As fish farms flourish, so does waste

STANFORD (US) — Aquaculture is one of the fastest-growing segments of livestock farming in the U.S. but the problem of controlling fish effluent may be growing even faster.

Simple ocean dilution may not be the answer.

“For many years, people have assumed that because of the ocean’s size, because of the energy in its currents, that any substance you introduced into the ocean would quickly be diluted into concentrations that were barely detectable,” says Jeff Koseff, professor of civil and environmental engineering at Stanford University.

A new computational model will allow researchers to predict where the effluent from a coastal fish farm will go. The answer may not always be appealing to down-current swimmers and surfers.

“We discovered that the state of the natural environment around fish pens can dramatically affect how far waste plumes travel from the source,” Koseff says. “This suggests that we should not simply assume ‘dilution is the solution’ for aquaculture pollution.”

The simulation incorporates the influence of variables such as tides, currents, the rotation of the Earth and the physical structure of the pens in calculating the dispersal pattern of the waste.

“These plumes actually remain quite coherent at very long distances from the source and could become a major pollution problem in coastal regions,” Koseff says.

The model should prove valuable in selecting appropriate sites for future fish farms because by knowing the amounts of feces and uneaten food that are generated by pens, researchers will be able to predict how dissolved waste will travel from a particular location, given local conditions.

The model will likely show that some locations previously thought appropriate for fish farms are actually not suitable, but that won’t necessarily be viewed as a bad thing. Having clearly defined boundaries of where aquaculture is acceptable will help the industry avoid conflict with other users of coastal waters.

“A lot of the industry people that I have talked to are not working against the environment, they are really trying to make aquaculture work, and this would provide a useful tool for them,” says  Roz Nayor, professor of environmental earth system science.

The findings are published online in the journal Environmental Fluid Mechanics.

The study is timely, in light of legislation in the works at both the state and federal levels. In 2006, California passed the Sustainable Oceans Act, aimed at protecting the biologically rich waters off the coast while also recognizing the importance and economic value of providing fresh seafood.

A draft of the regulations to implement that legislation is currently under review and the new modeling tool should help in setting guidelines for locating and monitoring aquaculture.

At the federal level, the National Oceanic and Atmospheric Administration is drafting a national aquaculture policy.

“After the bill is passed, rules and regulations will have to be written around it and what we are providing now is a tool to help with that,” Naylor says.

Koseff acknowledged that some people might balk at relying on a computer model to guide regulations. “We understand and recognize the limitations of the simulations,” he says. “But we have confidence that the physics that we are representing in the model are realistic and our results are very representative of what happens in a near-coastal environment.”

For an aquaculture operation to be economically feasible, several pens will likely have to be concentrated in one area, making waste a significant concern.

“I also work a lot in terrestrial livestock, and I think the dissolved wastes that come out are one of the worst aspects of intensive animal raising. If we are really thinking about getting our animal protein from fish in the future, and it is coming from net pens that are in the ocean, one of the big fears is, are we going to have feedlots of the sea?

“We would really like to completely avoid the problems we have seen in terrestrial livestock. That would be the ultimate goal and this model can help achieve that.”

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