As the GMO debate rages, biotechnologists are asked to fish or cut bait.

For countless millennia, humans have cast their nets, thrown their spears, and drawn their lines through the oceans, lakes, and streams of the world to feed their families in a watery throwback to the ancient hunter-gatherer society. But it is within the last two millennia that spears have been turned into metaphorical plowshares to till the aquatic soils. Whether in a koi pond on the island of Honshu or a trout bed in Arkansas, a new crop has been planted and its bounty is finally being reaped.

Traditional Methods
Traditionally, aquaculture (freshwater) and mariculture (saltwater) farmers have relied on many of the husbandry methods long practiced by their landlocked cousins. If you wanted larger fin- or shellfish (hereafter categorized together as fish), you simply bred larger males and females and culled underdeveloped or deformed fish from the stocks. If you wanted fish that would better tolerate colder water, you grew them under cooler conditions. But as long as the natural fish populations remained abundant, aquaculture had little more than a niche market. Within the last century, however, as an expanding human population depleted land resources and natural fish stocks were decimated both by overfishing (e.g., North Atlantic cod) and natural disasters (e.g., North Sea salmon), aqua- and mariculture (hereafter referred to collectively as aquaculture) grew in importance.

Figure 1. Aquatic Hunter-Gatherers. The last century has seen rapid growth in the world’s capture fisheries, but declines in wild stocks mean an increasingly larger role for aquaculture. (Source: Food and Agricultural Organization of the United Nations)

Over the past 20 years, there has been an explosion in the importance of aquaculture to world fish capture; aquaculture now represents almost one-quarter of fish caught for all purposes (see Figure 1). A report by the United Nations Food and Agriculture Organization suggests that this trend will continue such that, by 2025, aquaculture will account for more than half of the world’s fish supply. This point is echoed by Yonathan Zohar, director of the University of Maryland’s Center of Marine Biotechnology (COMB) in Baltimore.

“Demand [for fish] is projected to double supply by the year 2025,” he says, “and this gap needs to be filled by aquaculture. But for aquaculture to meet this challenge, it needs to become more efficient, cost effective, and so on.” And, he adds, it will have to increase production 3- to 4-fold in the next 25 years.

For Zohar and hundreds of researchers like him, biotechnology will provide at least one part of the key to this growth. And this is biotechnology in the broad sense that includes but also extends beyond the scope of genetically modified organisms (GMOs) and the furor that follows them (see sidebar, “A Fish for All Seasons”). Zohar sees biotechnology as a way of addressing several of the bottlenecks that have plagued the development of aquaculture.

Reproduction
“Many commercially important farmed fish do not reproduce at all or in a very predictable way when they are grown in captivity,” says Zohar. In part, this is because “they do not have the environmental conditions of the spawning grounds.” But efforts to simply simulate these conditions have largely met with failure, partially because of a lack of detailed knowledge about environmental changes, such as salinity, temperature, and water depth.

For this reason, Zohar’s group studied the hormonal changes that fish experience as they move toward reproduction. One of the main hormones in the process is gonadotropin-releasing hormone (GnRH), but early efforts to stimulate breeding using GnRH injections met with failure because the circulating hormone is rapidly degraded by an enzyme in the fish’s bloodstream. Zohar’s team then engineered the hormone by altering its labile site so that it was no longer sensitive to the enzyme, testing the synthetic agonist’s efficacy using cell culture. But even this approach suffered from the need for repeated injections, which were hard on the fish. Finally, the Baltimore researchers developed a controlled-release polymer system for the introduction of the tailored hormone, a system that could be modified for fish of specific species. This system is marketed as ReproBoost by VeriPharm International LLC (Richmond, ME).

One goal of this type of research is to produce an aquaculture system where spawning occurs throughout the year. Another benefit would be in the manipulation of gender by bathing the young fish in hormones. Thus, a pond could be controlled through the artificial maintenance of an all-male or all-female population.

Early Development
Biotechnology has also provided the groundwork for great strides in the development of specialty feeds for larval fish. Interestingly, improvements in this area came from work on the production of human infant formulas by Martek Biosciences (Columbia, MD). Human mother’s milk is highly enriched in polyunsaturated fatty acids, one of which is docosa-hexaenoic acid (DHA), one of the omega-3 fatty acids that have recently been shown to be so critical to human health. Commercial formulas, by contrast, lack DHA. Martek saw this difference as an opportunity. The company used its expertise in producing compounds from algal lysates to mutate and isolate a dinoflagellate species that would heterotrophically produce more than 50% of its fatty acids as DHA, thus providing a rich source of marine oils.

But this was only the first step in the process. Larvae need a source of small live food, typically rotifers and artemia. “But, while rotifers are easily cultured organisms,” says Allen Place, a colleague of Zohar’s at COMB, “they don’t have any DHA in them. So, you have to do an enrichment of the live feed before you feed it to the fish larvae.” And apparently, this technology has met with success. “It is literally the difference between 0% survival and 60% survival,” adds Place.

Food
The popularity of the omega-3 fatty acids also highlights another problem associated with aquaculture. How do you feed all of those fish?

“Other marine oils are made from taking the heads of tuna or other fish and rendering oil from them,” continues Place. “So the problem is that you are harvesting fish to feed fish or humans.” This has long been one of the arguments against the sustainability of modern aquaculture—that it was actually consuming more fish (by weight) than it was producing.

At a recent meeting of the American Association for the Advancement of Science (AAAS) in San Francisco, Rosamond Naylor, an economist from the Stanford Institute for International Studies, suggested that two pounds of wild fish are required for every pound of farmed fish produced. In a recent review, Naylor and colleagues argued that aquaculture must break with its reliance on wild fish and adopt more ecologically sound practices.

It has been estimated that food costs account for almost 50% of the cost of aquaculture and that by 2010, these costs will run to $10 billion per year. A full third of the output of the world’s fisheries goes to produce fish meal (approximately 6 million tonnes annually). It is hoped that algal feed sources will help eliminate this problem. Efforts to produce a feed system based on proteins and fatty acids from soy are also expected to yield good results.

These steps will also reduce the fear of contaminating one fish population with the problems of another—a situation that is all too sensitive in a world frightened by bovine spongiform encephalopathy (mad cow disease).

Ecology
The aquaculture of marine fish is dominated by the use of coastal sea pens—in effect, large floating baskets that allow the ocean water to circulate while keeping the cultivated fish population from mixing with the indigenous population. Although this technology works well, it is by no means foolproof and there are numerous examples of fish escaping from pens either through poor maintenance or from weather-related accidents. Another problem comes from the fact that waste material is allowed to pass untreated from the sea pens, damaging the local ecosystem and potentially promoting eutrophication and oxygen depletion. Finally, by being open to the vagaries of weather and sea currents, penned fish are susceptible to changes in water temperature, and recent experiences with “super chills”—severe temperature drops—have threatened to cripple the aquaculture industry of some countries.

For these and other reasons, many groups advocate a move away from ocean pens and are developing self-contained units that carefully control conditions such as salinity and temperature. But it is in the treatment of wastewater that scientists are making their greatest strides. At facilities like those of COMB, researchers are using biotechnology to engineer microbes that will bioremediate the chemical composition of wastewater—nitrifying or denitrifying as required and metabolizing organic waste products—in an effort to create a zero-impact operation. At the Baltimore site, polymer cylinders shaped like small wagon wheels float in secondary tanks as the water from the fish pens is circulated through the system. Embedded within these polymers are microbes that clean the water before it reenters the tanks or leaves the waterfront facility.

These land-based units do not come cheaply, though, and there is some question about whether there will be much of a market in developing countries that may not find it easy to or wish to devote economic resources to environmental concerns. Zohar, however, sees the adoption of this technology as inevitable, citing examples of industry collapse due to inefficient or poorly developed aquaculture systems. He also argues that the land-based facilities will remove the reliance on water bodies, as aquaculture can conceivably be set up in any warehouse with the right plumbing and power.

Disease
But perhaps the area in which biotechnology will have its greatest impact is the field of disease management. Nucleic acid amplification technologies, such as polymerase chain reaction (PCR), allow fish to be diagnosed rapidly for disease or infection by viruses, bacteria, or parasites. And the development of real-time PCR tools (Taqman) and portable machinery has moved this science from benchtop to poolside.

For aquacultural practices involving shrimp, for which no vaccines exist, early detection of viral infection can mean the difference between a reduced yield and no yield. A recent report by the World Resources Institute suggested that disease is the greatest limiting factor in shrimp cultivation. The report quoted a World Bank study that estimated the global cost of shrimp disease to be $3 billion annually.

As for humans, drug and vaccine delivery for fish continues to be an area of exploration. It is no longer tolerable to merely dump large quantities of antibiotics into a pen of fish and simply hope for the best. With regard to vaccines, the problems of biological half-life and delivery are exacerbated, as the immune memory of fish is shorter than that of mammals. Thus, vaccines must either be injected repeatedly—which (like hormone injections) is hard on the fish and costly in time and resources—or other methods of introduction must be developed. One such method is transdermal delivery using ultrasound, a method that is also being developed for the delivery of insulin to humans. This technique immerses the fish in a bath containing the vaccine. Then the low-intensity energy alters the permeability of the fish’s skin, enhancing vaccine uptake.

Conclusion
There are, of course, other ways in which biotechnology is revolutionizing aquaculture. As the need for improved and more reliable food supplies increases, it is likely that biotechnology will continue to play a growing role.

Further Reading

• Aquatic Biotechnology. Available at www.meds-sdmm.dfo-mpo.gc.ca/sealane/AOSB/ENGLISH/Biotechn.htm A report of the Canadian Department of Fisheries and Oceans: Ottawa, Canada.
• Colgan, C. S.; Baker, C. Prospects for Marine Biotechnology in Maine. A report for the Maine Department of Marine Resources: Augusta, ME, 2000.
• Matthews, E.; Hammond, A. Fish Consumption and Aquatic Ecosystems. Available at www.wri.org/pdf-docs.html A report of the World Resources Institute: Washington, DC, 2000.
• Naylor, R. L. et al. Effect of aquaculture on world fish supplies. Nature 2000, 405, 1017–1024.
• The State of World Fisheries and Aquaculture 2000. Available at www.fao.org/docrep/003/x8002e/x8002e00.htm A report of the Food and Agricultural Organization of the United Nations: Rome, 2000.
• Sustainable Aquaculture. Bardach, J. E., Ed.; John Wiley & Sons, Inc.: New York, 1997.

(All Web sites accessed June 2001.)

Randall C. Willis is an assistant editor of Today’s Chemist at Work. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th St N.W., Washington, DC 20036.