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Sustainable Energy

Aquaculture Cleans Up Its Act

Researchers investigate recirculating water from fish farms, and moving them farther offshore.

As commercial fisheries around the globe decline or collapse completely, aquaculture will become “the main source of seafood products in the future,” declares Kevan Main, a biologist at the Harbor Branch Oceanographic Institution in Fort Pierce, FL. “There is no doubt about it.”

Speaking earlier this month at the Oceanology International conference in Miami, Main discussed two primary thrusts in aquaculture work: protecting the surrounding environment and improving ocean-cage facilities.

Water Recycling

In the past 15 years, aquaculture use has doubled worldwide, but not without raising some environmental concerns. One major concern is that land-based aquaculture facilities that raise animals in pools or tanks produce large amounts of wastewater polluted with nitrogen and phosphorus, which can shift the natural balance in surrounding waters and even contribute to harmful algal blooms. (These wastes can be converted into fertilizer, but the process is too expensive to be widely adopted.)

One obvious solution is to shift from flow-through systems to recirculating systems that clean up wastewater and use it again. That can be easily done using technology similar to that used for sewage treatment. Water passes through a filter to remove solid waste and then into a tank filled with beads that support the growth of bacteria that naturally takes up pollutants.

However, this technology is presently too expensive for the vast majority of facilities to use and still turn a profit.

Harbor Branch researchers are developing systems that could remedy this problem by using cheaper materials. For example, they have built a system that slashes water-treatment costs for raising shrimp by using relatively inexpensive off-the-shelf products to treat about 95% of the water coming from the shrimp tanks. The rest is processed through a purpose-built wetland onsite where aquatic plants and microbes clean the water well enough for safe discharge.

Main says her group is seeking funding for scaling up these treatment systems. This approach “would be terrific,” claims Rebecca Goldburg, a senior scientist with Environmental Defense in New York.

Farms to Go

A diver within a cage that has not yet been stocked with fish. (Photo courtesy of Ocean Spar Technologies)

Another common aquaculture technique is to raise caged fish and sometimes shellfish in protected coastal waters. U.S. regulations severely restrict this practice, but it’s popular abroad.

However, its use is limited not only by the actual space available in a given area but also by environmental concerns. If, for instance, tides don’t adequately flush out a lagoon, the cages can dramatically increase pollution.

A deceptively simple solution is to move the cages out into the open ocean so that flushing is guaranteed and space is nearly unlimited. However, standard cages don’t work well in the often violent environment of open water. So companies such as Ocean Spar Technologies of Bainbridge Island, Washington, have been working on new ideas.

One solution, dubbed the SeaStation Sea Cage, dispenses with the standard design of a floating frame with suspended, weighted nets. Instead, the cage is built around a tough central spire with a large hoop around the center. Netting is strung taut from the top and bottom of the spire out to the hoop.

Because the cage does not depend on surface floats or gravity for support, operators can simply release air from its ballast tank to sink the cage below the turbulence of wave action. The cage also can be easily towed to other locations, the company claims.

Gary Loverich, an Ocean Spar senior engineer, says the company is developing automatic fish-feeders that can submerge with the cages. Currently the task is done by hand or with floating automatic feeders that are endangered by hazardous surface conditions. The basic SeaStation is available commercially and in use in Hawaii, the Philippines and other areas.

Floating Cities

While SeaStation cages escape most ocean turbulence, they are anchored and still subject to significant stress. Ultimately, Loverich says, Ocean Spar would like to take the offshore cage to a new level-a “floating city” of connected cages that would simply drift in areas with suitable currents.

This system would include an above-water platform for onboard operators-a necessity to ensure that various contingencies are dealt with quickly.

The cage city may take a while to hit the water, but Loverich says it could be built now if the right customer is found. He suggests that the Straits of Juan de Fuca in Puget Sound might prove a good area, since the tides there create regular back-and-forth currents that would not be strong enough to take the city all the way into land. The city would need some minimal form of propulsion to keep it seaworthy, he adds.

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