Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Evaluation of an Intensive Polyculture System Incorporating Control of Algal Off-Flavors and Water Quality
Peter W. Perschbacher
Increasing supplies of aquaculture products in a sustainable, environmentally friendly manner requires increased stocking density to conserve water and land, integration of plant production grown on fish waste to be consumed directly or fed to fish, and maintenance and/or improvement of water quality to reduce discharge from flushing and reduce any discharge pollution strength. These features keep with the U.S. definition for sustainable agriculture as “an integrated system of plant and animal production practices having a site-specific application that will, over the long term: satisfy human food and fiber needs; enhance environmental quality and the natural resource base upon which the agriculture economy depends; make the most use of nonrenewable resources and on-farm resources, and integrate, where appropriate, natural biological cycles and controls; sustain the economic viability of farm operations; and enhance the quality of life for farmers and ranchers, and society as a whole” (Food, Agriculture, Conservation, and Trade Act of 1990, Public Law 101-624, 101st Congress, 28 November 1990). In addition, flavor and product quality must be maintained or improved to encourage consumption and expand markets.
Production in commercial culture ponds of channel catfish Ictalurus punctatus is intensive; annual harvest averages 4,560 kg/ha, with no or little discharge or draining; however, algae populations are largely uncontrolled. The result is periodic and unpredictable boom-and-bust cycling and algal dominance by undesirable species. Rapid swings in algal biomass are associated with reduced water quality (primarily increased ammonia and decreased dissolved oxygen), and undesirable species are responsible for the majority of “off-flavors” that render the flesh unmarketable.
Algal flavor tainting was first described in natural waters and continues to be a major threat to drinking water and potentially to the quality of sport and commercial fish harvest as well as to aquaculture production (Perschbacher et al. 1996). Algae-related costs to catfish farmers may be on the order of 20% of the value of their production from disease outbreaks following algal fluctuations and from delayed harvest after undesirable flavor tainting. Without algal control, these problems may increase, as their incidence has increased relative to past production intensifications (Brown and Boyd 1982).
Blue-green algae (also known as cyanobacteria because of its similarities to bacteria) is the primary group proliferating in channel catfish production ponds. Certain cyanobacteria produce undesirable odors and tastes (off-flavor) at densities as low as 1,000–2,000 cells/mL (Jones and Korth 1995). They are known to produce primarily earthy and musty off-flavors. These same algae infest drinking-water reservoirs and cause sales of bottled water to soar, at unmeasured cost to the public. Blue-green algae also are thought to cause mortalities in minnow production ponds and in catfish ponds of 2–3 g/L salinity by causes as yet unknown.
The two major off-flavor compounds, 2- methylisoborneol (MIB) and geosmin, result primarily from several species of blue-green algae (Tucker and Martin 1991). Both are aromatic alcohols (volatilize at room temperatures with resulting odors) and are produced during algal metabolism. Thus maximum concentrations typically occur when the algae are actively growing.