Increased stocking density influences the acute physiological stress response of common carp Cyprinus carpio (L.) N M Ruane, E C Carballo & J Komen Fish Culture & Fisheries Group, Wageningen Institute of Animal Sciences (WIAS), Wageningen University, Wageningen, The Netherlands Correspondence: Neil Ruane, Fish Culture & Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University, PO Box 338,6700 AH Wageningen, The Netherlands. E-mail: Neil.Ruane@alg.venv.wau.nl Abstract The physiological response of common carp, Cypri- nus carpio (L.) to increased stocking density and an additionalacutenetconfinementstressorwasinves- tigated. Stocking densities were increased from 28.4 to 56.8 or 113.6kgm 3 by the use of crowding screens and fish were sampled from the crowded groups after 15,39 and 87hours of crowding (hc). A transient elevation of plasma cor- tisol was found in the higher density group after 15hc before values returned to control levels. In- creasedstockingdensityalsoincreasedplasmalevels of glucose, free fatty acids (FFA) and lactate during the experimental period. No effect was found on oxygen radical production in the blood, haemo- globin or haematocrit levels, but leucocrit values were lower after 39hc in both groups. Net confine- ment resulted in a significant increase in plasma cortisol levels in all groups. However, after 15hc, cortisol and FFA levels in both crowded plus con- finement groups were higher than in the control and confined groups. Confinement resulted in in- creased oxygen radical production in the crowded plus confined groups at all times. Results indicate that although carp responded with a mild stress response to increased stocking density and adapta- tion occurred by 87hc, the crowded fish were more sensitive to an additional acute stressor. Keywords: common carp, stress, plasma cortisol, secondary responses Introduction The physiological responses of fish to stressors are essentially adaptive, resulting in the mobilization of energy reserves and cardiovascular changes that help the animal overcome a disturbance. It is well established that when subjected to stressors over a long period, these responses become maladaptive, resulting in deleterious effects on growth, reproduc- tion and health status (Barton & Iwama 1991). Measuring stress responses in aquaculture species isanintegralpartofthedevelopmentofappropriate management procedures as it provides information on the health status of the animals with many simple and portable test kits now available for the easy measurement of stress levels on-site (Iwama, Morgan & Barton 1995). In the aquaculture envir- onment, exposure to stressors is commonplace be- cause of the regular management procedures such as weighing, grading, transporting or, for more eco- nomical reasons, increased rearing densities. How- ever, little is known about the effects of these more acute stressors on the health and stress response of fish reared at elevated densities. The rearing of fish under crowded conditions such as high loading (defined as the weight per unit flow rate, kgL 1 min) or stocking densities (defined as the weight per unit volume, kgm 3 ) have been shown to negatively affect growth (Pickering & Stewart 1984; Procarione, Barry & Malison 1999), influence metabolite levels (Vijayan, Ballantyne & Leatherland 1990) and modulate immune functions (Yin, Lam & Sin 1995; Tort, Aquaculture Research, 2002, 33, 777±784 ß 2002 Blackwell Science Ltd 777