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SYRACUSE, New York -- The discovery that fish can pick up — and physically record — evidence of "dead zones" in oceans and coastal waters could help scientists understand how fish deal with the growing problem of low-oxygen areas that threaten fisheries and tourism worldwide.
In a paper published this week in the Proceedings of the National Academy of Sciences, researchers report that fish are shown to pick up a chemical "signature" from dead zones.
The paper's lead author, Dr. Karin Limburg of the SUNY College of Environmental Science and Forestry (ESF) in Syracuse, N.Y., said the discovery could be a significant tool for researchers.
"It has the potential to revolutionize the way we understand fish interactions with this growing environmental problem," she said.
Leading an international team drawn from the diverse fields of fisheries science, archaeology, biogeochemistry, and synchrotron physics, Limburg studied chemical patterns in the earstones, or otoliths, of cod from the Baltic Sea.
She and her team discovered that manganese, which becomes dissolved in bottom sediments under hypoxic (low oxygen) conditions, enters the otoliths of cod in proportion to the intensity of their exposure to hypoxic waters. In addition, two other trace elements, strontium and barium, are also taken up in proportion to environmental conditions.
"These other elements tell us about the salinity and temperature of the different habitats lived in by the fish over their lives," Limburg said.
Dead zones have long been a source of concern around the world, including in the Baltic Sea, the Black Sea and Chesapeake Bay. In the United States, the most widely recognized dead zone is in the Gulf of Mexico, where the Mississippi River deposits fertilizer runoff from agricultural regions in the Midwest.
These areas became known as dead zones because the accumulation of fertilizers, sewage, and other sources of nutrients generated by humans deplete the oxygen and make them hostile to many life forms.
Otoliths are small, calcified structures that form part of a fish's hearing and balance system. They grow by depositing increments every day, creating a chronological record of fish age and growth. The trace elements and isotopes taken up by otoliths also reflect conditions in the fish's environment.
Otoliths, therefore, create a sort of life story for each individual fish, telling the tale of how it lived its life, how long it was exposed to hypoxia (or not), how long it resided as a juvenile in a nursery habitat, and when and where it migrated to offshore, deeper waters.
In addition to sampling modern fish, the team has a rare collection of cod otoliths from the Neolithic Stone Age, about 4,500 years ago. When put through the same analysis, fish from that period did not show evidence that they had experienced hypoxia. But of two otoliths analyzed from the more recent Iron Age, about 700 to 1,000 years ago, one showed exposure to hypoxic conditions, confirming geological evidence that hypoxia has long occurred in the Baltic Sea.
Limburg has found that other species of fish can show the same uptake by otoliths of manganese in oxygen-poor environments, such as polluted lakes or estuarine embayments.