Measuring cortisol in the water as an indicator of stress caused by increased loading density in common carp (Cyprinus carpio)

The influence of a high loading density for a period of 28 days on growth, water quality and stress parameters in common carp (Cyprinus carpio) was investigated. In addition, cortisol levels in the water were measured during the experiment to investigate if a relationship exists between loading density and cortisol levels. No differences in the relative growth rates and food conversion were found between fish reared at the high (2.6 kg l(-1) min) and low loading rates (0.6 kg l(-1) min). Water quality was reduced in tanks with a higher loading density but not to an extent which was detrimental to the health of the fish. Plasma cortisol levels were only significant between high and low loading densities on day 3. However, cortisol levels in the water were significantly elevated on days 1, 3, 8 and 28 in the highdensity groups compared to the low. Loading densities did not effect plasma glucose and free fatty acid (FFA) levels, although an effect of the initial handling was found for FFA. Results show that carp adapt well to high loading densities and that measuring cortisol levels in the water may prove to be a useful indicator of stress levels in fish without inducing any disturbance. (C) 2003 Elsevier Science B.V. All rights reserved.