Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Hypoxia and 'Flat Fish'

This article was written by Nathan Gilles, for the magazine Confluence, a former publication of Oregon Sea Grant. Confluence back issues

Lorenzo Cianelli

August 30, 2012

Moving swiftly over the ocean floor, a metal chain attached to a large net hangs between two metal doors. The center of the chain is submerged in the sandy bottom, kicking up sand, rock, and occasionally a bottom-dwelling flat fish as it travels along.

This process, called trawling, is a great way to catch bottom-dwelling creatures, such as the speckled sanddab or English sole.

From his office in Burt Hall, Oregon Sea Grant researcher Lorenzo Ciannelli points to the video on his computer monitor of a recent trawling expedition. Ciannelli is a biologist at OSU's College of Earth, Ocean and Atmospheric Sciences who focuses on ocean fisheries. The video on his computer represents hours of ship time and shows a species affected by hypoxia, English sole, which the researcher is eager to learn more about. As the chain kicks up the sand in the video, the startled fish emerges. Acting quickly, the fish darts for safety. The animal appears to escape unharmed, but the next one isn't so lucky. Ciannelli explains that just behind the chain is a large net that will catch the less-than-energetic fish. When the chain reaches it, the pancake-shaped English sole hits the chain, flipping over it like a hot cake being turned on a griddle.

After capturing the video, students from OSU's Research Experience for Undergraduates program will carefully sift through the frames and count exactly how long it takes each fish to flee from the chain and net. After three years, hundreds of hours at sea, and hundreds of hours of video, Ciannelli and his students have discovered something interesting: the reaction time of each animal is correlated to the amount of dissolved oxygen present in the water - the more oxygen in the water, the faster and longer the fish swim. Likewise, when the water contains very low oxygen, the fish tend to be a little sluggish. When this happens, "Swoosh!" says Ciannelli, moving his hand in a sweeping gesture, "The fish almost immediately fall back [behind the chain]." And when they fall back, the net gets them.

Ciannelli has a keen interest in the biological effects of hypoxia and has been looking into how the marine community off Oregon's coast has been responding to low oxygen.

From 2008 to 2010, Ciannelli examined the larval and juvenile stages of species from plankton to vertebrates and invertebrates, including larval and juvenile flatfish, such as the butter and English sole as well as larval and juvenile anchovies and rockfish. What the biologist found is that creatures that experienced low oxygen not only tended to move more slowly than populations that hadn't experienced low oxygen, but there also seemed to be fewer of them. Ciannelli observed that populations that encountered low oxygen also tended to have fewer larvae than populations that hadn't faced low oxygen. While he says it is too soon to tell whether the low oxygen levels caused the low larvae counts, he does says that data does suggest some species appear to fare better in lower oxygen environments than others.

For example, Ciannelli says, the English sole (that bottom-dwelling creature whose speed of escape he has worked so hard to measure) is not as abundant in Oregon's coastal waters as it once was, though the researcher says he can't say for certain whether low oxygen levels are the direct cause of the animal's decreased numbers.

Speckled Sanddab

Commonly referred to as a flat fish because of the fish's flat body shape, the English sole, like other flat fish including halibut and flounders, has evolved to live comfortably supine on the ocean's floor. When it comes to hypoxia, this evolutionary adaptation works to the animal's disadvantage. The ocean floor naturally tends to be a low-oxygen environment, says Ciannelli. The deeper the water, the less oxygen it often contains.

Turning from his computer, Ciannelli explains his research. Just examining how quickly newly settled juvenile English sole could flee his net was not enough, he says. It might give him a general sense of how much energy the creatures had, but in designing his research, Ciannelli determined something more was needed. The scientist knew measuring just the speed of the flat fish might be criticized for not being rigorous enough. After all, the flight response isn't an aerobic, or oxygen-dependent, reaction but an anaerobic reaction, based on accumulated energy storage. Ciannelli determined that, in much the same way a human can make a quick sprint without taking a breath, a fish could do the same. So he decided to put his juvenile flat fish under the microscope.

On the screen in front of him, Ciannelli cycles through pictures of animals he has taken from the sea back to his lab. There are fish, crabs, and, in one poorly framed picture obviously taken with one free hand, a small, translucent octopus sits in the palm of one of his assistants' hands. Animals not lucky enough to escape Ciannelli's trawl and net, including this octopus, ended up sorted, bagged, frozen on dry ice, and sent back to OSU, where his graduate and undergraduate assistants weighed and measured the creatures.

In the lab, Ciannelli discovered that the aquatic animals raised in lower-oxygen environments were physiologically different in a number of ways from fish raised in waters containing higher oxygen levels. By measuring the lipid content of animals caught in his net, Ciannelli noted that juveniles that grew up in more-oxygenated waters had higher lipid counts than ones that grew up in low-oxygen environments. The researcher also noted that animals that spent more time in more-oxygenated waters also had higher levels of triglycerides over sterol lipids, suggesting that animals that developed in low-oxygen environments had fewer reserves to draw on than animals that grew up in higher-oxygen environments. This, says Ciannelli, explains why the juvenile English sole he recorded seemed so languid.

Ciannelli had determined two things: there were fewer larvae than usual, and the juvenile fish were not only slower, they were also physiologically different from juveniles raised in higher-oxygen environments. For his future research, the biologist says he is in the process of planning a series of controlled laboratory experiments. Ciannelli says this will help him determine how dissolved oxygen, as well as other variables such as water temperature, is affecting fish behavior and physiology.

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