William A. Wurts

Sustainable Aquaculture in the Twenty-First Century

There have been many efforts to create a conceptual framework for understanding and defining sustainable aquaculture. A recent consensus or stakeholder view (Caffey et al., 1998) approached sustainability from three perspectives: environmental, economic, and sociological. Often, stakeholder views are snapshot or present oriented. The multiple variables affecting sustainability and viability are considered from a here-and-now perspective rather than considering the effects that significant change in one or several variables might cause. Aquatic nitrogen loads generated from the sewage effluent of a growing, global human population (15 billion vs. 6 billion people) may prevent the legal discharge of any aquacultural effluents. Much of the intensive aquaculture industry has a highly centralized structure with respect to production and distribution. This centralized development has flourished around energy-rich — at times extravagant — cultures and economies. How will increased costs or shortages of electricity, gasoline, and diesel fuel affect the sustainability or survival of the current production system? Does a large, centralized industry provide more jobs and profit or a better quality of life (per capita) than widely dispersed, small-scale operations producing at local or county levels? Nutrient recycling (converting nitrogen back to protein) through different polyculture systems could be more practical and efficient than controlling or treating the effluents associated with traditional, intensive monoculture practices. Phytoplankton and zooplankton occupy sizable respiratory (oxygen consumption) niches in the production pond environment — and have no market value. Careful selection of suitable filter feeding fish and mollusks for polyculture could open up these niches for the production of species with greater economic value. It might be more desirable to culture channel catfish with paddlefish and some species of freshwater mussel than to face bankruptcy because it has become illegal to discharge effluents from production ponds used for intensive monoculture. Ultimately, sustainability may be the aquaculture industrys ability to adapt on a planet with an ever-increasing human population that continues to consume its limited supply of nonrenewable resources at an alarming rate.

Effects of calcium and magnesium hardness on acute copper toxicity to juvenile channel catfish, Ictalurus punctatus

Abstract Two experiments were conducted to evaluate the effects of calcium or magnesium hardness on the acute toxicity of copper sulfate to juvenile channel catfish ( Ictalurus punctatus ) in low alkalinity environments. A preliminary bioassay determined the 48-h LC 50 of copper sulfate to be 1.25 mg l −1 for juvenile catfish placed in water with calcium hardness and total alkalinity set at 20 mg l −1 CaCO 3 . In the first experiment, catfish were exposed to 1.25 mg l −1 copper sulfate in environments where calcium hardness was varied from 10–400 mg l −1 CaCO 3 . Total alkalinity was 20 mg l −1 CaCO 3 . As calcium hardness increased, copper-induced catfish mortalities decreased significantly from 90% at 10 mg l −1 CaCO 3 to 5% at 400 mg l −1 CaCO 3 . In the second experiment, catfish were exposed to 1.25 mg l −1 copper sulfate in environments containing either calcium or magnesium hardness, 20 and 400 mg l −1 CaCO 3 , with total alkalinity set at 20 mg l −1 CaCO 3 . Survival rates in calcium hardness treatments were consistent with those in the first experiment. However, 100% mortality was observed in both treatments containing magnesium-based hardness. These data suggest a calcium-specific mechanism with respect to acute copper toxicity in channel catfish.

Effects of bicarbonate alkalinity and calcium on the acute toxicity of copper to juvenile channel catfish (Ictalurus punctatus)

Abstract Three experiments were conducted to evaluate the relative importance of calcium hardness and bicarbonate alkalinity to the acute response of juvenile channel catfish ( Ictalurus punctatus ) exposed to a toxic concentration of copper sulfate. A preliminary bioassay revealed 28 mg·l −1 copper sulfate caused 50% mortality within 48 h (48-hLC 50 ) in juvenile channel catfish placed in water with calcium hardness and bicarbonate alkalinity, set at 75 mg·l −1 CaCO 3 . Catfish were then exposed to 28 mg·l −1 copper sulfate concentrations in environments where hardness or alkalinity concentrations were varied. Bicarbonate alkalinities above 75 mg·l −1 CaCO 3 , with calcium hardness held at 20 mg·l −1 CaCO 3 , significantly reduced catfish mortalities from 97–100% to 63–70%. Copper-induced mortalities were 100% for all fish placed in calcium hardness treatments (20–250 mg·l −1 CaCO 3 ) in which bicarbonate alkalinity was held at 20 mg·l −1 CaCO 3 . When bicarbonate alkalinity was held constant at 75 mg·l −1 CaCO 3 and calcium hardness was varied from 20 to 250 mg·l −1 CaCO 3 , copper related catfish mortalities displayed high variability and means ranged from 6.7 to 60%. Mortalities decreased as calcium concentrations increased. Although differences in mortalities were not statistically significant, the latter hardness findings appear to suggest a biologically significant calcium effect on copper toxicity in the presence of sufficient alkalinity concentrations.

Responses of Red Drum (Sciaenops ocellatus) to Calcium and Magnesium Concentrations in Fresh and Salt Water

Abstract Experiments were conducted to determine some of the chemical aspects of water quality required by cultured red drum in fresh or salt water. Two studies were conducted in fresh and two in salt water differing in concentrations of calcium and magnesium. Red drum weighing 1–3 g each were stocked at 15 fish per 114-l tank. Treatments were replicated three times. Environmental calcium significantly affected red drum survival in fresh and salt water; magnesium produced no discernible effect on performance. In saltwater [35 g/l total dissolved solids (TDS)] experiments, fish in experimental water containing less than 176 mg/l calcium exhibited 100% mortality within 96 h. Highest survivals were observed in water containing 340–465 mg/l calcium. Red drum stocked in fresh water (0.56–1.9 g/l TDS) with calcium concentrations 1.7 mg/l or less performed poorly (0–33% survival after 96 h). Growth and survival were not significantly affected when calcium was between 9 and 407 mg/l. These data are consistent with the recognized physiological effects of calcium on membrane permeability and its postulated function in pore or channel mechanisms. Generally, fish from the first fresh- and saltwater experiments had significantly better longterm (42 days) survival than those from the second set of experiments. This appeared to be related to the use of acid-washed biofiltration media (hypothetically related to the removal of essential trace components) in the second trials.

Growth Response of Blue Tilapias to Selected Levels of Dietary Menhaden and Catfish Oils

Abstract Blue tilapia (Tilapia aurea) fingerlings were fed a series of nine semipurified diets containing 0, 2.5, 5.0, 7.5 or 10% lipid in the form of menhaden oil or catfish oil over a 10-week experimental period. The study, conducted in flow-through aquaria, demonstrated that best growth and food conversions were obtained from fish fed the 10% menhaden oil diet. Total levels of the linolenic family (ω-3) of fatty acids were highest in the menhaden oil diets (up to 2.5%). In no instance did linolenic acid reach 1.0% of the total diet. That result supports the conclusion that growth reductions observed in tilapia and other fish species fed diets approaching 1% dietary linolenic acid are due to that specific fatty acid, and are not caused by the addictive effect of dietary ω-3 fatty acids.

AN HYPOTHESIS ON THE LIGHT REQUIREMENTS FOR SPAWNING PENAEID SHRIMP, WITH EMPHASIS ON PENAEUS SETIFERUS

Wurts, W.A. and Stickney, R.R., 1984. An hypothesis on the light requirements for spawning penaeid shrimp, with emphasis on Penaeus setiferus. Aquaculture, 41: 93-98. Using standard equations, we have concluded that the light intensities which have been used by investigators attempting to induce maturation and spawning are up to 4000 times higher than would occur over the spawning grounds of Penaeus setiferus, a species for which successful controlled spawning has yet to be achieved. We recommend that future attempts at captive maturation and spawning with that species be conducted at light levels of no more than 12 µW cm -2 and that the intensity at which other species of shrimp are matured and spawned be examined relative to natural light levels on their spawning grounds. It is further recommended that the need for light of blue or green color be evaluated for its role as a promotor of maturation and spawning. The intent of these modifications in current spawning procedures is to eliminate the need for eyestalk ablation in captively spawned penaeid shrimp.