Keywords: Water quality, Species and food webs, Invertebrates, Marine habitat, Dungeness crabs, Hypoxia, Nutrient pollution, Eutrophication

As observed in Hood Canal, low-oxygen conditions can upend the lives of Dungeness crabs trying to stay alive. Levels of dissolved oxygen can alter predator-prey relationships for a multitude of species, affecting populations throughout the food web. Part two of our series "Oxygen for life" examines a crab case study.

In the low-oxygen waters of Hood Canal, Dungeness crabs were on the move. But where exactly were they going?

The scuttling crabs, equipped with tracking devices, were the central players in an expansive research project designed by Halley Froehlich, a University of Washington graduate student at the time. Froehlich had been studying how marine creatures change behaviors when oxygen in the water drops to uncomfortable levels, a condition known as hypoxia.

“In the overall scheme of things, we tend to ignore hypoxia,” said Froehlich, now assistant professor at the University of California, Santa Barbara. “When hypoxia does make it to the front page of the newspaper, it is usually because of fish kills.”

Southern Hood Canal is known for its naturally low oxygen conditions, which occasionally turn deadly for fish and other creatures. Froehlich was able to show how a chronic lack of oxygen limits the species that can live there, compared to northern Hood Canal where oxygen conditions are more hospitable. Each species has its own general tolerance for low oxygen, a tolerance that can vary when a local population adapts to harsh conditions.

Froehlich became interested in how marine species respond — both physiologically and behaviorally — when oxygen conditions decline but not to the point of death. Such internal and external responses by fish, crabs and a multitude of other creatures can affect growth, reproduction and predator-prey relationships, as revealed by Froehlich’s research in 2009 and 2010 along with dozens of other studies before and since. Such changes can alter the food web and shift entire populations.

“Hood Canal turned out to be the perfect kind of natural field laboratory, with a fairly consistent control site in the north and a treatment site in the south,” she said. “We were able to explore these questions on a much larger scale than working in a small laboratory setting.”

As climate change and human impacts increase the problem of hypoxia throughout the world, studies in Hood Canal have become part of a growing body of scientific evidence related to the biological effects of low oxygen on all sorts of marine life.

Climate studies have shown that areas of the world with historically low oxygen conditions are growing in size. At the same time, new low-oxygen areas are being formed. One study estimates that the total volume of deadly-low-oxygen waters has quadrupled since 1960.

Crabs on the move

Froehlich’s studies of Dungeness crabs in Hood Canal, a genetically unique population, revealed that the crabs increased their movements along the bottom when oxygen levels declined.

“Most of our focus was on mobile organisms that could move when conditions got bad,” she said. “They try to get out of Dodge, but they don’t necessarily know where the good conditions are, so they increase their activity when oxygen levels drop.”

A photo of a single crab with a transmitter glued to its back, and another photo of a woman on a boat wearing an orange vest and gloves, holding a rope; open water and land in the background.

Acoustic transmitters, like the one shown at left, were attached to Dungeness crabs to track their movements in Hood Canal. Photo: Anne Beaudreau. On the waters of Hood Canal, at right, Halley Froehlich pulls up an instrument that measures water quality at various depths during her studies of low-oxygen effects on marine life. Photo: Nolan Gross.

The experimental crabs, caught in crab pots in both northern and southern Hood Canal, were fitted with acoustic transmitters glued to their backs. The crabs were then released within 24 hours at the same place they were caught. Acoustic receivers set up in deep water recorded the changing locations of each crab as well as their depth, thanks to attached pressure sensors.

When oxygen levels dropped, the crabs increased their rate of travel. Interestingly, their general direction was south — opposite the direction of better oxygen conditions. They also tended to move toward shore into shallower water, where oxygen conditions were better.

As time went on, the crabs tended to congregate along a line just above the hypoxic zone, where they could survive but in less-than-ideal conditions.

“They seemed to hang on the edge of hypoxia and follow it,” Froehlich said. “There is some evidence they were scooping up polychaetes that may have been coming up for air.”

Polychaetes are a large class of segmented worms, some of which are known to swim in an undulating fashion to seek out food and better conditions. Some have gills; others breathe through their skin. Movements by all sorts of mobile organisms shows how hypoxia can alter the food web as they search out better oxygen conditions.

A green, segmented worm with multiple legs seen underwater resting on the sea floor next to rocks and seaweed.

Dungeness crabs may follow and feed on polychaetes, like the one shown here, as the segmented worms seek better oxygen conditions. Movements by all sorts of mobile organisms shows how hypoxia can alter food webs. Photo: Paul & Hien Brown, (CC BY-NC 2.0)

Why the crabs in Hood Canal would move generally south instead of north remains a mystery, Froehlich said. She wonders if they might be responding to other stimuli, such as food or bottom conditions.

A world of invertebrates

Froehlich, long interested in physiological responses to environmental conditions, found herself immersed in studies of sea creatures without backbones, but only after she failed to catch enough flatfish in northern Hood Canal.

“I had been told that you can’t throw a rock into Puget Sound without hitting an English sole,” she said, which is one reason she decided to study this particular species of bottom fish. She was quite hopeful when she set out on a fishing boat at 4 a.m. on her first day of graduate school. But after a long day of grueling work, she was in a mild state of despair by 5 p.m., when the number of fish was far below what she needed for the project.

Her adviser, Tim Essington, was surprised but told her not to worry, since she had enough crabs for that part of the study. His words, as she recalls: “Congratulations, you are going to be an invertebrate ecologist.”

“That’s how I got into the world of crabs and invertebrates,” Froehlich said, adding that her career path veered into a domain of some pretty amazing species, including those able to survive under extreme conditions.

This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality.

Up next: Our series continues with a look at the potential impacts of climate change on oxygen levels in Puget Sound. 

View the entire series. 


A purple sea star attached to a rock covered with mussels and seaweed.

Scientists are reporting a decline in oxygen-rich waters throughout the world. Causes for the decline vary from place to place but may involve climate change and increasing discharges of tainted water. In Puget Sound, low oxygen levels can occur naturally or due to eutrophication from human-caused pollution. In this five-part series, we describe the critical nature of oxygen to Puget Sound sea life. Scientists are finding that changes in oxygen levels can lead to physiological adjustments, shifts in predator-prey relationships and other repercussions throughout the food web.


View from underwater of bubbles rising to the surface of the ocean with sunlight above.

In time, lower dissolved oxygen worsened by climate change could increase the abundance of rare species in Puget Sound while putting populations of more common species into a tailspin. Part three of our series "Oxygen for life" looks at how warmer waters will gradually make it harder for many sea creatures to breathe. 


A crab pot (circular mesh cage) with an oxygen sensor (a white tube inside the cage) is held off the side of a boat as it is about to be dropped into the water.

The search goes on for a set of definitions and thresholds to represent low-oxygen concentrations that threaten various aquatic creatures. Over the years, ecologists have relocated, reshaped and revised the word “hypoxia” to describe these conditions. In part four of our series "Oxygen for life" we look at how scientists determine whether oxygen levels are low enough to be considered harmful to sea life. 


View of Puget Sound with red-orange water near the shoreline and blue sky with clouds above land in the distant background.

How do excess nutrients trigger low oxygen conditions in Puget Sound and what do those conditions mean for the species that live here?

About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute.

Oxygen for life: The biological impacts of low dissolved oxygen

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