition, distribution, and abundance of the Bay's demersal fish community. Continued monitoring will be critical for detecting how the Bay ecosystem responds to continued stresses from fishing, development, and climate change. It's an essential component to a successful management strategy for the marine resources of Chesapeake Bay and the coastal Atlantic."