World Aquaculture 51 Stock densities, growth and survival for pacu (Piaractus mesopotamicus) Gustavo Wicki1, L. Luchini2, L. Romano2 and S. Panné Huidobro2 Stocks of the indigenous freshwater pacu (Piaractus mesopotamicus) and other omnivorous species have been adversely affected by environmental changes in the La Plata river basin in Argentina; in particular, dam construction and pollution from industrial, agriculture and organic matter. These species have almost disappeared from Uruguay River, while they have been reported as scanty to medium and low in the Paraná river (Quiros 1990). According to Machado-Allison (1980), pacu must be considered as an omnivorous species because they eat mainly fruits, seeds, vegetables and invertebrates; mainly crustaceans, molluscs and insects. Two factors have contributed to an interest in culturing pacu: a remarkable decrease in wild stocks and its high demands in the local markets and restaurants in the country (Figure1). Its culture is also being considered as an option for those interested in agriculture diversification. Yields of over 400 tons annually (2004) can be obtained in semi-intensive systems. Cultured pacu have been sold whole, frozen, as boneless fillets, as burgers, as well as alive for stocking recreational fishing ponds. Pacu is offered in several expensive and exclusive restaurants in Buenos Aires. The aim of this study at Centro Nacional de Desarrollo Acuícola (CENADAC) was to carry out experimental culture research concerning pacu production at different densities with the goal of producing large individuals in the shortest culture period. Two diets (35 percent crude protein), successfully used in previous experiences were employed. Experimental feeds were formulated with locally available ingredients according to nutritional requirements set in Brazil by Cantelmo and Souza (1988) and Cantelmo (1993). Fish growth differs according to culture density. Aggressiveness seems to increase at lower densities and decrease at higher ones in some species, while others show greater levels of aggression and even cannibalism when density increases (Stickney 1994). There also appears to be a clear relationship among culture density, oxygen requirements and metabolic waste production. Piper et al. (1986) reported that density reduction yielded better quality even though there was no apparent environmental stress caused by crowding. Hepher (1993) pointed out that as the biomass in a pond increases, there is a greater requirement for DO, as well as an increase in metabolic wastes, mainly ammonia nitrogen, excreted by the fish, which restrains growth on account the toxicity. Fig. 1. Pacu sampled at commercial size. Study Methods Experimental pacu monoculture studies were carried out at the CENADAC in the northeastern region of Argentina (27º32´S, 58º30´W; Figure 2). Studies were carried out in 35 excavated earthen ponds (300 and 500 m2, each). The culture period ran for 16 months from late December 1999 to late April 2001, in a semi-intensive system, with water only being supplied in case of leakage or to make up for evaporative losses. Before stocking, the ponds were fertilized with poultry manure (400 kg/ha) and inorganic manure (urea and triple superphosphate at 1.5 to 5.0 kg/ ha (Boyd 1998). Previous lime treatment was not necessary because of the favorable soil characteristics. Adult herbivorous carp (500-1,000 g initial weight) were stocked at a density of 150/m2 to control weed growth, with excellent results. The water supply came from a deep well at pH 7.9, alkalinity of 9.1 mg/L, total hardness of 8.6 mg/L and DO of 2.85 ppm at the wellhead. The study began with densities of 1.0 (A), 0.5 (B) and 0.3 (C) pacu/m2. Experimental feed was based on two isocaloric and isonitrogenous diets, with a total of 35 percent crude protein, six percent lipid and 40 percent carbohydrate. The energy level was 3.36 kcal/kg. The diet contained 12 percent ash and nine percent moisture. Periodic samples to determine feed composition were analyzed in the laboratory. Ingredients included fishmeal, soybean,
RkJQdWJsaXNoZXIy MjExNDY=