WWU Grad Student Working to Turn Mussel Waste into Aquaculture Gold

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By Milthon Lujan

by John Thompson*
Western Washington University graduate student Casey Pruitt is like the alchemists of olden times — scientists who worked to uncover the secret of turning lead into gold. Whereas their toil produced nothing but hundreds of years of failure and frustration, Pruitt is on to something that could be a benefit to both the environment and the economy.

At its core, Pruitt, who also got his bachelor’s degree from Western in Environmental Science, is working with a concept that seems almost too good to be true: What if you could take the waste or byproducts produced by one organism and use them as food for another, grown alongside the first, in a symbiotic relationship that benefits both?

While dairy farmers, for example, have long used the waste of their cows as fertilizer for their hayfields to produce quicker growing and healthier hay that is then fed back to their cows, Pruitt’s work is different for a number of reasons. First, his project doesn’t require any waste to be moved, diluted, or transported to the second crop – they live right next to each other. Secondly, this process isn’t occurring in a hay field – it’s happening beneath the waves in the long fjords of Puget Sound.

Pruitt is researching, in cooperation with his corporate partner Taylor Shellfish Farms, how Taylor’s commercial mussel-growing operations might benefit from the addition of sea cucumbers – a soft, slow-moving bottom-dweller closely related to sea stars – living beneath their floating mussel aquaculture pens. Mussels, which along with clams are one of the most-consumed mollusks in North America – are a hugely important commercial species that grow readily as an aquaculture crop. And like every living organism, mussels produce waste.

“This is where the sea cucumber really shines,” said Pruitt, who is pursuing his master’s degree while still working at his job for the state Department of Natural Resources Aquatic Assessment & Monitoring Team. “The mussels are producing something the sea cucumbers want to feed on.”

It’s a process called Integrated Multi-Trophic Aquaculture, or IMTA, and more and more often, it’s being put to use by both commercial companies as well as environmental agencies. And while a different bottom-dwelling species could be substituted in this project – Pruitt said sea urchins would a logical choice for an alternative second crop – the most important thing is to find a second species that can be just as commercially viable as the mussels. While sea cucumbers or sea urchins aren’t necessarily commercially viable locally, both are very valid target species for consumers in Asian markets like Japan, which has decimated its sea cucumber species through overfishing.

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“It’s the two-species aspect which is of most interest to the commercial sector, and finding ways to both add to their bottom line while improving water quality in the Sound has real appeal to both sides of this equation,” Pruitt said. “And more often than ever before, consumers are more interested in a sustainable product.”

Gordon King, the director of Mussel Farms for Taylor Shellfish, said the decision to allow Pruitt to work on his research at Taylor’s facilities comes from both the company’s culture as well as using the collaboration as an opportunity build valuable relationships.

“These relationships not only give us access to new and interesting ideas and data related to shellfish farming and the ecosystem we work in, but opens up a dialog with capable people in the field that matter to us,” said King. “It is valuable for young agency employees to get some understanding of an aquaculture farm from the inside, and for them to see some of the permitting difficulties and contradictions we as farmers have to cope with all the time.”

Brian Bingham, Pruitt’s faculty advisor and the director of Marine and Coastal Sciences at Western, agreed with King.

“Casey’s research is helping us better understand basic ecological processes while supporting efforts of a commercial aquaculture business to operate more efficiently and sustainably. Such public/private partnerships are critical to addressing real-world questions in our changing world,” he said.

IMTA is being utilized more and more often by other partnerships similar to the one Pruitt has with Taylor. Some of the most common involve a combination of fish and seaweed or kelp; in Canada’s Bay of Fundy between New Brunswick and Nova Scotia, one group is working with Atlantic salmon with mussels; off of Chile another group is working on the relationship between farmed trout, oysters, and seaweed. It’s a concept that is clearly gaining momentum in both scientific and commercial circles, and Pruitt’s work is at the forefront of the effort.

A Trip to the Lab

It’s a sunny day in August, with smoke from the Oregon wildfires casting a hazy gauze over the deep blue water and low green hills of the South Sound as we motor down Totten Inlet northwest of Olympia. Pruitt is at the helm of a Taylor Shellfish workboat he has borrowed while working on his project; it’s basically a large aluminum tub with a motor, its bottom littered with nets, rope, Pruitt’s equipment and other bits and pieces.

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We pull up to two sets of floating pens, each about half the size of a basketball court. A group of harbor seals basks in the sun on the pens, most barely raising their heads to look our way; they have seen Pruitt’s boat many times at this point.

“Yeah, they keep us company while we get our data,” he said.

Pruitt pulls up a wire trap from under the mussels, its top coated with slimy mussel waste – but inside are a happy pair of sea cucumbers, which roll, wriggle, shrink and expand as Pruitt measures and weighs them. They are bright red, spiky, and as their name would suggest, about the size and shape of a cucumber. Meet Parastichopus californicus, the largest of the sea cucumber species common to Puget Sound.

“They are really pretty interesting creatures,” Pruitt said as the larger of the two sea cucumbers metamorphosed in his hand from something like an eggplant to what looked like a spiky red orange. “We tried tagging them, but they have shed every kind of tag we could think of – they just slowly push it out of their body. And every winter they absorb their whole digestive tract, so they can go all winter on reserves and not need to eat anything at all.”

It’s easy to see why this is a perfect place for Pruitt to do his research, which at its most basic involves understanding how sea cucumbers that are fed this steady diet of food produced by the mussels living in the pens above them either thrive or don’t thrive. As part of his study, Pruitt compares these specimens with others he has in an open-water control group that isn’t being fed the nutrients from the mussels.

While it’s still too early to know definitively, there isn’t any denying that these two sea cucumbers, at least, seem robust, happy, and thrilled with their dietary options.

Pruitt said he sees more opportunities to put the framework he has developed for this research, which is supported by a grant from the Department of Natural Resources, into play studying other species: he is pondering a similar study on local kelp populations, which are in trouble across the Salish Sea.

“Kelp is another viable commercial species that could benefit from a similar partnership, and it could make a for a fascinating study,” he said. “But I need to get done this one first!”

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Another benefit to Pruitt’s grant: the ability to hire on an undergrad assistant to work with him all summer on collecting data, and Yelm resident and Environmental Science pre-major Chloe Cason got the job.

“Working with Casey this summer has allowed me to really learn more of what being a researcher is all about. Hitting the water at 7 a.m., the ‘working’ was more like all-day learning in an outdoor classroom,” she said. “I have learned about a variety of relationships between species that were happening right under my nose, currents, knots, and just as importantly, the best ways to wash sea-cucumber poop out of my clothes.”

The seals, finally, have had enough of our company, and plunge into the water as a group, popping up to gaze at us as we slowly motor back up Totten Inlet on the way to the Taylor Shellfish dock.

Pruitt sighs as we pull up to the beach to unload; today he showed us the process for collecting the data from just one container of the sea cucumbers; tomorrow he will back to repeat the process 24 times, once for each captive set of the creatures.

“I guess if it was easy, somebody else would have done it already,” he said.


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