UNO Biologists Release Study on Effects of Hypoxia on Salt Marsh Fish

Most animals require oxygen for part or all of their life cycle, yet many fish and other aquatic animals may encounter low oxygen, or hypoxia, in their natural environment for periods ranging from a few hours to several weeks.

How do they survive and what are the biological consequences of prolonged exposure to low oxygen?  These are questions addressed by researchers at the University of New Orleans and their colleagues at the University of Delaware’s School of Marine Science and Policy in a study released this month in the Journal of Fish Biology, a leading international peer-reviewed journal.

In aquatic habitats, oxygen can drop to low levels, even approaching zero, resulting in so-called “dead zones”, for example in the Gulf of Mexico, the Dead Sea, and other locations around the world. Marshes also undergo cycles of dissolved oxygen, said lead researcher Bernard Rees, a professor of biological sciences at UNO.

Although some variation in oxygen is natural, human activities have increased the size of areas affected, how low the oxygen goes, and the length of time that the water is hypoxic.

In fact, some scientists consider hypoxia to be the single most pressing environmental problem in aquatic habitats today.

Animals that occur in these areas respond in several ways, said Rees. Some can tolerate the low levels of oxygen while others cannot. The all-too-familiar fish kills or sudden die-offs of fish or other aquatic life seen in dead zones are most commonly caused by reduced oxygen levels.

Among animals that tolerate hypoxia, decreased oxygen can negatively impact growth and reproduction, effects which may translate to changes in populations of aquatic species.

In their research, Rees and his collaborators studied a common salt marsh resident fish known as the mummichog to assess the effects of prolonged exposure to low oxygen on growth and body composition. The mummichog is a common marsh fish along the Atlantic Coast and a closely related species, the cocahoe minnow, is found in Louisiana’s coastal marshes.

These fish are important in the food-web of marsh habitats – they are prey for many commercially important or recreational sport fish species. In addition, much is known about the ecology and population biology of the mummichog, making it a good model for studies of environmental biology, Rees said.

Researchers held fish at low oxygen for up to one month then measured how fast they grew and evaluated the biochemical makeup of skeletal muscle. They found that fish exposed to low oxygen for up to two weeks lost weight and during this time period resembled starving fish. Between two and four weeks, growth rate rebounded and was similar to that of fish exposed to normal oxygen levels.

The mummichog’s capacity to recover after being held under low oxygen for longer periods suggests these fish are more tolerant of hypoxia than some other common marsh fishes, researchers said.

“This well-developed hypoxia tolerance might help explain why this is such an abundant fish in these habitats,” Rees said.

Nevertheless, short-term hypoxia had measurable negative effects on growth, which indicates that even a tolerant species is adversely affected by low oxygen levels, he said.

In addition to revealing the biology of hypoxia tolerance, the scientists’ research may help resource or conservation managers set levels for dissolved oxygen in making regulatory or management decisions.

“Knowing the level of oxygen deprivation that would impact a tolerant species could indicate a worst case scenario: if oxygen is low enough to negatively impact the mummichog, then it is likely too low for many other less tolerance species to thrive”, said Rees.

Rees’s laboratory is also working to understand the molecular signals that allow the mummichog to tolerate hypoxia and other environmental stresses.

“By studying model organisms like these fish, we’re trying to learn basic mechanisms that allow all animals, including humans, to respond to these stressors,” said Rees.

He pointed out that low oxygen is a feature of many human pathologies, such as stroke, heart attack and cancer. Chronic obstructive pulmonary disease is also an important human health issue with hypoxia as a major symptom.

“What is learned from model organisms often has direct implications for biomedicine and may ultimately lead to novel therapies for human diseases,” Rees said.

In the meantime, Rees, his students and colleagues continue to probe the secrets of hypoxia tolerance in the mummichog and other fish species.