Interaction of Water Alkalinity and Stocking Density on Survival and Growth of Silver Catfish, Rhamdia quelen, Juveniles

The use of lime to increase water pH, hardness, and alkalinity can decrease mortality of acidic-sensitive species (Weatherley 1988) because it reduces ion loss through the gills (Flik et al. 1996; Wilkie et al. 1999) and allows recovery of normal ion plasma levels (Rosseland et al. 1986). Enhancement of water quality also reduces incidence of diseases in fishes (Singhal et al. 1986) and improves active cellular transport during the embryonic phase as well as eggs hatching (Hwang et al. 1994). Alkalinity is a measure of the acid-neutralizing capacity of water, and it is important because it protects organisms from pH changes (Boyd 1990; Zweig et al. 1999). Alkalinity levels below 30 mg CaCO3/L limits primary production in well-fertilized ponds, and in unfertilized ponds, the primary production increased with alkalinity levels up to 120 mg CaCO3/L (Boyd 1990). According to Zweig et al. (1999), there are no direct effects of alkalinity on fish, but adequate values of water alkalinity can improve growth of Nile tilapia, Oreochromis niloticus, larvae (Rojas and Rocha 2004). The use of high stocking densities to maximize available space for production might impair water quality because of excess of nitrogen residues from fish excretion (Jobling 1994). In addition, some species try to establish territories and do not feed properly (Alanara 1994), compromising water quality. Silver catfish, Rhamdia quelen (Quoy and Gaimard, 1824; Heptapteridae), accepts artificial food and possesses high fertility, fast growth, and good acceptance in the fish market (Baldisserotto 2004), and consequently, its culture is increasing in Brazil. Juveniles of this species survive in soft water without problems within the 4.0–9.0 pH range (Zaions and Baldisserotto 2000), and the increase of water hardness to 100–150 mg CaCO3/L improves survival at very acidic and alkaline waters (Townsend and Baldisserotto 2001). Growth of this species is lower in acidic or alkaline soft water compared to neutral water (Copatti et al. 2005), but no studies with alkalinity were made. A previous study verified that stocking density of 0.5 fish/L in a closed system provided better growth of silver catfish juveniles than lower stocking densities (Piaia and Baldisserotto 2000), but higher densities were not tested. Therefore, this study investigated the influence of water alkalinity and stocking density for the culture of silver catfish juveniles in a closed system.