Fish in the Chesapeake Bay and other estuaries face a greater risk from climate change than previously suspected, a new study suggests, as they’re more likely to become disoriented and die in water that is both starved of oxygen and has become more acidic.
Scientists with the Smithsonian Environmental Research Center in Edgewater, MD, found that the acidification of water, which is caused by higher carbon dioxide levels in the air, can make fish more sensitive to the lack of dissolved oxygen, which can kill them directly or force them to the surface where they are more easily caught by predators.
The findings, published in Marine Ecology Progress Series, pose a particular challenge for estuaries like the Chesapeake, where fish are already stressed by nutrient pollution which drives oxygen levels down in the water. Every spring and summer, the Bay experiences a large “dead’’ zone where fish and crabs have a hard time breathing.
Globally, low oxygen and acidity are both expected to worsen as ocean temperatures rise. But until now, most research has looked at how fish deal with one or the other. This study suggests a kind of double whammy: As water becomes more acidic, fish may die at dissolved oxygen thresholds now considered safe.
In the current issue of Marine Ecology Progress, Smithsonian researchers describe how they tested the individual and combined effects of acidification and low oxygen on two species of ecologically important forage fish, known as silversides.
“They’re a really important member of the Bay’s food web,” explained Seth Miller, lead author of the paper and a post-doctoral research fellow. “They’re superabundant in estuaries.”
Low oxygen, known as hypoxia, and acidification are each known to impact aquatic organisms in varying ways, including death and altered behavior. One thing fish tend to do as oxygen concentrations in water decline is swim to the surface, where they can sustain themselves on oxygen mixing into the water from the air.
In tests run during two separate years, fish kept in tanks with water that was both oxygen-poor and low in pH — a measure of high acidity — swam to the surface to catch their breath and died at oxygen levels not normally low enough to kill fish by themselves. In tests run last year, researchers also saw fish exposed to both water conditions lose their equilibrium, or become disoriented, at oxygen levels that usually don’t cause such problems.
Miller said researchers hadn’t expected to see such distinctive behavioral changes, and suggested that indirect mortality from the two stressors may be as great as or greater than the direct die-off. Many fish not killed outright by lack of oxygen, he explained, would nevertheless be picked off by larger fish, crabs or even birds as they expose themselves on the water’s surface or flounder around, disoriented.
“It starts getting a little woozy,” Miller said “That fish can’t swim very well; when a fish has lost its equilibrium, it’s as good as dead because it can’t escape predators.”
It wasn’t clear to the Smithsonian team why fish reacted the way they did to the two conditions. Further research is needed, they said.
And while the experiment only looked at silversides, it’s possible the combined effects of acidification and low oxygen could take a similar toll on other fish and crabs. Even if the impacts were limited to silversides, that could have widespread, indirect effect on the rest of the Bay’s creatures.
“All those fish people like to catch eat silversides,” Miller pointed out, “so if there are fewer silversides, there are fewer bigger fish to catch.”