When fish farms are built along the coast, where does the waste go?

Feb 15, 2009 By Dan Stober

If you are a fish eater, it's likely that the salmon you had for dinner was not caught in the wild, but was instead grown in a mesh cage submerged in the open water of oceans or bays. Fish farming, a relatively inexpensive way to provide cheap protein to a growing world population, now supplies, by some estimates, 30 percent of the fish consumed by humans.

Two hundred and twenty species of finfish and shellfish are now grown in farms.

Intuitively, it seems a good idea—the more fish grown in pens, the fewer need be taken from wild stocks in the sea. But marine aquaculture can have some nasty side effects, especially when the pens are set near sensitive coastal environments. All those fish penned up together consume massive amounts of commercial feed, some of which drifts off uneaten in the currents. And the crowded fish, naturally, defecate and urinate by the tens of thousands, creating yet another unpleasant waste stream.

The wastes can carry disease, causing damage directly. Or the phosphate and nitrates in the mix may feed an algae bloom that sucks the oxygen from the water, leaving it uninhabitable, a phenomenon long associated with fertilizer runoff.

It has been widely assumed that the effluent from pens would be benignly diluted by the sea if the pens were kept a reasonable distance from shore, said Jeffrey Koseff, a professor of civil and environmental engineering and co-director of Stanford's Woods Institute for the Environment. But early results from a new Stanford computer simulation based on sophisticated fluid dynamics show that the icky stuff from the pens will travel farther, and in higher concentrations, than had been generally assumed, Koseff said.

"What we've basically debunked is the old adage that 'The solution to pollution is dilution,' " he said. "It's a lot more complicated."

The computer modeling (with new Stanford software that goes by the acronym SUNTANS) was conducted by Oliver Fringer, an assistant professor of civil and environmental engineering. He created a virtual coastal marine area resembling California's Monterey Bay.

Previous software, he said, has not been up to the task of accurately predicting where the unhealthy effluent from fish pens will end up, and should probably not be used by state or federal regulators when they approve locations for fish farms.

Existing software is typically derived from models that attempt to describe the drift of effluent from sewage outfall pipes, even though the substances and situations are different from fish farms. (Sewage outflow, for example, is often warmer than the ocean water.)

The fine details of modeling the flow of dissolved fish poop from a submerged cage are not as simple as they may seem. The design of the cage itself can affect the outcome. How much of the current flows through the cage, and how much goes around? Does the moving water swirl into eddies at the edges of the pen? Even the effects of the rotation of the earth on the waste plume comes into play.

The fish farmer would prefer that currents flush out his pens frequently, but as those currents take out the garbage they might unfortunately deliver it to a mangrove ecosystem or a public beach. On the other hand, insufficient flow through the pen can create a "dead zone" on the ocean floor as the fecal matter and uneaten food pile up beneath the fish.

Fringer is designing his software so that it can be used to asses any site—Puget Sound, perhaps—where sufficient digital mapping of the area already exists. SUNTANS comes just in time, said Stanford oceans expert Rosamond Naylor, as federal and local officials begin spelling the details of new health and environmental regulations for fish pens.

Also participating in the research was former postdoctoral researcher Subhas Karan Venayagamoorthy, now at Colorado State University.

Source: Stanford University

Explore further: US delays decision on Keystone pipeline project

add to favorites email to friend print save as pdf

Related Stories

Fish farm waste can drift to distant shores

Apr 07, 2011

Concentrated waste plumes from fish farms could travel significant distances to reach coastlines, according to a study to be published in an upcoming issue of the journal Environmental Fluid Mechanics, available online now. R ...

Fears for South Africa's lions

Jan 16, 2013

Lions may be the well-reputed kings of the savannah, but South Africa's lucrative trophy-hunting industry means the regal cats are more likely to know the inside of a paddock ringed with an electric fence ...

Recommended for you

US delays decision on Keystone pipeline project

20 hours ago

The United States announced Friday a fresh delay on a final decision regarding a controversial Canada to US oil pipeline, saying more time was needed to carry out a review.

New research on Earth's carbon budget

Apr 18, 2014

(Phys.org) —Results from a research project involving scientists from the Desert Research Institute have generated new findings surrounding some of the unknowns of changes in climate and the degree to which ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

MikeB
5 / 5 (1) Feb 16, 2009
If we are going to have fish farms, I hereby declare that the fish farmers must equip every fish with diapers to eliminate any fish feces from polluting our natural environment. I believe that it would also be prudent to use fishy bibs to keep the unwanted extra food from our oceans.

I know that the article points out that these nasty fish are eating in the water, and putting "icky stuff" in the water. However, what other sick and perverted things are these fish doing in our water?
Velanarris
not rated yet Feb 17, 2009
Your average aquarist knows how this cycle works. The fish create ammonia (NH3) in water with a pH higher than 7, Nitrosomonas bacteria oxidize the ammonia into Nitrite, Nitrobacter bacteria further change the Nitrite into Nitrate which is then consumed by plants, and some species of bacteria.

Considering the fish stocks of the area that the article is referencing are severely depleted there is virtually no harm being committed here. This is doubly true seeing as the mentioned bacteria here is primordial and omnipresent on our planet.

More news stories

Magnitude-7.2 earthquake shakes Mexican capital

A powerful magnitude-7.2 earthquake shook central and southern Mexico on Friday, sending panicked people into the streets. Some walls cracked and fell, but there were no reports of major damage or casualties.

China says massive area of its soil polluted

A huge area of China's soil covering more than twice the size of Spain is estimated to be polluted, the government said Thursday, announcing findings of a survey previously kept secret.

Airbnb rental site raises $450 mn

Online lodging listings website Airbnb inked a $450 million funding deal with investors led by TPG, a source close to the matter said Friday.

Health care site flagged in Heartbleed review

People with accounts on the enrollment website for President Barack Obama's signature health care law are being told to change their passwords following an administration-wide review of the government's vulnerability to the ...