The Stress of four commercial farming practices, feeding, counting, grading and harvesting, in farmed rainbow trout, Oncorhynchus mykiss

Abstract

Plasma cortisol concentrations in farmed rainbow trout (Oncorhynchus mykiss) were used to determine the stress caused by feeding, counting, grading and harvesting. The effect of carrying out these practices with the addition of an aerator was also determined. The cortisol concentration in trout plasma was assessed using enzyme immunoassay (EIA). Pre-feeding levels were found to be 3–4 ng/ml. Feeding, counting, grading and harvesting produced significant elevations in plasma cortisol. The presence of an aerator during these practices significantly reduced this cortisol response. The plasma cortisol response during winter grading was significantly less (p<0.0001) compared to summer grading. Grading was also found to be a more stressful practice than feeding or counting. The cortisol response to grading was dependent on fish size (p = 0.0027). Winter harvesting was more stressful than summer harvesting (p = 0.0134), suggesting that lower temperatures may prolong the loss of consciousness. This study suggests that stress incurred by the trout during fish farming practices can be significantly reduced by oxygenating the water.